Monday, August 21, 2017
Tuesday, June 8, 2010
Questionnaire on Cellular concepts
Questionnaire on Cellular concepts
1. The available frequency spectrum in GSM is
a) 45 MHz
b) 30 MHz
c) 200 KHz
d) 25 MHz
2. Number of carriers available in GSM frequency Band are
a) 32
b) 8
c) 124
d) 132
3. The major reason for introducing cellular concepts in GSM is
a) to reduce interference
b) to increase Subscriber Capacity
c) to utilize scarce frequency spectrum
4. Cell reuse is based on one of the following.
a) Reuse of Cell no.
b) Frequency reuse
c) Transmit Power reuse
d) Subscriber identification no. reuse
5. The theoretical shape of a cell is
a) Circular
b) Pentagonal
c) Triangular
d) Hexagonal
6. K-factor in cellular system is
a) Cell size
b) Cluster size
c) Cell Radius
d) Ratio of Subscriber capacity to no. of carriers
8. The frequency reuse ratio in a cluster of cells is determined by
a) no. of frequency carriers in the cluster
b) no. of subscribers in the cluster
c) no. of cells in the cluster
d) no. of Power levels
9. The basic parameter governing the minimum distance between two reused cells is
a) co-channel interference
b) adjacent channel interference
c) mobile station Transmit Power
d) received signal level
10. The cell sectorization is used
a) to increase coverage
b) to increase system capacity
c) to reduce co-channel interference
d) to improve speech quality
11. 1 Erlang of traffic is
a) 60 seconds of use per busy hour
b) 90 seconds of use per busy hour
c) 30 minutes of use per busy hour
d) 60 minutes of use per busy hour
12. Grade Of Service (GOS) for 2% blockage is
a) measure of speech quality
b) measure of probability of a mobile subscriber of accessing a free channel
c) measure of probability of system to access mobile
d) measure of system capacity
Layer 3 Questionnaire
1. The basic function of layer 3 in OSI model is
a) physical addressing
b) frame segmentation
c) logical addressing and Routing
d) flow Control and data retransmission
2. The sub-layers in air interface layer 3 at MS are
a) Radio Resource (RR), Mobility Management (MM) and Connection Management (CM)
b) Call Control (CC), Short Message Service (SMS) and Supplementary Services (SS)
c) LAPD, BTSM & MTP
d) None
3. The functions of RR management sub-layer are
a) register updating, authentication and TMSI reallocation
b) establishment, operation and release of radio channel
c) base Station Radio Power Management
d) call connection, maintenance and release
4. The functions of MM management sub-layer are
a) register updating, authentication and TMSI reallocation
b) establishment, operation and release of radio channel
c) base Station Radio Power Management
d) call connection, maintenance and release
5. The function of Transaction Identifier (TI) field in a typical layer 3 message are
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
6. The function of Protocol Discriminator (PD) field in a typical layer 3 message is
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
7. The function of Message Type (MT) field in a typical layer 3 message is
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
8. The function of Information Elements (IE) field in a typical layer 3 message is
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
9. Mobility Management (MM) sub-layer provides services to which of the following entities?
a) Radio Resource (RR), Mobility Management (MM) and Connection Management (CM)
b) Call Control (CC), Short Message Service (SMS) and Supplementary Services (SS)
c) LAPD, BTSM & MTP
d) None from above
Layer 2 Questionnaire
1. The basic function of layer 2 (Data link layer) in general is
a) logical addressing and routing
b) transaction identification and data retransmission
c) frame formation by message segmentation & reassembly
d) logical verification of data
2. The data link layer in air interface is
a) LAPD (Link Access Procedure for ISDN data channel)
b) MTP (Message Transfer Part)
c) LAPDm (modified Link Access Procedure for ISDN data channel)
d) SCCP (Signaling Connection Control Part)
3. The fields of a LAPDm frame are
a) Network Field, Guard Field, Access Field & Response Field
b) Address Field, Control Field, Length Indicator, Information Field & Fill Bits
c) Data Field, Request Field, Stealing Bits, Access Field & End Bits
d) None
4. The Address Field of LAPDm frame contains
a) reserved bit, LPD, SAPI, C/R & EA fields
b) flag bit, BSN, BIB, FSN & FIB fields
c) flag bit, len, spare, service, status & CRC fields
d) none from above
5. The function of LPD field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
6. The function of C/R field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
7. The function of SAPI field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
8. The function of EA field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
9. The flag bits at the start & end to delimit the frame are not used in
a) LAPD
b) LAPDm
c) MTP2
d) MAP
10. The frame size in LAPDm for TCH is
a) 21 octets
b) 19 octets
c) 8 octets
d) 23 octets
11. The maximum length of frames on the Abis interface is
a) 256 octets
b) 64 octets
c) 264 octets
d) 272 octets
12. The maximum length of frames on the A interface is
a) 256 octets
b) 64 octets
c) 264 octets
d) 272 octets
13. Error detection for LAPDm is carried out by
a) adding 16 redundancy bits to each frame according to a coding scheme chosen for its error detection
b) combination of convolutional & block coding at the physical layer
c) None from above
14. SAPI 0 is used in LAPDm to indicate
a) transfer of signaling messages
b) short Message services
c) supplementary service messages
d) none from above
15. SAPI 3 is used in LAPDm to indicate
a) transfer of signaling messages
b) Short Message services
c) Supplementary service messages
d) none from above
1. The available frequency spectrum in GSM is
a) 45 MHz
b) 30 MHz
c) 200 KHz
d) 25 MHz
2. Number of carriers available in GSM frequency Band are
a) 32
b) 8
c) 124
d) 132
3. The major reason for introducing cellular concepts in GSM is
a) to reduce interference
b) to increase Subscriber Capacity
c) to utilize scarce frequency spectrum
4. Cell reuse is based on one of the following.
a) Reuse of Cell no.
b) Frequency reuse
c) Transmit Power reuse
d) Subscriber identification no. reuse
5. The theoretical shape of a cell is
a) Circular
b) Pentagonal
c) Triangular
d) Hexagonal
6. K-factor in cellular system is
a) Cell size
b) Cluster size
c) Cell Radius
d) Ratio of Subscriber capacity to no. of carriers
8. The frequency reuse ratio in a cluster of cells is determined by
a) no. of frequency carriers in the cluster
b) no. of subscribers in the cluster
c) no. of cells in the cluster
d) no. of Power levels
9. The basic parameter governing the minimum distance between two reused cells is
a) co-channel interference
b) adjacent channel interference
c) mobile station Transmit Power
d) received signal level
10. The cell sectorization is used
a) to increase coverage
b) to increase system capacity
c) to reduce co-channel interference
d) to improve speech quality
11. 1 Erlang of traffic is
a) 60 seconds of use per busy hour
b) 90 seconds of use per busy hour
c) 30 minutes of use per busy hour
d) 60 minutes of use per busy hour
12. Grade Of Service (GOS) for 2% blockage is
a) measure of speech quality
b) measure of probability of a mobile subscriber of accessing a free channel
c) measure of probability of system to access mobile
d) measure of system capacity
Layer 3 Questionnaire
1. The basic function of layer 3 in OSI model is
a) physical addressing
b) frame segmentation
c) logical addressing and Routing
d) flow Control and data retransmission
2. The sub-layers in air interface layer 3 at MS are
a) Radio Resource (RR), Mobility Management (MM) and Connection Management (CM)
b) Call Control (CC), Short Message Service (SMS) and Supplementary Services (SS)
c) LAPD, BTSM & MTP
d) None
3. The functions of RR management sub-layer are
a) register updating, authentication and TMSI reallocation
b) establishment, operation and release of radio channel
c) base Station Radio Power Management
d) call connection, maintenance and release
4. The functions of MM management sub-layer are
a) register updating, authentication and TMSI reallocation
b) establishment, operation and release of radio channel
c) base Station Radio Power Management
d) call connection, maintenance and release
5. The function of Transaction Identifier (TI) field in a typical layer 3 message are
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
6. The function of Protocol Discriminator (PD) field in a typical layer 3 message is
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
7. The function of Message Type (MT) field in a typical layer 3 message is
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
8. The function of Information Elements (IE) field in a typical layer 3 message is
a) to distinguish between messages belonging to RR, MM & CM procedures
b) to identify function of the message being sent
c) as an optional field to carry some information in the message
d) to distinguish multiple parallel activities within 1 mobile station
9. Mobility Management (MM) sub-layer provides services to which of the following entities?
a) Radio Resource (RR), Mobility Management (MM) and Connection Management (CM)
b) Call Control (CC), Short Message Service (SMS) and Supplementary Services (SS)
c) LAPD, BTSM & MTP
d) None from above
Layer 2 Questionnaire
1. The basic function of layer 2 (Data link layer) in general is
a) logical addressing and routing
b) transaction identification and data retransmission
c) frame formation by message segmentation & reassembly
d) logical verification of data
2. The data link layer in air interface is
a) LAPD (Link Access Procedure for ISDN data channel)
b) MTP (Message Transfer Part)
c) LAPDm (modified Link Access Procedure for ISDN data channel)
d) SCCP (Signaling Connection Control Part)
3. The fields of a LAPDm frame are
a) Network Field, Guard Field, Access Field & Response Field
b) Address Field, Control Field, Length Indicator, Information Field & Fill Bits
c) Data Field, Request Field, Stealing Bits, Access Field & End Bits
d) None
4. The Address Field of LAPDm frame contains
a) reserved bit, LPD, SAPI, C/R & EA fields
b) flag bit, BSN, BIB, FSN & FIB fields
c) flag bit, len, spare, service, status & CRC fields
d) none from above
5. The function of LPD field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
6. The function of C/R field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
7. The function of SAPI field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
8. The function of EA field in LAPDm frame is
a) to specify either a command or response frame
b) to identify the user of the service provided by the protocol
c) to specify a particular recommendation of the use of LAPDm
d) as an extended address bit to specify the length of LAPDm address field as either 1 or 2 bytes long
9. The flag bits at the start & end to delimit the frame are not used in
a) LAPD
b) LAPDm
c) MTP2
d) MAP
10. The frame size in LAPDm for TCH is
a) 21 octets
b) 19 octets
c) 8 octets
d) 23 octets
11. The maximum length of frames on the Abis interface is
a) 256 octets
b) 64 octets
c) 264 octets
d) 272 octets
12. The maximum length of frames on the A interface is
a) 256 octets
b) 64 octets
c) 264 octets
d) 272 octets
13. Error detection for LAPDm is carried out by
a) adding 16 redundancy bits to each frame according to a coding scheme chosen for its error detection
b) combination of convolutional & block coding at the physical layer
c) None from above
14. SAPI 0 is used in LAPDm to indicate
a) transfer of signaling messages
b) short Message services
c) supplementary service messages
d) none from above
15. SAPI 3 is used in LAPDm to indicate
a) transfer of signaling messages
b) Short Message services
c) Supplementary service messages
d) none from above
INTERVIEW QUESTIONS ON GSM
Questions On GSM
History Of GSM
1) Describe in short the evolution of GSM?
Year Mobile System
1981 Nordic Mobile Telephone (NMT) 450
1983 American Mobile Phone System (AMPS
1985 Total Access Communications System (TACS)
1986 Nordic Mobile telephony (NMT) 900
1991 American Digital Cellular (ADC)
1991 Global System for Mobile Communications (GSM)
1992 Digital Cellular Systems (DCS) 1800
1994 Personal Digital Cellular (PDC)
1995 PCS-1900 Canada
1996 PCS- United States
2) What are the standard bodies for the development and nurturing of the GSM standard?
CEPT (Conference Europeenne des Postes et Telecommunications)
MoU (Memorandum Of Understanding)
CCITT (Comite Consultatif International Telegraphique et telephonique)
General Characteristics Of GSM Technology
1) What is Time Division Multiple Access?
TDMA is a digital transmission technology, which works by dividing a radio frequency into time slots and then allocating slots to each user within each channel. In this way, a single frequency can support multiple, simultaneous data channels.
2) What is FDMA?
Frequency Divison Multiple access is a scheme in which the entire frequency band is divided into channels, each channel corresponding to a particular frequency range. With FDMA each channel can be assigned to one user at a time.
3) Give details of the uplink and downlink band of GSM?
The uplink band of GSM is from 890MHz to 915MHz and the downlink band of
is from 935MHz to 960MHz
4) What is the bandgap between the uplink and downlink carrier in GSM?
45 MHz.
5) What is the bandgap between 2 consecutive carriers?
200KHz
6) How many carriers are present in the GSM band and how many channels are there
in each carrier?
124 carriers and 8 channels per carrier
7) What type of modulation is used in GSM?
Gaussian Minimum Shift Keying (GMSK)
8) What is ARFCN?
In cellular mobile communications the radio channels are identified by their
Absolute Radio Frequency Channel Number. In GSM ARFCN 1 to 124 are used.
GSM Network Architecture
9) What are the components of the GSM network? Draw a schematic diagram of the
GSM Network architecture?
B
D
Um BSS Abis A
C H
E F
The GSM network architecture consists of the following components
a. Mobile Station (MS)
b. Base Transceiver Station (BTS)
c. Base Station Controller (BSC)
d. Mobile Switching Center (MSC)
e. Home Location Register (HLR)
f. Visitor Location Register (VLR)
g. Authentication Center (AuC)
h. Equipment Identity Register (EIR)
10) Describe in brief the functionalities of the Base Transceiver Station (BTS)?
The BTS is a part of the Base Station Sub system and is in contact with the MS
through the radio interface. The BTS is in charge of management of transmission and
reception on the radio interface.
11) Describe in brief the functionalities of the Base Station Controller (BSC)?
The BSC is connected on one side to one or several BTSs and on the other side to the
MSC. The main function of the BSC is allocation and release of radio channels and
the handover management.
12) What is the function of the Mobile switching center (MSC)?
The MSC performs the basic function of switching. The main function of the MSC is to co-ordinate the setting up of calls to and from GSM users and the external network The MSC has interface with the BSS on one side and the external network on the other side.
13) What is the function of the Home Location Register (HLR)?
The Home Location Register (HLR) together with the MSC, provide the callrouting
and roaming capabilities of GSM. The HLR contains all the
administrative information of each subscriber registered in the corresponding GSM
network, along with the current location of the mobile.
14) What is the function of the Visitor Location Register (VLR)?
The Visitor Location Register contains roaming information. Once the visited system
detects the mobile, the VLR of that system enquires the HLR to make sure you are a
valid subscriber. It temporarily stores the last location area visited by the MS, the
power the mobile uses, the special services the MS is subscribed to and so on.
15) What is the function of the Equipment Identity Register (EIR)?
The Equipment Identity Register (EIR) is a database that stores data related to the
mobile equipment. It contains a list of all valid mobile equipment on the network,
where each mobile station is identified by International Mobile Equipment Identity
(IMEI).This is useful when searching for stolen mobile equipment or when
monitoring misuse of mobile stations.
16) What is the function of the Authentication Center (AuC)?
The Authentication Center is a database that stores a copy of the secret key
stored in each subscriber's SIM card, which is used for authentication and ciphering
of the radio channel.
17) What is the function of the TRAU?
18) What is SIM? What are the functions of the SIM?
The Subscriber Identity Module is a card inside a mobile equipment which contains subscriber related data. The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information.
19) Describe how authentication is performed in GSM?
Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key (Ki), one copy of which is stored in the SIM card and the other in the Authentication Center. During authentication, the AuC generates a random number (RAND) that it sends to the mobile. Both the mobile and the AuC then use the random number, in conjunction with the subscriber's secret key and a ciphering algorithm called A3, to generate a number (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated.
GSM Air Interface
19) What are the two types of logical channels?
The two types of logical channels are Traffic Channels (TCH) and Control Channels
20) What are the various types of Control Channels?
There are three types of control channels mainly broadcast Control Channels,
Common Control Channels and Dedicated Control Channels.
21) What are the various types of Broadcast Control Channels?
The three types of Broadcast Control Channels are Broadcast Control Channel,
Synchronization Channel and Frequency Correction Channel.
22) What are the various types of Common Control Channels?
The three types of Common Control Channels are Access Grant Channel, Paging
Channel and Random Access Channel.
23) What are the various types of Dedicated Control Channels?
The three types of dedicated control channels are Slow Associated Control Channel,
Fast Associated Control Channel and Stand Alone Dedicated Control Channel
24) What is the function of the FCCH?
This downlink channel continuously transmits unmodulated carrier frequency for
frequency correction of the MS.
25) What is the function of the SCH?
The Synchronization Channel is used for frame synchronization of the Mobile Station. It contains information about TDMA frame number and the BSIC.
26) What is the function of the BCCH?
The Broadcast Control Channel broadcasts general information about the cell viz. neighbor cell information, MS maximum transmit power on CCHs and local area identification.
27) What is the function of the FACCH?
The FACCH is used as a main signaling link for the transmission of data eg. Handover commands. The FACCH is required for every call set up and release. The FACCH is transmitted in the burst by pre-empting a portion of the speech/user data information bits during active call.
28) What is the function of the AGCH?
The AGCH is used for sending access grant messages, which assign the MS to an SDCCH or directly to a TCH in response to a request placed on RACH by MS.
29) What is the function of the RACH?
The RACH is used to transfer uplink messages in response to the call initiation request placed by the MS or in response to a paging request or automatically as part of a location update.
30) What is the function of the Paging Channel (PCH)?
The PCH is used to page the MS during mobile terminated call setup. Paging messages are sent to the area where the recipient MS is located.
31) What is the function of the SDCCH?
The SDCCH carries all signaling between the BTS and the MS when no TCH is allocated. It is used for service requests (eg. SMS), location updates, subscriber authentication, ciphering, initiation, equipment validation and assignment to a TCH.
32) Draw and explain the protocol layer between the Mobile Station and the BTS?
Um
33) What are the functions of the Radio resource Management Layer in GSM?
The main function of the Radio resource management layer is to establish and release stable connections between mobile stations and the MSC for the duration of the call and to maintain the connection despite user movements.
34) What are the functions of the Mobility Management sub layer in GSM?
The mobility Management layer manages the location updating, handovers, and registration procedures. The machines concerned with mobility management are mainly the MS, the HLR and VLR. The security function is performed by the AuC.
35) What are the functions of the Communications Management sub layer in GSM?
Communications Management sub layer terminates at the MSC and contains entities that currently consist of CC including call-related supplementary services, SMS, and
call independent supplementary services support (SS).
36) What is Idle Mode?
A mobile is said to be in idle mode when it is active (powered on) but is not allocated any traffic channel. In the idle mode the MS listens to broadcast channels in order to intercept paging messages, monitor the radio environment in order to evaluate its quality and choose the most suitable base station.
37) What are layer 2 and 3 messages?
38) What is Base Station Identity Code?
The BSIC is a color code which the MSs use to be able to discriminate between the cells transmitting their beacon channels on the same frequency. Many cells bear the same BSIC and it is common for neighboring cells to have the same BSIC.
39) What is IMSI/TMSI?
Every Subscriber is assigned an IMSI associated with its SIM card. The IMSI is known only to the subscriber and is kept internal and transmitted as rarely as possible for security reasons keeping the identity of the subscriber confidential (in case some one is listening on the air interface). The IMSI consists of Mobile Country code (MCC) which identifies home country of the subscriber, the Mobile Network Code (MNC) which identifies the PLMN of the subscriber and the Mobile Station Identification Number (MSIN) which identifies the subscriber within a PLMN.
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The MSC/VLR allocates a TMSI temporarily to a subscriber present in the geographical area served by the MSC/VLR. Only the VLR stores the TMSI not the HLR. The TMSI serves to identify the MS when it needs to communicate with the network. It is used instead of the IMSI to avoid transmitting the IMSI. Several MSC/VLR’s can use the same TMSI
40) What is IMSI attach /Detach?
When a MS station is switched off(or when the SIM is removed by the user), call towards that MS station cannot be completed. Hence important resources are consumed for nothing. To alleviate this load, the IMSI attach and IMSI detach procedure is used. The subscribers record in the MSC/VLR contains a binary information indicating whether or not it is useful to try to complete the call toward the subscriber. The IMSI detach procedure will set the binary bit to “ Not Useful To Try” whereas the IMSI attach procedure will do the reverse.
41) What is the structure of IMSI?
3 Digits 2 Digits
Mobile Country Mobile Network Mobile Subscriber Identification
Code Code Number
42) What is MSISDN?
Mobile Station ISDN Number This is the subscriber phone number. It is the identity of the subscriber known by the external world. It is the number dialed when someone needs to call the mobile subscriber. The MSISDN consists of the Country Code (CC), National Destination Code (NDC) and Subscriber Number.
43) What is Mobile Station Roaming Number (MSRN)?
The MSRN is used to identify a subscriber when routing the call from the GMSC to the visited MSC for mobile terminating calls.. It is allocated by the MSC/VLR currently serving the subscriber on a call basis (temporary assignment)
44) What is discontinuous Reception?
For the sake of battery consumption in MS, it is important to minimize the amount of information the MS has to receive, demodulate and analyze when it is in idle mode. Therefore the downlink common control channel is divided into several paging sub channels and all messages pertaining to a given subscriber are sent on the same sub channel. Such a scheme allows the MS to restrict the monitoring of paging messages to their own paging sub channel, thereby increasing the life time of the battery at the expense of a small increase in the delay for the setting up of incoming calls.
45) What is Discontinuous Transmission?
Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation by turning the transmitter off during silence periods. Power is conserved at the mobile unit by appling Discontinuous Transmission. Whether DTX should be applied or not is decide by the MSC while its execution is controlled by the BSC.
46) What is Location Updating?
When a MS moves to a new location area or is switched on in a new location area, it must register with the network to indicate its current location. A location update message is sent to the new MSC/VLR, which records the location area information, and then sends the location information to the subscriber's HLR
47) What are the various types of bursts in GSM?
Normal Burst
Synchronization Burst
Dummy Burst
Frequency Synchronization Burst.
48) What is the structure of a normal Burst?
49) Draw and Explain the Protocol architecture of the GSM Network?
a. RR Management sub layer
Manages the Radio Interface
Terminates at BSS from MS
b. MM sub Layer
Terminated at MSC
Messages from or to the MSC are relayed transparently from BSS
c. Communications Management sub Layer
Terminates at MSC
Contains entities that consist of CC including call related supplementary services, SMS and call independent Supplementary Services (SS) support.
d. TCAP and MAP
These are the interfaces between the MSC and HLR/VLR
e. BSSAP
Used to implement all procedures between the MSC and the BSS that require interpretation and processing of information related to single calls and resource management.
f. SCCP and MTP
SCCP and MTP protocols are used to implement the data link layer and layer 3 transport functions for carrying the call control and mobility management signaling message son the BSS_MSC link.
GSM Network Planning & Optimization
1) What is the basic procedure for RF planning using RF planning tool in GSM?
Planning Tool
Loading of digital map data of the city on the planning tool, Model tuning for proper prediction on the tool ,Nominal cell planning for the coverage ,frequency planning and interference study.
Final system planning.
2) What are the different types of antennas used in a GSM network?
Antennas of different types based on antenna beamwidth and gain,
In GSM We have space diversity antenna and cross polorised antenna.Omni antennas,
3) How do the power, orientation, beamwidth, tilt and height of the antenna affect the coverage?
Power increases the coverage, Proper Orientation, Beamwidth and tilt improves the coverage of the planned area. Increase in antenna height improves the coverage.
4) What is EIRP? How is it calculated?
Effective isotropic radiated power is the total o/p power of Base station
EIRP=BTS (O/P) power+Antenna gain-cablelosses-other (combiner)losses
5) What is frequency planning? Why is frequency reuse pattern used?
Frequency planning is to be done for the GSM network for a given operator with
A set of GSM frequency band.Frequency reuse pattern is used for capacity planning.
To minimize frequency interference problems.
6) What are all the patterns available? How is frequency planning done on planning tool?
4/12 pattern.3/9 pattern .Automatic frequency planning can be done in planning tool.
7) What is co-channel interference and adjacent channel interference?
The interference caused by using same channels in a network is called cochannel interference and interference caused by adjacent channels of is called adjacent channel interference.
8) How do you minimize co-channel interference?
proper frequency planning and proper reuse pattern.
9) How do you minimize adjacent channel interference?
Proper frequency planning.
10) What is model tuning? Why is it used? Okarama Hatta model?
Model tuning is done for any planning tool to obtain proper prediction expected from
planning tool
as there will be difference in actual and predicted data for planning tool. One of the model is Okarama Hatta
Hatta Model is used which considers the signal propagation losses.
11) What is link Budget Analysis used for in GSM?
Link budget analysis is used for the path balance between both uplink (Transmitter)
Downlink (Receiver) part of the network.
12) How is path loss taken into calculation?
This is the total path loss occurred due to multipath propagation of the signal between transmitted signal and the received signal level.
13) What is capacity planning?
This is process of increasing traffic by adding Trxs traffic channels and proper use of frequency reuse pattern. Addition of BTSs
14) How is site selection done for planning and site acquisition?
After nominal cell planning with the geographical coordinates identifying best site candidate in a geographical area is the process of site selection.
15) What is optimization? What are the tools used for optimization?
Once GSM network is integrated in order to achieve proper planning prediction
RF parameters Optimization is done. Drive test tool,Planning tool and Post processing tools are
Used for optimization.
16) How do we use the above antenna patterns for optimization?
Proper antenna orientation and tilts and antenna types can be used
for optimizing network.
17) How do we handle poor C/I, C/A, handover problems and blocking?
Checking proper neighbor list, changing frequency planning and proper no of radio channels availability.
18) What is system information?
System information is from BTS sent to Mobile for the idle mode and dedicated mode of the call for call management.
19) What is bench marking in GSM?
Bench Marking used for comparing performance of different network for quality and call performance parameters.
20) What is Erlang table?
This is the table for calculating Traffic in erlang for no of channels.
21) What is grade of service?
Grade of service is the blocking for the given traffic channels.
22) How do you optimize a network using OMCR performance data?
Proper increase in call success, and increase in handover success performance and drop call reduction performance.
23) What is daily report and traffic?
This is the report obtained in OMC for the performce of all BTS about traffic .
24) What is cell_reselect_hysterisis?
This is process of handover done by mobile in idle mode from selected cell to another cell.
25) What is path loss criterion (C1)? How is it calculated?
This is the signal strength measured in idle mode for selecting best serving channel.
History Of GSM
1) Describe in short the evolution of GSM?
Year Mobile System
1981 Nordic Mobile Telephone (NMT) 450
1983 American Mobile Phone System (AMPS
1985 Total Access Communications System (TACS)
1986 Nordic Mobile telephony (NMT) 900
1991 American Digital Cellular (ADC)
1991 Global System for Mobile Communications (GSM)
1992 Digital Cellular Systems (DCS) 1800
1994 Personal Digital Cellular (PDC)
1995 PCS-1900 Canada
1996 PCS- United States
2) What are the standard bodies for the development and nurturing of the GSM standard?
CEPT (Conference Europeenne des Postes et Telecommunications)
MoU (Memorandum Of Understanding)
CCITT (Comite Consultatif International Telegraphique et telephonique)
General Characteristics Of GSM Technology
1) What is Time Division Multiple Access?
TDMA is a digital transmission technology, which works by dividing a radio frequency into time slots and then allocating slots to each user within each channel. In this way, a single frequency can support multiple, simultaneous data channels.
2) What is FDMA?
Frequency Divison Multiple access is a scheme in which the entire frequency band is divided into channels, each channel corresponding to a particular frequency range. With FDMA each channel can be assigned to one user at a time.
3) Give details of the uplink and downlink band of GSM?
The uplink band of GSM is from 890MHz to 915MHz and the downlink band of
is from 935MHz to 960MHz
4) What is the bandgap between the uplink and downlink carrier in GSM?
45 MHz.
5) What is the bandgap between 2 consecutive carriers?
200KHz
6) How many carriers are present in the GSM band and how many channels are there
in each carrier?
124 carriers and 8 channels per carrier
7) What type of modulation is used in GSM?
Gaussian Minimum Shift Keying (GMSK)
8) What is ARFCN?
In cellular mobile communications the radio channels are identified by their
Absolute Radio Frequency Channel Number. In GSM ARFCN 1 to 124 are used.
GSM Network Architecture
9) What are the components of the GSM network? Draw a schematic diagram of the
GSM Network architecture?
B
D
Um BSS Abis A
C H
E F
The GSM network architecture consists of the following components
a. Mobile Station (MS)
b. Base Transceiver Station (BTS)
c. Base Station Controller (BSC)
d. Mobile Switching Center (MSC)
e. Home Location Register (HLR)
f. Visitor Location Register (VLR)
g. Authentication Center (AuC)
h. Equipment Identity Register (EIR)
10) Describe in brief the functionalities of the Base Transceiver Station (BTS)?
The BTS is a part of the Base Station Sub system and is in contact with the MS
through the radio interface. The BTS is in charge of management of transmission and
reception on the radio interface.
11) Describe in brief the functionalities of the Base Station Controller (BSC)?
The BSC is connected on one side to one or several BTSs and on the other side to the
MSC. The main function of the BSC is allocation and release of radio channels and
the handover management.
12) What is the function of the Mobile switching center (MSC)?
The MSC performs the basic function of switching. The main function of the MSC is to co-ordinate the setting up of calls to and from GSM users and the external network The MSC has interface with the BSS on one side and the external network on the other side.
13) What is the function of the Home Location Register (HLR)?
The Home Location Register (HLR) together with the MSC, provide the callrouting
and roaming capabilities of GSM. The HLR contains all the
administrative information of each subscriber registered in the corresponding GSM
network, along with the current location of the mobile.
14) What is the function of the Visitor Location Register (VLR)?
The Visitor Location Register contains roaming information. Once the visited system
detects the mobile, the VLR of that system enquires the HLR to make sure you are a
valid subscriber. It temporarily stores the last location area visited by the MS, the
power the mobile uses, the special services the MS is subscribed to and so on.
15) What is the function of the Equipment Identity Register (EIR)?
The Equipment Identity Register (EIR) is a database that stores data related to the
mobile equipment. It contains a list of all valid mobile equipment on the network,
where each mobile station is identified by International Mobile Equipment Identity
(IMEI).This is useful when searching for stolen mobile equipment or when
monitoring misuse of mobile stations.
16) What is the function of the Authentication Center (AuC)?
The Authentication Center is a database that stores a copy of the secret key
stored in each subscriber's SIM card, which is used for authentication and ciphering
of the radio channel.
17) What is the function of the TRAU?
18) What is SIM? What are the functions of the SIM?
The Subscriber Identity Module is a card inside a mobile equipment which contains subscriber related data. The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information.
19) Describe how authentication is performed in GSM?
Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key (Ki), one copy of which is stored in the SIM card and the other in the Authentication Center. During authentication, the AuC generates a random number (RAND) that it sends to the mobile. Both the mobile and the AuC then use the random number, in conjunction with the subscriber's secret key and a ciphering algorithm called A3, to generate a number (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated.
GSM Air Interface
19) What are the two types of logical channels?
The two types of logical channels are Traffic Channels (TCH) and Control Channels
20) What are the various types of Control Channels?
There are three types of control channels mainly broadcast Control Channels,
Common Control Channels and Dedicated Control Channels.
21) What are the various types of Broadcast Control Channels?
The three types of Broadcast Control Channels are Broadcast Control Channel,
Synchronization Channel and Frequency Correction Channel.
22) What are the various types of Common Control Channels?
The three types of Common Control Channels are Access Grant Channel, Paging
Channel and Random Access Channel.
23) What are the various types of Dedicated Control Channels?
The three types of dedicated control channels are Slow Associated Control Channel,
Fast Associated Control Channel and Stand Alone Dedicated Control Channel
24) What is the function of the FCCH?
This downlink channel continuously transmits unmodulated carrier frequency for
frequency correction of the MS.
25) What is the function of the SCH?
The Synchronization Channel is used for frame synchronization of the Mobile Station. It contains information about TDMA frame number and the BSIC.
26) What is the function of the BCCH?
The Broadcast Control Channel broadcasts general information about the cell viz. neighbor cell information, MS maximum transmit power on CCHs and local area identification.
27) What is the function of the FACCH?
The FACCH is used as a main signaling link for the transmission of data eg. Handover commands. The FACCH is required for every call set up and release. The FACCH is transmitted in the burst by pre-empting a portion of the speech/user data information bits during active call.
28) What is the function of the AGCH?
The AGCH is used for sending access grant messages, which assign the MS to an SDCCH or directly to a TCH in response to a request placed on RACH by MS.
29) What is the function of the RACH?
The RACH is used to transfer uplink messages in response to the call initiation request placed by the MS or in response to a paging request or automatically as part of a location update.
30) What is the function of the Paging Channel (PCH)?
The PCH is used to page the MS during mobile terminated call setup. Paging messages are sent to the area where the recipient MS is located.
31) What is the function of the SDCCH?
The SDCCH carries all signaling between the BTS and the MS when no TCH is allocated. It is used for service requests (eg. SMS), location updates, subscriber authentication, ciphering, initiation, equipment validation and assignment to a TCH.
32) Draw and explain the protocol layer between the Mobile Station and the BTS?
Um
33) What are the functions of the Radio resource Management Layer in GSM?
The main function of the Radio resource management layer is to establish and release stable connections between mobile stations and the MSC for the duration of the call and to maintain the connection despite user movements.
34) What are the functions of the Mobility Management sub layer in GSM?
The mobility Management layer manages the location updating, handovers, and registration procedures. The machines concerned with mobility management are mainly the MS, the HLR and VLR. The security function is performed by the AuC.
35) What are the functions of the Communications Management sub layer in GSM?
Communications Management sub layer terminates at the MSC and contains entities that currently consist of CC including call-related supplementary services, SMS, and
call independent supplementary services support (SS).
36) What is Idle Mode?
A mobile is said to be in idle mode when it is active (powered on) but is not allocated any traffic channel. In the idle mode the MS listens to broadcast channels in order to intercept paging messages, monitor the radio environment in order to evaluate its quality and choose the most suitable base station.
37) What are layer 2 and 3 messages?
38) What is Base Station Identity Code?
The BSIC is a color code which the MSs use to be able to discriminate between the cells transmitting their beacon channels on the same frequency. Many cells bear the same BSIC and it is common for neighboring cells to have the same BSIC.
39) What is IMSI/TMSI?
Every Subscriber is assigned an IMSI associated with its SIM card. The IMSI is known only to the subscriber and is kept internal and transmitted as rarely as possible for security reasons keeping the identity of the subscriber confidential (in case some one is listening on the air interface). The IMSI consists of Mobile Country code (MCC) which identifies home country of the subscriber, the Mobile Network Code (MNC) which identifies the PLMN of the subscriber and the Mobile Station Identification Number (MSIN) which identifies the subscriber within a PLMN.
`
The MSC/VLR allocates a TMSI temporarily to a subscriber present in the geographical area served by the MSC/VLR. Only the VLR stores the TMSI not the HLR. The TMSI serves to identify the MS when it needs to communicate with the network. It is used instead of the IMSI to avoid transmitting the IMSI. Several MSC/VLR’s can use the same TMSI
40) What is IMSI attach /Detach?
When a MS station is switched off(or when the SIM is removed by the user), call towards that MS station cannot be completed. Hence important resources are consumed for nothing. To alleviate this load, the IMSI attach and IMSI detach procedure is used. The subscribers record in the MSC/VLR contains a binary information indicating whether or not it is useful to try to complete the call toward the subscriber. The IMSI detach procedure will set the binary bit to “ Not Useful To Try” whereas the IMSI attach procedure will do the reverse.
41) What is the structure of IMSI?
3 Digits 2 Digits
Mobile Country Mobile Network Mobile Subscriber Identification
Code Code Number
42) What is MSISDN?
Mobile Station ISDN Number This is the subscriber phone number. It is the identity of the subscriber known by the external world. It is the number dialed when someone needs to call the mobile subscriber. The MSISDN consists of the Country Code (CC), National Destination Code (NDC) and Subscriber Number.
43) What is Mobile Station Roaming Number (MSRN)?
The MSRN is used to identify a subscriber when routing the call from the GMSC to the visited MSC for mobile terminating calls.. It is allocated by the MSC/VLR currently serving the subscriber on a call basis (temporary assignment)
44) What is discontinuous Reception?
For the sake of battery consumption in MS, it is important to minimize the amount of information the MS has to receive, demodulate and analyze when it is in idle mode. Therefore the downlink common control channel is divided into several paging sub channels and all messages pertaining to a given subscriber are sent on the same sub channel. Such a scheme allows the MS to restrict the monitoring of paging messages to their own paging sub channel, thereby increasing the life time of the battery at the expense of a small increase in the delay for the setting up of incoming calls.
45) What is Discontinuous Transmission?
Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation by turning the transmitter off during silence periods. Power is conserved at the mobile unit by appling Discontinuous Transmission. Whether DTX should be applied or not is decide by the MSC while its execution is controlled by the BSC.
46) What is Location Updating?
When a MS moves to a new location area or is switched on in a new location area, it must register with the network to indicate its current location. A location update message is sent to the new MSC/VLR, which records the location area information, and then sends the location information to the subscriber's HLR
47) What are the various types of bursts in GSM?
Normal Burst
Synchronization Burst
Dummy Burst
Frequency Synchronization Burst.
48) What is the structure of a normal Burst?
49) Draw and Explain the Protocol architecture of the GSM Network?
a. RR Management sub layer
Manages the Radio Interface
Terminates at BSS from MS
b. MM sub Layer
Terminated at MSC
Messages from or to the MSC are relayed transparently from BSS
c. Communications Management sub Layer
Terminates at MSC
Contains entities that consist of CC including call related supplementary services, SMS and call independent Supplementary Services (SS) support.
d. TCAP and MAP
These are the interfaces between the MSC and HLR/VLR
e. BSSAP
Used to implement all procedures between the MSC and the BSS that require interpretation and processing of information related to single calls and resource management.
f. SCCP and MTP
SCCP and MTP protocols are used to implement the data link layer and layer 3 transport functions for carrying the call control and mobility management signaling message son the BSS_MSC link.
GSM Network Planning & Optimization
1) What is the basic procedure for RF planning using RF planning tool in GSM?
Planning Tool
Loading of digital map data of the city on the planning tool, Model tuning for proper prediction on the tool ,Nominal cell planning for the coverage ,frequency planning and interference study.
Final system planning.
2) What are the different types of antennas used in a GSM network?
Antennas of different types based on antenna beamwidth and gain,
In GSM We have space diversity antenna and cross polorised antenna.Omni antennas,
3) How do the power, orientation, beamwidth, tilt and height of the antenna affect the coverage?
Power increases the coverage, Proper Orientation, Beamwidth and tilt improves the coverage of the planned area. Increase in antenna height improves the coverage.
4) What is EIRP? How is it calculated?
Effective isotropic radiated power is the total o/p power of Base station
EIRP=BTS (O/P) power+Antenna gain-cablelosses-other (combiner)losses
5) What is frequency planning? Why is frequency reuse pattern used?
Frequency planning is to be done for the GSM network for a given operator with
A set of GSM frequency band.Frequency reuse pattern is used for capacity planning.
To minimize frequency interference problems.
6) What are all the patterns available? How is frequency planning done on planning tool?
4/12 pattern.3/9 pattern .Automatic frequency planning can be done in planning tool.
7) What is co-channel interference and adjacent channel interference?
The interference caused by using same channels in a network is called cochannel interference and interference caused by adjacent channels of is called adjacent channel interference.
8) How do you minimize co-channel interference?
proper frequency planning and proper reuse pattern.
9) How do you minimize adjacent channel interference?
Proper frequency planning.
10) What is model tuning? Why is it used? Okarama Hatta model?
Model tuning is done for any planning tool to obtain proper prediction expected from
planning tool
as there will be difference in actual and predicted data for planning tool. One of the model is Okarama Hatta
Hatta Model is used which considers the signal propagation losses.
11) What is link Budget Analysis used for in GSM?
Link budget analysis is used for the path balance between both uplink (Transmitter)
Downlink (Receiver) part of the network.
12) How is path loss taken into calculation?
This is the total path loss occurred due to multipath propagation of the signal between transmitted signal and the received signal level.
13) What is capacity planning?
This is process of increasing traffic by adding Trxs traffic channels and proper use of frequency reuse pattern. Addition of BTSs
14) How is site selection done for planning and site acquisition?
After nominal cell planning with the geographical coordinates identifying best site candidate in a geographical area is the process of site selection.
15) What is optimization? What are the tools used for optimization?
Once GSM network is integrated in order to achieve proper planning prediction
RF parameters Optimization is done. Drive test tool,Planning tool and Post processing tools are
Used for optimization.
16) How do we use the above antenna patterns for optimization?
Proper antenna orientation and tilts and antenna types can be used
for optimizing network.
17) How do we handle poor C/I, C/A, handover problems and blocking?
Checking proper neighbor list, changing frequency planning and proper no of radio channels availability.
18) What is system information?
System information is from BTS sent to Mobile for the idle mode and dedicated mode of the call for call management.
19) What is bench marking in GSM?
Bench Marking used for comparing performance of different network for quality and call performance parameters.
20) What is Erlang table?
This is the table for calculating Traffic in erlang for no of channels.
21) What is grade of service?
Grade of service is the blocking for the given traffic channels.
22) How do you optimize a network using OMCR performance data?
Proper increase in call success, and increase in handover success performance and drop call reduction performance.
23) What is daily report and traffic?
This is the report obtained in OMC for the performce of all BTS about traffic .
24) What is cell_reselect_hysterisis?
This is process of handover done by mobile in idle mode from selected cell to another cell.
25) What is path loss criterion (C1)? How is it calculated?
This is the signal strength measured in idle mode for selecting best serving channel.
INTERVIEW QUESTIONS
GSM Air Interface
1.What is the uplink frequency range for GSM?
a. 935-960MHz
b. 945-960MHz
c. 890-915Mhz
d. 880-915MHz
2. What is the Downlink frequency range for GSM?
a. 935-960MHz
b. 945-960MHz
c. 890-915Mhz
d. 880-915MHz
3.How many carriers are present in GSM?
a. 8
b. 124
c. 25
d. 16
4. How many Channels are present in each carrier?
a. 8
b. 124
c. 25
d. 16
5. What type of modulation is used in GSM?
a. GMSK
b. BPSK
c. QPSK
d. MSK
6. What is the period of 1 burst?
a. 20ms
b. 120ms
c. 0.320ms
d. 0.577ms
7. What is the spacing between each Channel?
a. 25MHz
b. 200KHz
c. 1.25MHz
d. 100KHz
8. What is the interface between the Mobile Station and BTS called?
a. Um interface
b. Abis Interface
c. A Interface
d. B Interface
9. The Channel that is used to alert the Mobile Station of an incoming call is
a. Random Access CHannel
b. Paging CHannel
c. Synchronization CHannel
d. Access Grant CHannel
10. The Channel that is used to carry all signaling information when no traffic Channel
has been allocated is
a. Random Access CHannel
b. Broadcast Control CHannel
c. Fast Associated Control CHannel
d. Stand Alone Dedicated Control CHannel
11. The Channel that is used to transfer uplink messages in response to paging
request is
a. Random Access CHannel
b. Access Grant CHannel
c. Slow Associated Common Control CHannel
d. Fast Associated Control CHannels
12. Neighboring cell information is carried on
a. Broadcast Control CHannel
b. Paging CHannel
c. Random Access CHannel
d. Slow Associated Common Control CHannel
13. Frame synchronization of the MS is done by
a. Broadcast Control CHannel
b. Paging CHannel
c. Access Grant CHannel
d. Synchronization CHannel
14. The Channel that is transmitted in burst stealing mode is
a. BCCH
b. RACH
c. FACCH
d. SDCCH
15. How many slots make up a frame?
a. 4
b. 8
c. 12
d. 16
16. How many frames make up a TCH multiframe?
a. 8
b. 16
c. 26
d. 51
17. How many frames make up a CCH multiframe?
a. 8
b. 16
c. 26
d. 51
17. What are the different broadcast control Channels?
a. Paging CHannel, broadcast control CHannel and Synchronization CHannel
b. Broadcast control CHannel, common control CHannels, random access CHannel
c. Broadcast control CHannel, synchronization CHannel and frequency correction CHannel
d. Broadcast control CHannel, access grant CHannel and random access CHannel
18. What are the various types of common control Channels?
a. Paging CHannel, broadcast control CHannel and Synchronization CHannel
b. Broadcast control CHannel, common control CHannels, random access CHannel
c. Synchronization CHannel, frequency correction CHannel and access grant CHannel
c. Paging CHannel, random access CHannel and access grant CHannel
19. What are the different dedicated control Channels?
a. Paging CHannel, broadcast control CHannel and Synchronization CHannel
b. Broadcast control CHannel, access grant CHannel and random access CHannel
c. Paging CHannel, random access CHannel and access grant CHannel
d. Stand alone dedicated control CHannel, slow associated control CHannel and fast associated control CHannel.
20. What is the spacing between an uplink and downlink carrier?
a. 1.25MHz
b. 25MHz
c. 45MHz
d. 60MHz
21. What is the basic GSM speech rate?
a. 13Kbps
b. 14Kbps
c. 15Kbps
d. 16Kbps
22. What is the basic GSM data rate?
a. 13Kbps
b. 9.6Kbps
c. 4Kbps
d. 10.2Kbps
23. A MS is said to be in dedicated mode when
a. A traffic Channel is allocated to it
b. A traffic Channel is not allocated to it
24. A traffic Channel (TCH) is always associated with
a. AGCH
b. RACH
c. PAGC
d. SACCH
25. Which of these Channels is an uplink Channel?
a. FCCH
b. RACH
c. BCCH
d. PAGCH
26. What is the speech coding algorithm used in GSM?
GSM Protocol Architecture
1. The layer 2 protocol on the air interface is
a. LAPDm
b. LAPD
c. SCCP
d. TCAP
2. The layer 2 protocol on the Abis interface is
a. LAPDm
b. LAPD
c. SCCP
d. TCAP
3. The interface between the MSC and VLR/HLR are
a. MTP
b. BSSAP and MTP
c. TCAP and MAP
d. MAP and MTP
4.The layer responsible for the management of the radio resource management is
a. Radio Resource management layer (RR)
b. Mobility Management layer (MM)
c. Communication Control Layer (CM)
5. The layer which is responsible of handling call related supplementary services is
a. RR
b. MM
c. CM
6.The function of handovers is performed by?
a. RR
b. MM
c. CM
GSM Network Architecture
1. The network entity which is in direct contact with the Mobile Station (MS) is
d. Base Transceiver Station (BTS)
e. Base Station Controller (BSC)
f. Mobile Switching center (MSC)
g. Home location Register (HLR)
2. The network entity which is in charge of managing one or more BTS is
a. MSC
b. BSC
c. AuC
d. HLR
3. The entity which is in charge of managing the radio resources is
a. BTS
b. BSC
c. MSC
d. HLR
4. The code which identifies the Mobile Equipment is
a. Subscriber identity Module (SIM)
b. International Mobile Subscriber identity (IMSI)
c. International Mobile Equipment Identity (IMEI)
d. Temporary Mobile Subscriber Identity (TMSI)
5. The code which is used instead of IMSI for security reasons is
a. IMEI
b. TMSI
c. SIM
d. MSISDN
6. The directory number used to call up the subscribers is
a. MSISDN
b. IMSI
c. TMSI
d. IMEI
7. Both MSISDN and IMSI contain an identification of the country and the network
within the country
a. True
b. False
8. Subscriber information relevant to the to the provision of telecom services is held by?
a. MSC
b. VLR
c. HLR
d. BSC
9.What is the fucnction of the TRAU?
The TRAU is used for transcoding and rate adaptation. Basically speech on air interface is encoded as 13Kbps whereas speech on ISDN lines is standard 64Kbps. This translation from 13Kbps on air interface to 64Kbps standard ISDN rate is done by the TRAU.
10. The database which stores data related to mobile equipment is
a. AuC
b. BSC
c. EIR
d. MSC
GSM Layer 3
1. When there is no transmission taking place for a MS it is said it be in
e. Dedicated mode
f. Idle Mode
g. Handover
h. Sleep mode
2. The process in which the actual transmission on radio part is reduced to minimum during silence periods is
a. Handover Process
b. Discontinuous Reception
c. Paging Procedure
d. Discontinuous Transmission
1.What is the uplink frequency range for GSM?
a. 935-960MHz
b. 945-960MHz
c. 890-915Mhz
d. 880-915MHz
2. What is the Downlink frequency range for GSM?
a. 935-960MHz
b. 945-960MHz
c. 890-915Mhz
d. 880-915MHz
3.How many carriers are present in GSM?
a. 8
b. 124
c. 25
d. 16
4. How many Channels are present in each carrier?
a. 8
b. 124
c. 25
d. 16
5. What type of modulation is used in GSM?
a. GMSK
b. BPSK
c. QPSK
d. MSK
6. What is the period of 1 burst?
a. 20ms
b. 120ms
c. 0.320ms
d. 0.577ms
7. What is the spacing between each Channel?
a. 25MHz
b. 200KHz
c. 1.25MHz
d. 100KHz
8. What is the interface between the Mobile Station and BTS called?
a. Um interface
b. Abis Interface
c. A Interface
d. B Interface
9. The Channel that is used to alert the Mobile Station of an incoming call is
a. Random Access CHannel
b. Paging CHannel
c. Synchronization CHannel
d. Access Grant CHannel
10. The Channel that is used to carry all signaling information when no traffic Channel
has been allocated is
a. Random Access CHannel
b. Broadcast Control CHannel
c. Fast Associated Control CHannel
d. Stand Alone Dedicated Control CHannel
11. The Channel that is used to transfer uplink messages in response to paging
request is
a. Random Access CHannel
b. Access Grant CHannel
c. Slow Associated Common Control CHannel
d. Fast Associated Control CHannels
12. Neighboring cell information is carried on
a. Broadcast Control CHannel
b. Paging CHannel
c. Random Access CHannel
d. Slow Associated Common Control CHannel
13. Frame synchronization of the MS is done by
a. Broadcast Control CHannel
b. Paging CHannel
c. Access Grant CHannel
d. Synchronization CHannel
14. The Channel that is transmitted in burst stealing mode is
a. BCCH
b. RACH
c. FACCH
d. SDCCH
15. How many slots make up a frame?
a. 4
b. 8
c. 12
d. 16
16. How many frames make up a TCH multiframe?
a. 8
b. 16
c. 26
d. 51
17. How many frames make up a CCH multiframe?
a. 8
b. 16
c. 26
d. 51
17. What are the different broadcast control Channels?
a. Paging CHannel, broadcast control CHannel and Synchronization CHannel
b. Broadcast control CHannel, common control CHannels, random access CHannel
c. Broadcast control CHannel, synchronization CHannel and frequency correction CHannel
d. Broadcast control CHannel, access grant CHannel and random access CHannel
18. What are the various types of common control Channels?
a. Paging CHannel, broadcast control CHannel and Synchronization CHannel
b. Broadcast control CHannel, common control CHannels, random access CHannel
c. Synchronization CHannel, frequency correction CHannel and access grant CHannel
c. Paging CHannel, random access CHannel and access grant CHannel
19. What are the different dedicated control Channels?
a. Paging CHannel, broadcast control CHannel and Synchronization CHannel
b. Broadcast control CHannel, access grant CHannel and random access CHannel
c. Paging CHannel, random access CHannel and access grant CHannel
d. Stand alone dedicated control CHannel, slow associated control CHannel and fast associated control CHannel.
20. What is the spacing between an uplink and downlink carrier?
a. 1.25MHz
b. 25MHz
c. 45MHz
d. 60MHz
21. What is the basic GSM speech rate?
a. 13Kbps
b. 14Kbps
c. 15Kbps
d. 16Kbps
22. What is the basic GSM data rate?
a. 13Kbps
b. 9.6Kbps
c. 4Kbps
d. 10.2Kbps
23. A MS is said to be in dedicated mode when
a. A traffic Channel is allocated to it
b. A traffic Channel is not allocated to it
24. A traffic Channel (TCH) is always associated with
a. AGCH
b. RACH
c. PAGC
d. SACCH
25. Which of these Channels is an uplink Channel?
a. FCCH
b. RACH
c. BCCH
d. PAGCH
26. What is the speech coding algorithm used in GSM?
GSM Protocol Architecture
1. The layer 2 protocol on the air interface is
a. LAPDm
b. LAPD
c. SCCP
d. TCAP
2. The layer 2 protocol on the Abis interface is
a. LAPDm
b. LAPD
c. SCCP
d. TCAP
3. The interface between the MSC and VLR/HLR are
a. MTP
b. BSSAP and MTP
c. TCAP and MAP
d. MAP and MTP
4.The layer responsible for the management of the radio resource management is
a. Radio Resource management layer (RR)
b. Mobility Management layer (MM)
c. Communication Control Layer (CM)
5. The layer which is responsible of handling call related supplementary services is
a. RR
b. MM
c. CM
6.The function of handovers is performed by?
a. RR
b. MM
c. CM
GSM Network Architecture
1. The network entity which is in direct contact with the Mobile Station (MS) is
d. Base Transceiver Station (BTS)
e. Base Station Controller (BSC)
f. Mobile Switching center (MSC)
g. Home location Register (HLR)
2. The network entity which is in charge of managing one or more BTS is
a. MSC
b. BSC
c. AuC
d. HLR
3. The entity which is in charge of managing the radio resources is
a. BTS
b. BSC
c. MSC
d. HLR
4. The code which identifies the Mobile Equipment is
a. Subscriber identity Module (SIM)
b. International Mobile Subscriber identity (IMSI)
c. International Mobile Equipment Identity (IMEI)
d. Temporary Mobile Subscriber Identity (TMSI)
5. The code which is used instead of IMSI for security reasons is
a. IMEI
b. TMSI
c. SIM
d. MSISDN
6. The directory number used to call up the subscribers is
a. MSISDN
b. IMSI
c. TMSI
d. IMEI
7. Both MSISDN and IMSI contain an identification of the country and the network
within the country
a. True
b. False
8. Subscriber information relevant to the to the provision of telecom services is held by?
a. MSC
b. VLR
c. HLR
d. BSC
9.What is the fucnction of the TRAU?
The TRAU is used for transcoding and rate adaptation. Basically speech on air interface is encoded as 13Kbps whereas speech on ISDN lines is standard 64Kbps. This translation from 13Kbps on air interface to 64Kbps standard ISDN rate is done by the TRAU.
10. The database which stores data related to mobile equipment is
a. AuC
b. BSC
c. EIR
d. MSC
GSM Layer 3
1. When there is no transmission taking place for a MS it is said it be in
e. Dedicated mode
f. Idle Mode
g. Handover
h. Sleep mode
2. The process in which the actual transmission on radio part is reduced to minimum during silence periods is
a. Handover Process
b. Discontinuous Reception
c. Paging Procedure
d. Discontinuous Transmission
RF Optimization Scenarios
RF Optimization Scenarios
This is a brief description of the common scenarios that the RF team experience during optimization. Also described are the databases that needs checking.
Problem Parameters Physical
Dropped_Calls 1. Wrong parameter settings (perform consistency checks on adjcell/adjres, hoctrl, pwrctrl, and bts objects.) 1. Wrong azimuth orientation (check site/antenna config database, also MapInfo maps)
2. Verify if default parameters matches with the current database of omc or check for mismatches between customer settings and omc settings. 2. Wrong antenna/feeder cable connections (requires site visit)
3. Analyze drive data using TEMS 3. Faulty hardware installation (see site documentation)
4. Verify the neighbor list (this falls on consistency checks) 4. Site down (check omc site alarms and status)
5. In case of interference, check BCCH database or design data 5. Analyze data, look for interference
Handover Failures 1. Wrong parameter settings (perform consistency checks on adjcell/adjres, hoctrl, pwrctrl, and bts objects.) 1. Investigate coverage plots (verify coverage holes)
2. Check LAC assignment on the release_omc or perform consistency checks on lac listings. 2. Check antenna configuration/orientation database
3. Check Pwrctrl parameters (check mismatches between default and omc settings) 3. Analyze drive test data.
Multiple Handovers 1. Verify Hoctrl parameters, PwrBgtAvePeriod, HO_Margin etc. 1. Investigate coverage plots (verify coverage holes)
2. Check Adjcell list (verify RxLevMinAccess and HO_Margin values) 2. Check antenna configuration/orientation database
3. Analyze drive test data.
Interference 1. Check BCCH listings 1. Check site reports for possible hardware problems
2. Check cluster listings and verify cells with co-channel and adjacent channel 2. Use TEMS/other tools to analyze and distinguish internal and external interference
3. Check antenna ht.database 3. Check maps and locate sites with interfering BCCHs
Wrong (Invalid) Handovers 1. Check the neighbor list and verify mutuality between adjacent cells. 1. Check antenna azimuths and tilts
Blocking 1. Check PMS statistics (daily and weekly reports) 1. Add RTs
2. Check site database and verify number of RTs .
Methods and Tools:
Cluster Testing is done to fine tune the fixed network and identify coverage issues. Identifying what type of cluster methodology is used.
Data Analysis and Sorting (Radio Link and Cluster Performance Test)
Drive Test Data indicates the status of the cluster before, and after, the addition of sites. Also indicates coverage issues.
Customer Complaints (Trouble Tickets) are collected from the Customer Service Department and RF related issues are sent to the Network Planning department. The complaints are sorted into clusters and are given to the RF engineers to work on.
PMS (Performance Measurement Sub-system) monitors network performance and creates reports that are then analyzed by the RF engineers during the performance testing.
OMC Data is accessed and used to assess current network status information
Cluster Drive Testing involves:
1. Defining Drive Routes
2. Drive Test Scheduling:
Night Drive(off-peak time testing)
Day Drive (peak-time testing)
3. Data Analysis and Recommendations
Analysis concentrates on coverage and quality issues. The drive test data analyzed is RXLevel, RXQual, MSTXPWR, and intra-Cluster connectivity.
4. Fine Tuning:
Changes in parameters after analysis have been made.
5. Determining RF Coverage holes
6. Post- Processing of Drive Test Data using MapInfo and GIMS
7. Analysis using TEMS
8. Site visits if necessary
9. Printing and analyzing drive test plots for analysis and presentation
Main Issues of Concern:
Dropped calls
Call setup failures
Poor coverage
Handover Failures
Transmission and Synchronization
Interference
Blocking, etc.
Actions:
Workorders are produced for implementation of new services, and changes to the existing networks.
Drive Test and Analysis Tools:
MapInfo – desktop/pc mapping software
GIMS – TEMS post processing software in MapInfo
Ericsson TEMS mobile phone (1 or 2)
GPS and external antennas
Notebook computer and accessories
Switch Terminal/AUI/GUI – real time switch terminal necessary for retrieving current network status information, cell parameters, BSS parameters, etc.
Initial Optimization
The purpose is to verify the basic performance of the radio-network and identify severe RF problems and possible hardware problems.
Responsibilities of the RF engineering group:
Preparation phase:
• Reviewing frequency plan
• Preparing and checking of RF related OSS BSS data
• Handover performance
• Handover order
• Inter BCE Handover
• Inter MSC handovers
• Coverage
• Service quality
In contrast, ongoing RF optimization can cover issues such as resolving the main reasons of the degradation of the radio-network performance, which are the following:
• Congestion
• Oscillating Handovers
• Actual network reconfiguration
• Interference caused by increasing traffic
A summary of Radio Performance Test and RF Optimization Tasks:
Task Radio Performance Test RF Optimization
Call setup possibility Checking the feasibility of setting up
MS originated calls and MS
terminated calls During RF optimization it is
possible to keep track of call setup
performance, because sufficient
number of network statistics data
and drive test data will be available.
Handover performance Verification of proper handover
function for each defined neighbor
relationships - identifying handover areas
between all neighboring site(s)
- identify and prioritize all abnormal
handovers
- adjustments for the elimination
of oscillating handovers
- the handover areas are identified
on a map for future optimization
Handover order check correct handover sequence
Traffic distribution Shifting or reducing the traffic from
congested site(s) to its neighboring
sites and optimize traffic flow in the
network, handle hot-spot areas
Frequency plan
adjustment When new site is going to be integrated into network the initial frequency plan will be reviewed and
readjusted if necessary Due to the sufficient amount of
network statistics data and drive
test data more efficient frequency
will be possible to be implemented
and achieve higher capacity of the
network
Coverage During each site testing it will be
possible to check correct per cell
coverage distribution and identify-
resolve general service degrading
coverage problems Due to the more extensive drive
tests detailed mapping of the cove-
rage will be available. In this case
collected data will be sufficient for
fine tune of antenna configuration in
order to achieve evenly distributed
coverage along clusters of sites
Service Quality Any potential or identified interfe-
rence issues, during the site tests,
are analyzed and minimized Increasing traffic and capacity up-
grades of the network will result in
new interference issues that can be
identified and resolved only by regu-
lar monitoring of the network from
network statistics and drive test
data inputs.
This is a brief description of the common scenarios that the RF team experience during optimization. Also described are the databases that needs checking.
Problem Parameters Physical
Dropped_Calls 1. Wrong parameter settings (perform consistency checks on adjcell/adjres, hoctrl, pwrctrl, and bts objects.) 1. Wrong azimuth orientation (check site/antenna config database, also MapInfo maps)
2. Verify if default parameters matches with the current database of omc or check for mismatches between customer settings and omc settings. 2. Wrong antenna/feeder cable connections (requires site visit)
3. Analyze drive data using TEMS 3. Faulty hardware installation (see site documentation)
4. Verify the neighbor list (this falls on consistency checks) 4. Site down (check omc site alarms and status)
5. In case of interference, check BCCH database or design data 5. Analyze data, look for interference
Handover Failures 1. Wrong parameter settings (perform consistency checks on adjcell/adjres, hoctrl, pwrctrl, and bts objects.) 1. Investigate coverage plots (verify coverage holes)
2. Check LAC assignment on the release_omc or perform consistency checks on lac listings. 2. Check antenna configuration/orientation database
3. Check Pwrctrl parameters (check mismatches between default and omc settings) 3. Analyze drive test data.
Multiple Handovers 1. Verify Hoctrl parameters, PwrBgtAvePeriod, HO_Margin etc. 1. Investigate coverage plots (verify coverage holes)
2. Check Adjcell list (verify RxLevMinAccess and HO_Margin values) 2. Check antenna configuration/orientation database
3. Analyze drive test data.
Interference 1. Check BCCH listings 1. Check site reports for possible hardware problems
2. Check cluster listings and verify cells with co-channel and adjacent channel 2. Use TEMS/other tools to analyze and distinguish internal and external interference
3. Check antenna ht.database 3. Check maps and locate sites with interfering BCCHs
Wrong (Invalid) Handovers 1. Check the neighbor list and verify mutuality between adjacent cells. 1. Check antenna azimuths and tilts
Blocking 1. Check PMS statistics (daily and weekly reports) 1. Add RTs
2. Check site database and verify number of RTs .
Methods and Tools:
Cluster Testing is done to fine tune the fixed network and identify coverage issues. Identifying what type of cluster methodology is used.
Data Analysis and Sorting (Radio Link and Cluster Performance Test)
Drive Test Data indicates the status of the cluster before, and after, the addition of sites. Also indicates coverage issues.
Customer Complaints (Trouble Tickets) are collected from the Customer Service Department and RF related issues are sent to the Network Planning department. The complaints are sorted into clusters and are given to the RF engineers to work on.
PMS (Performance Measurement Sub-system) monitors network performance and creates reports that are then analyzed by the RF engineers during the performance testing.
OMC Data is accessed and used to assess current network status information
Cluster Drive Testing involves:
1. Defining Drive Routes
2. Drive Test Scheduling:
Night Drive(off-peak time testing)
Day Drive (peak-time testing)
3. Data Analysis and Recommendations
Analysis concentrates on coverage and quality issues. The drive test data analyzed is RXLevel, RXQual, MSTXPWR, and intra-Cluster connectivity.
4. Fine Tuning:
Changes in parameters after analysis have been made.
5. Determining RF Coverage holes
6. Post- Processing of Drive Test Data using MapInfo and GIMS
7. Analysis using TEMS
8. Site visits if necessary
9. Printing and analyzing drive test plots for analysis and presentation
Main Issues of Concern:
Dropped calls
Call setup failures
Poor coverage
Handover Failures
Transmission and Synchronization
Interference
Blocking, etc.
Actions:
Workorders are produced for implementation of new services, and changes to the existing networks.
Drive Test and Analysis Tools:
MapInfo – desktop/pc mapping software
GIMS – TEMS post processing software in MapInfo
Ericsson TEMS mobile phone (1 or 2)
GPS and external antennas
Notebook computer and accessories
Switch Terminal/AUI/GUI – real time switch terminal necessary for retrieving current network status information, cell parameters, BSS parameters, etc.
Initial Optimization
The purpose is to verify the basic performance of the radio-network and identify severe RF problems and possible hardware problems.
Responsibilities of the RF engineering group:
Preparation phase:
• Reviewing frequency plan
• Preparing and checking of RF related OSS BSS data
• Handover performance
• Handover order
• Inter BCE Handover
• Inter MSC handovers
• Coverage
• Service quality
In contrast, ongoing RF optimization can cover issues such as resolving the main reasons of the degradation of the radio-network performance, which are the following:
• Congestion
• Oscillating Handovers
• Actual network reconfiguration
• Interference caused by increasing traffic
A summary of Radio Performance Test and RF Optimization Tasks:
Task Radio Performance Test RF Optimization
Call setup possibility Checking the feasibility of setting up
MS originated calls and MS
terminated calls During RF optimization it is
possible to keep track of call setup
performance, because sufficient
number of network statistics data
and drive test data will be available.
Handover performance Verification of proper handover
function for each defined neighbor
relationships - identifying handover areas
between all neighboring site(s)
- identify and prioritize all abnormal
handovers
- adjustments for the elimination
of oscillating handovers
- the handover areas are identified
on a map for future optimization
Handover order check correct handover sequence
Traffic distribution Shifting or reducing the traffic from
congested site(s) to its neighboring
sites and optimize traffic flow in the
network, handle hot-spot areas
Frequency plan
adjustment When new site is going to be integrated into network the initial frequency plan will be reviewed and
readjusted if necessary Due to the sufficient amount of
network statistics data and drive
test data more efficient frequency
will be possible to be implemented
and achieve higher capacity of the
network
Coverage During each site testing it will be
possible to check correct per cell
coverage distribution and identify-
resolve general service degrading
coverage problems Due to the more extensive drive
tests detailed mapping of the cove-
rage will be available. In this case
collected data will be sufficient for
fine tune of antenna configuration in
order to achieve evenly distributed
coverage along clusters of sites
Service Quality Any potential or identified interfe-
rence issues, during the site tests,
are analyzed and minimized Increasing traffic and capacity up-
grades of the network will result in
new interference issues that can be
identified and resolved only by regu-
lar monitoring of the network from
network statistics and drive test
data inputs.
NETWORK PLANNING CONCEPTS
NETWORK PLANNING OVERVIEW
NPLOVE
RADIO NETWORK PLANNING
NETWORK PLANNING TRAINING PROGRAM
1 Network Planning Overview Course
Training group
Number of participants
Object
Prerequisite knowledge
Duration
Form
Subject
Planning principles and theory
Practical tasks Persons dealing with the network planning
3 - 20
To provide the trainees with a general knowledge of GSM/PCN network planning
SYSTRA and general knowledge of Radio Systems and Radio-wave Propagation
1 day
Lectures
Duration (h)
5
1
CONTENTS
GSM/PCN RADIO NETWORK PLANNING OVERVIEW
1 Radio Network Planning of GSM/PCN Cellular System
2 Radio Network Planning Procedure
3 Radio Network Planning Tools
3.1 NPS/X
3.2 NMS/X
NETWORK DIMENSIONING
1 Network Dimensioning Input
2 Capacity Calculations
3 Frequency Reuse Schemes
4 Power Budget Calculations
5 Cell Size Calculation
6 Antennas
7 Commercial Aspects and Business Planning
8 Network Layout
9 Exercise
COVERAGE PLANNING AND SITE SELECTION
1 Introduction
2 Site Selection
3 Location Probability
4 Propagation and Propagation Models
5 Coverage Predictions
5.1 Digital Maps
5.2 Point to Point and Cell Coverage
6 Field Strength Measurements
7 Propagation Model Tuning
8 Microcellular planning
8.1 Microcellular Network Applications
8.2 The two-layer Microcellular network Planning Process
8.3 BSC Features for Microcellular Network
9 Indoor Coverage Planning
9.1 Indoor Environment and Propagation
9.2 Indoor Solutions
10 Extension of Cell Coverage Area
10.1 Extended cell (E-Cell)
11 Intelligent Underlay-Overlay
FREQUENCY PLANNING
1 Introduction
2 Interference Calculations
2.1 Co-channel interference
2.2 Time Dispersion
2.3 Digital maps based co-channel interference
3 Heuristic Algorithms
4 Frequency Hopping
4.1 Frequency Hopping Behaviour
4.2 Baseband Hopping
4.3 Synthesized Hopping
4.4 Discontinuous Transmission (DTX)
BSS PARAMETER PLANNING
1 GSM Radio Path
1.1 Common Channels
1.2 Dedicated Channels
1.3 Usage of the Channels
1.4 Radio Path Measurements
2 Power Control and Handover
2.1 Power Control Strategy and Parameters
2.2 Handover Strategies and Parameters
2.3 HO and PC Interaction
2.4 BSS Parameters
NETWORK VERIFICATION AND OPTIMIZATION
1 Network Verfication
1.1 Network Quality Criteria
1.2 Measurement Procedure
1.3 Analysis of the Results
2 Network Optimization
2.1 BSS Default Parameter Assessment
2.2 Small Quality Cycle
2.2.1 Basic Configuration Analysis
2.2.2 Basic Network Optimisation
2.3 Full Optimisation Service
2.4 Optimisation Tools
GSM/PCN RADIO NETWORK PLANNING OVERVIEW
1 Radio Network Planning of GSM/PCN Cellular System
Network planning is a process where the planning engineers have the following main targets:
* to achieve the required radio coverage with maximum time and location probability (more than 90%)
* to maximize the network capacity (Erl/km2) with limited frequency band (MHz) by reusing same frequencies
* to reach good quality of service (QOS) with a minimum level of interference
* to minimize the number of network elements (MSC, BSC, BTS, ...) and therefore the cost of the network infrastructure
The described network planning process is an optimization process which needs information about network elements, system properties (GSM/PCN), planning environment, topography of the service area, existing facilities of the operator, distribution of the subscribers, and estimated subscriber future growth.
In rural areas the aim is to get large coverage with high transmission power and high antenna height. Omnidirectional and directional antennas are used, the latter ones for two-sector cells along the roads.
In city areas with high traffic density the aim is to get high capacity with small or micro cell structure by using low transmission power and low antenna height. Directional antennas are used and most of the sites are sectorized.
2 Radio Network Planning Procedure
The planning work starts by collecting all the relevant information. The basis is the data obtained from the operator, but in most cases it is not sufficient, so the network planner must use topographical maps and statistical books.
Network dimensioning is done so that the coverage and capacity requirement based on subscriber growth forecast are fulfilled. Once the number of network elements and commercial aspects are available the business plan can be made.
The selection of base station site candidates is based on the terrain topography and the existing facilities at those sites. This means that the coverage area of an existing site can be limited due to some terrain fluctuations or other obstacles and still to be preferable, when the cost of establishing a new site is taken into account.
Visiting the sites gives accurate information and is done when making the final decisions on site selection. This task is called field or site survey. The radio wave propagation environment is studied, and field strength measurements can be done to clarify the actual coverage or radio interference. The existing antenna mast (if any), space for antennas, space for equipment and expandability, air conditioning, power source and grounding, transmission lines and capacity are studied during the survey.
A computer aided design system is used for coverage predictions, interference analysis and frequency allocation. A measurement system is used for coverage and interference measurements and for predictions verification. Nokia Cellular Systems (NCS) has developed own tools for network planning and measurement:
* NPS/X Network Planning System
* NMS/X Network Measurement System
The network planning engineers in NCS have extensive experience in designing the network structure. The latest technical properties of the network elements are taken from the manufacturers - a good knowledge of this is needed. The transmission planning is an optimization task related to network planning, and the results constitute considerable savings for the operator.
The last activity of network planning is the base station sub-system (BSS) parameter planning and radio network verification and optimization. This is based on experience gained during the trial period of the network. The successful hand-overs as well as the hand-over and power control algorithms will greatly affect the speech quality of the network.
The general results and documents from network planning are:
* List of sites and network elements
* Coverage predictions and measurements
* Frequency plan and interference analysis
* Capacity calculations
* BSS parameter plan
* Transmission network plan
During the network existence the knowledge about its operation and subscriber behaviour will improve. The network will also expand in terms of capacity and coverage because of the continuous subscriber growth. Therefore the network planning is also a continuous process which tries to minimize the modifications to the existing network while preparing extension plans. Testing and tuning is also needed at each new stage of the network to ensure good quality of service.
3 Radio Network Planning Tools
3.1 NPS/X
The Network Planning System NPS/X is an integrated software package for cellular network design. It provides interactive and easy-to-use tools for network planning and documentation from rough sketches to accurate designs. NPS/X runs on a Sun(TM) or Hewlett-Packard(TM) workstations, using all the benefits of UNIX networking capabilities. The use of pop-up menus and input/output panels with action buttons make NPS/X easy to work with.
Cellular planning with NPS/X is based on utilization of digitized map and measurement results. The design database includes the parameters of the base sta¬tions, antennas, propagation models and system parameters.
Coverage predictions can be based on different propagation models such as Okumura-Hata or Walfish-Ikegami. The model parameters can be modified and tuned for a particular propagation environ¬ment.
The basic package includes:
* coverage area calculation
* composite coverage area and dominance
* point-to-point calculation, BS->MS
* interference area calculation
Options:
* automatic frequency allocation
* digitizing and editing system for digitized maps
* measurement system interface (to NMS/X) and route calculation
* utilization of traffic density map
* micro cell modelling
* cellular simulation
3.2 NMS/X
The Network Measuring System NMS/X is a field strength measuring system with an integrated navigation equipment. The NMS/X consists of a navigation system, a field strength receiver or a mobile and a portable computer which collects geographical coordinates and field strength values. The system can be easily installed in a measuring vehicle.
Measurement results can be further processed in two ways: field strength curves with location marks can be printed out with a separate printing program or results can be transferred in diskettes to NPS/X (Network Planning System) for analysis.
NETWORK DIMENSIONING
Network Dimensioning (ND) is usually the first task to start the planning of a given cellular network. The main result is an estimation of the equipment necessary to meet the following requirements:
* capacity
* coverage
* quality
ND cannot replace the detailed network planning and vice versa. It gives an overall picture of the network and is used as a base for all further planning activities.
1 Network Dimensioning Input
The requirements mentioned above imply the input information for ND task as follows:
The input data is normally supplied by the operator, but use of defaults is also possible. However, the marked items are the absolute minimum to be given in advance.
The technical parameters and characteristics of the equipment to be used are another very important part of the input. This includes the basic network modules (MSC, BSC, BTS) as well as some additional elements (antennas, cables ...).
2 Capacity Calculations
The capacity of a given network is measured in terms of the subscribers or the traffic load that it can handle. The former requires knowledge of subscriber calling habits (average traffic per subscriber), while the latter is more general. The network capacity calculation can be performed in several simple steps:
* Find the maximum number of carriers per cell that can be reached for the different regions, based on the frequency reuse patterns and the available spectrum.
* Given the blocking probability and the number of carriers, calculate the capacity of a given cell.
* Find the system capacity as a sum of all cell capacities.
ERLANG B TABLE
Chs 1% 2% 3% 5% Chs 1% 2% 3% 5%
1 0.01 0.02 0.03 0.05 21 12.80 14.00 14.90 16.20
2 0.15 0.22 0.28 0.38 22 13.70 14.90 15.80 17.10
3 0.46 0.60 0.72 0.90 23 14.50 15.80 16.70 18.10
4 0.87 1.09 1.26 1.52 24 15.30 16.60 17.60 19.00
5 1.36 1.66 1.88 2.22 25 16.10 17.50 18.50 20.00
6 1.91 2.28 2.54 2.96 26 17.00 18.40 19.40 20.90
7 2.50 2.94 3.25 3.75 27 17.80 19.30 20.30 21.90
8 3.13 3.63 3.99 4.54 28 18.60 20.20 21.20 22.90
9 3.78 4.34 4.75 5.37 29 19.50 21.00 22.10 23.80
10 4.46 5.08 5.53 6.22 30 20.30 21.90 23.10 24.80
11 5.16 5.84 6.33 7.08 31 21.20 22.80 24.00 25.80
12 5.88 6.61 7.14 7.95 32 22.00 23.70 24.90 26.70
13 6.61 7.40 7.97 8.83 33 22.90 24.60 25.80 27.70
14 7.35 8.20 8.80 9.73 34 23.80 25.50 26.80 28.70
15 8.11 9.01 9.65 10.60 35 24.60 26.40 27.70 29.70
16 8.88 9.83 10.50 11.50 36 25.50 27.30 28.60 30.70
17 9.65 10.70 11.40 12.50 37 26.40 28.30 29.60 31.60
18 10.40 11.50 12.20 13.40 38 27.30 29.20 30.50 32.60
19 11.20 12.30 13.10 14.30 39 28.10 30.10 31.50 33.60
20 12.00 13.20 14.00 15.20 40 29.00 31.00 32.40 34.60
To do the second task we need the well-known Erlang B tables or formulas and the number of traffic channels for different number of carriers. The result we get is the traffic capacity in Erlangs, which can easily be transferred into the number of subscribers.
3 Frequency Reuse Schemes
A cellular network can easily be drawn as a combination of hexagons or circles by the help of regular grids. The hexagonal world is quite different from the real one, but is still the best approximation at the ND stage. One of the benefits is the possibility to try different frequency reuse patterns (clusters) and calculate the expected C/I ratio. The goal is to assign a frequency reuse number (cluster size) to any of network regions or area types. It is clear that the high density regions (big cities) are the most problematic parts of the network.
For the city areas in a GSM network, where sectorization is expected, the optimistic estimate of the cluster size is 9 (3x3) while the pessimistic estimate is 12 (4x3).
4 Power Budget Calculations
To guarantee a good quality in both directions (uplink and downlink) the powers of BTS and MS should be in balance at the edge of the cell. The main idea behind the power budget calculations is to receive the maximum output power level of BTS transmitter as a function of BTS and MS sensitivity levels, MS output power, antenna gain (RX and TX), diversity reception, cable loss, combiner loss, etc.
The Power budget calculation provides following useful results:
BTS transmitted power. BTS transmitted power is adjusted to provide a balanced radio link (i.e. Uplink and Downlink radio link performance is the same) for given BTS and MS receiver performance, MS transmitter performance, antenna and feeder cable characteristics.
Isotropic path loss. This is the maximum path loss between BTS and MS according to given radio system performance requirements.
Coverage threshold. Downlink signal strength at coverage area border for given location probability. Coverage threshold value can be used in coverage prediction with NPS/X (Network Planning System).
Cell range for Indoor and Outdoor coverage. This is an rough indication about cell range in different area types and can be used for network dimensioning. It can also be used for comparing the effect of different equipment specifications and antenna heights for the cell range.
The following table explains the meaning of each of the parameters used in the Power Budget calculation.
Parameter Description Reference
MS CLASS Mobile power class GSM 05.05
RX RF-input sensitivity For MS this is updated according to given MS class. For BTS this can be adjusted according to the performance of Nokia BTS type in question. GSM 05.05
Cable loss + connector Antenna feeder cable and connector losses for BTS site and for car mounted MS. For handheld MS this is 0 dB. In dimensioning some default value is selected for BTS site, e.g. 3 dB
RX antenna gain Gain (dBi) of used BTS and MS antenna types. For handheld MS, default is 0 dB. Antenna product specification
Diversity gain RX diversity gain. For BTS, the practical range is 0...5 dB depending on environment and antenna installation (separation). When BTS RX diversity is used, the default value is 4 dB for urban areas.
Isotropic power Minimum signal strength (dBm) in the receiving end calculated from the above factors. GSM 03.03
Field strength Minimum field strength requirement (dBV/m) converted from isotropic power.
TX RF output peak power For MS this comes from selected MS class. For BTS this is calculated according to balanced radiolink requirement. GSM 05.05
Isolator + combiner + filter Attenuation of the BTS RF components between TX unit and antenna feeder connector of BTS. The exact value depends on BTS model and configuration, typical values are 3,5 .... 5 dB. BTS product specification
RF peak power (Combiner) output Transmit power at BTS/MS antenna connector
TX antenna gain According to selected RX antenna gain
Peak EIRP Effective Isotropic Radiated Power level
Isotropic path loss EIRP - minimum RX field strength
Standard deviation Standard deviation of the signal strength (outdoors). Typical value is 6 ... 8 dB depending on the environment.
BPL average Average building penetration loss in the target area (for indoor coverage calculation), typically between 8 ... 18 dB.
BPL deviation Signal deviation indoors (for indoor coverage calculation), typically 8 ... 12 dB.
Area type correction Area type correction factors for the Okumura-Hata model
5 Cell Size Calculations
The calculation of the cell radius at the ND stage is usually based on simple and easy to use propagation loss formulas: Okumura-Hata, Juul-Nyholm, Walfish-Ikegami (city areas). Since some of the steps are similar, both radio link budget and cell size calculations can be done simultaneously, see the power budget and cell sizes table.
There are many parameters affecting the cell size such as BTS power, antenna heights (BS and MS), and BTS TX antenna gain, but the area type (dense urban, urban, suburban, rural, forest, etc.) is of great importance. The expected outcome are some average cell sizes for all the different area types. It is also possible to obtain specific cell sizes for the most often used site configurations: omni, two-sectors (road) and three-sectors (city).
6 Antennas
The antenna is an element that radiates an electric signal into the air. Typically antennas are characterized by their gain, beamwidth and band of operation.
To have a reference an isotropic antenna is used, which has 0 dBi gain. This is a theoretical antenna that radiates the same power in all 3D directions. The gain gives the maximum power trasmittiter in one (directional antennas) or several (omnidirectional) directions.
The radiation pattern shows how the power is transmitted into the air in the differnt directions. Because it’s 3 dimensional 2 diagrams are given, the horizontal ( like a top view of a building ) and the vertical ( side view ).
The beamwidth is defined by the points of 3 dB atenuation compare to the maximum.
Usually the lower the frequency the bigger the antennas to achieve the same gain and the most difficult to make high gain antennas.
In the 900 Mhz (GSM) and 1800 (DCS) Mhz bands the antenna gains vary between 10 dBi (omni) and 18 dBi (directional)
7 Commercial Aspects and Business Plans
The final step in the ND task is to add the prices of the equipment and links and to calculate the overall network cost for a given period of time. This is important not only for the operator but also for the network planner. Adding the cost of transmission links may lead to some changes in the network structure, especially in the number of BSCs. The optimization of the network structure is now based on cost function minimisation.
It is possible to combine capacity, coverage and cost calculations in so-called business network plan application. In this way the ND procedure can start with cell sizes and using the results go directly to the business network plan.
8 Network Layout
The output from the ND task can differ from case to case but some of the items are common. These are average cell sizes, frequency reuse schemes, network evolution figures (area, subscribers, equipment), and sketches of the network or parts of it.
9 Exercise
1. Fill the following power budget calculations
Omni Directional
GENERAL INFO
Frequency (MHz): 1800 System:
RECEIVING END: BS MS
RX RF-input sensitivity dBm -104.00 -100.00 A
Cable loss + connector dB 4.00 C
Rx antenna gain dBi 12.00 0.00 D
Diversity gain dB 0.00 E
Isotropic power dBm F=A+C-D-E
TRANSMITTING END: MS BS
TX RF output peak power W 1.00
(mean power over RF cycle) dBm 30.00 K
Isolator + combiner + filter dB 4.0 L
RF-peak power, combiner output dBm M=K-L
Cable loss + connector dB N
TX-antenna gain dBi O
Peak EIRP W
(EIRP = ERP + 2dB) dBm P=M-N+O
Isotropic path loss dB Q=P-F
Directional
GENERAL INFO
Frequency (MHz): 1800 System:
RECEIVING END: BS MS
RX RF-input sensitivity dBm -104.00 -100.00 A
Cable loss + connector dB C
Rx antenna gain dBi 17.00 0.00 D
Diversity gain dB E
Isotropic power dBm F=A+C-D-E
TRANSMITTING END: MS BS
TX RF output peak power W 1.00
(mean power over RF cycle) dBm K
Isolator + combiner + filter dB 4.00 L
RF-peak power, combiner output dBm M=K-L
Cable loss + connector dB N
TX-antenna gain dBi O
Peak EIRP W
(EIRP = ERP + 2dB) dBm P=M-N+O
Isotropic path loss dB Q=P-F
2. Calculate the cell range and the number of needed bases stations for coverage:
Cell Range
Parameters
MS Antenna Height (m) 1.5
Frequency (MHz) 1800
Area (Sq.km) 1000
Road Lenght (km) 500
Urban S_Urban Rural(sec) Rural(omni) Roads
BTS Antenna Height 35 30 40 40 40
Correction Factor 0 -5 -12 -12 -12
Isotropic Path Loss
Cell Range omni(km)
Cell Range dir (km)
Cell Area (sq.km)
Some hints!
Path loss Range
140 2.40
141 2.56
142 2.74
150 1.97 2.55 4.68 4.68
151 2.11 2.72 5.00 5.00
152 2.25 2.91 5.35 5.35
153 2.40 3.10 5.72 5.72
BTS Distribution Urban S_Urban Rural Roads
BTS#0 0 % 0 % 20 % 0 %
BTS#D#00 0 % 0 % 0 % 100 %
BTS#D#000 100 % 100 % 80 % 0 %
Area Distribution 15 % 35 % 50 % 100 %
Sites Needed
3. Capacity calculation
Traffic
In air interface Erlang B table can be used
Traffic
TRXs TCHs CCs 1 % 2 % 5 %
1 7 1 2.50 2.94 3.25
2 15 1 8.11 9.01 9.65
3 22 2 13.70 14.90 15.80
4 30 2 20.30 21.90 23.10
5 38 2 27.30 29.20 30.50
6 45 3 33.43 35.60 39.55
7 53 3 40.60 43.06 47.53
8 61 3 47.86 50.59 55.57
9 69 3 55.19 58.18 63.65
10 76 4 61.65 64.85 70.75
11 84 4 69.08 72.52 78.89
12 92 4 76.55 80.23 87.05
Subscribers (000) 100000
Traffic per Subs 20 mErl
Blocking 2 %
Channel Spacing 200 kHz
Frequency Reuse 12
Band Width 4.8 MHz
Max. Carriers/Sector 2
Omni 2-sector 3-sector
Terrain Type Subs Dist. # of Subs Traffic # of BTSs # of BTSs # of BTSs
Urban 50 % 50000 1000
S_Urban 35 % 35000 700
Rural 15 % 15000 300
Total (Area) 100 % 100000 2000
Total (Roads): 100 % 10000 200
4. Taking into account the 2 previous resluts estimate the total amount of equipment need:
SUMMARY
Urban S_Urban Rural Roads Total
Sites, max(coverage, capacity)
Sectors
TRXs
Traffic
Traffic/Site
TRXs/Site
TRXs/Sector
Number of BSCs
BSC#032
BSC#064
BSC#128
MSC#050
MSC#100
COVERAGE PLANNING AND SITE SELECTION
1 Introduction
The objective of coverage planning phase in coverage limited network areas is to find a minimum amount of cell sites with optimal locations for producing the required radio coverage for the target area. This phase includes the selection of practical site candidates being an iterative process until the final site locations are frozen and initial coverage related cell parameters selected.
Coverage planning is normally performed with prediction modules on digital map databases. The task is guided with the requirements coming from Purchaser and from System Configuration and Dimensioning. The basic input information for coverage planning includes:
• Coverage regions
• Coverage threshold values on per region (Outdoor, In-car,Indoor)
• Antenna (tower) height limitations
• Preferred antenna line system specifications
• Preferred BTS specification
Activities such as propagation modelling, field strength predictions and measurements are usually referred to as coverage planning. The coverage is statistical in nature and always related to a given level of field strength, location and time probability. For the mobile radio the dependence on time can be neglected since the standard deviation of the field strength time distribution is relatively small. The radio path description includes fast (Rayleigh) fading and slow (log-normal) fading. The former is caused by multipath propagation and is typical for land mobile service. The slow fading, so-called "shadowing" is caused by terrain fluctuations and obstacles (buildings).
2 Site Selection
Coverage planning and site selection are performed in parallel with the site acquisition in interactive mode. Both network planning team and site acquisition team should have well defined responsibilities and means to communicate. This chapter covers the case where Nokia does the network planning and the Purchaser takes care of the site acquisition, however the exact procedure and interface are subject of agreement between the two parties.
An initial set of site candidates, if available, is provided . The data module for each site candidate should contain at least the coordinates and the antenna height range.
Coverage analysis and site surveys is performed for perspective candidates. The initial candidates are separated in groups depending on the analysis results, for example: Accepted, Possible, Rejected. Based on the coverage and capacity needs which are still unsatisfied, a list of wanted sites including coordinates and antenna height is produced.
3 Location Probability
To "have a coverage" in a given cell means that the field strength (predicted or measured) exceeds E dB in more than L% of the locations, where E and L are predefined. For the cellular networks L=90% is a widely-used choice, while the field strength threshold E is a function of parameters like area type, mobile sensitivity, mobile antenna gain. The location probability L should be related to the cell area (La), while from the measurements or field strength predictions we can obtain a location probability related to the distance (Ld). In order to transfer Ld to La an integration over the cell area must be done : for example area location probability La=90% corresponds to about Ld=75% location probability at the edge of the cell (distance equal to the cell range). Therefore, the field strength planning threshold value should include a correction for the Ld transition from 50% to 75%, referred to as a slow fading margin, see Power Budget and Cell Sizes tables. Using 7 dB standard deviation of the field strength distribution, the slow fading margin is about 4.5 dB.
Since the location probability for the cellular radio is always area related, the notation L% will be assumed equivalent to La% unless specially noted otherwise.
To "have a coverage" in a given cell means that the field strength (predicted or measured) exceeds E dB in more than L% of the locations, where E and L are predefined. For the cellular networks L=90% is a widely-used choice, while the field strength threshold E is a function of parameters like area type, mobile sensitivity, mobile antenna gain. The location probability L should be related to the cell area (La), while from the measurements or field strength predictions we can obtain a location probability related to the distance (Ld). In order to transfer Ld to La an integration over the cell area must be done : for example area location probability La=90% corresponds to about Ld=75% location probability at the edge of the cell (distance equal to the cell range). Therefore, the field strength planning threshold value should include a correction for the Ld transition from 50% to 75%, referred to as a slow fading margin, see Power Budget and Cell Sizes tables. Using 7 dB standard deviation of the field strength distribution, the slow fading margin is about 4.5 dB.
Since the location probability for the cellular radio is always area related, the notation L% will be assumed equivalent to La% unless specially noted otherwise.
4 Propagation and Propagation Models
Propagation in land mobile services at frequencies from 300 to 1800 MHz is affected in varying degrees by topography, morphography, ground constants and atmospheric conditions. A very common way of propagation loss presentation is the usage of so-called propagation curves, normally derived from measurement data . This approach is not very suitable for computation, therefore some approximation formulas have been introduced: Hata , Juul-Nyholm , Walfish-Ikegami .
The propagation models normally include a basic propagation loss and different correction factors, see Hata's formula for urban areas (small or medium city) as an example.
5 Coverage Predictions
We have already discussed the possibilities for rough coverage calculations, based only on propagation curves or formulas. These average values are not enough for the detailed network planning, therefore many computer-aided tools based on digital maps usage have been developed to improve the quality of the predictions.
5.1 Digital Maps
There are different types of information that can be digitized and used for the coverage predictions. The most important from the network planning point of view are topography (terrain heights), morphography (area types), roads, and traffic density. There exist also different ways of data presentation, but the raster format seems to be the most popular. The raster unit (pixel) can be rectangle or square, having size in the 50m to 500m range for the cellular radio.
For the micro cell modelling, which is required in a dense urban environment, more information and higher resolution maps should be used. Information about the buildings and streets is essential, so the pixel size from 5m to 25m is reasonable. The streets can be stored and used in vector format.
5.2 Point to Point and Cell Coverage
Using a given digital map it is not difficult to obtain the path profile between any two points, say BS and MS. Furthermore the profile can be related to the corresponding area types, thus making possible the calculation of specific propagation loss. Normally different corrections, such as the diffraction loss or mixed land-sea path correction are added to the basic propagation loss.
The result of such point to point calculations can be used for cell coverage prediction. There are two basic approaches:
* radial calculations
* pixel by pixel calculations
The latter one gives better possibilities for the interference predictions, so the results should be transferred to the raster format even if the radial approach is used.
6 Field Strength Measurements
The field strength measurements are needed for determination of coverage areas as well as for tuning the propagation model of a network planning system.
In case of measurements before base station installation the site should be equipped with a test transmitter. Possible test transmitter configurations are mobile station, base station channel unit and signal generator with power amplifier.
The selection of routes to be measured depends on the purpose of the measurements. The most critical routes are typically located in urban or hilly areas, where it is difficult to predict the field strength value with high accuracy. For indoor coverage determination a portable measurement system have to be used.
During the field strength measurement the measuring system normally takes samples from the signal received by the antenna. The field strength samples are recorded by a control computer with time, distance or location marks. Using the field strength samples it is possible to calculate some average values . The recommendation for the averaging interval is 40 wavelengths, corresponding to about 13m at 900 MHz and 7m at 1.8 GHz. Furthermore, it is possible to make a detailed statistical analysis including mean value, standard deviation and different percentile value calculation.
7 Propagation Model Tuning
Practice has shown that the propagation models are not universal. The predictions must be verified by measurements and the models tuned accordingly. The model testing and tuning is a very sophisticated and challenging task, which requires detailed knowledge of the propagation nature. It should be done for every area type in a given country or region before the detailed network planning is started.
8 Microcellular Planning
Because of high subscriber growth rates and sometimes very limited frequency resources, the capacity issue is becoming increasingly important especially in the crowded urban areas and in markets where expanded coverage is needed.
For the cellular industry to sustain growth and to meet the demand for higher capacity, the cellular coverage must be continuously improved and the network capacity must be increased. One method of increasing the capacity of a cellular system is applying Microcellular technology. Microcells are especially suitable in urban areas and other traffic "hot spots". In addition to offering high traffic capacity in the crowded spots, microcells will in many cases improve the coverage, for example, by covering "dead spots" of the macrocellular network.
There are a few common characteristics for microcellular networks. Typically the cell range is less than one kilometer, often about 300 meters. The coverage of microcells is very limited and highly guided by surrounding buildings. The transmission power required is low, in range of 10 to 100 mW. The antennae are normally placed below the average roof level so that the radio wave propagation will be limited along the streets.
Microcells are typically used in high traffic density spots, in urban areas such as shopping centers and downtown areas, and in places that macrocells cannot cover, such as tunnels.
8.1 Microcellular network applications
One of the main applications for microcellular network is to use it for capacity fill-in purposes in city and high traffic density areas. The small cell size ensures enough capacity needed in high traffic density spots such as shopping centers and office areas. A small cell needs only 1-2 TRXs to fulfill the capacity requirement of its coverage area.
Theoretically, microcells achieved by macrocell splitting provide high capacity. Building of continuous coverage by using single micro layer is however difficult. Larger overlapping areas and thus higher transmission powers or higher antennas are usually needed. This means often reduction in the efficiency of frequency reuse. Besides a tight single layer network may cause troubles for fast moving mobiles.
A two-layer network gives means to build the network in flexible and efficient way. Umbrella cells provide the backbone of GSM service and general coverage, and microcells on the underlay network supply the high traffic capacity where needed. Two-layer solution also offers the operator a possibility to segment subscribers according to the service offered to them, and thus charge subscribers differently. The service offered to the mass market could be the right to use the microcellular network within its coverage. By paying a separate fee a subscriber could be entitled to use the overlay network as well, and extend his or her coverage area.
The network elements should support a microcellular network which can be implemented under the existing GSM network and can be extended according to the capacity needed. The infrastructure implemented should give an answer at least to the following aspects:
- What happens, when a fast moving mobile, connected to a macrocell, drives through a microcell area?
- How to prevent call drops when subscribers move from indoors to outdoors or outdoors to indoors?
- How to balance the load between cell layers to ensure available macrocell capacity?
Sophisticated handover algorithms are essential for radio resource management in a microcellular network. Fast moving mobiles in a microcellular network set special requirements for BSC handover features.
In order to segment subscribers to e.g. business and private users, some new features for MSC are needed.
8.2 The two-layer Microcellular Network planning process
Implementation of a microcellular network sets new challenging requirements for the network planning. In order to build an efficient two-layer network structure, the capacity, coverage and the used frequencies of the existing network must be taken into account together with the problematic areas calling for microcells. Moreover, special care should be paid to parameter planning in order to avoid trouble when subscribers move from one cell to another - especially when the movement is out of the coverage of a microcell: the transition should be smooth, without service breaks and dramatic changes.
Coverage Planning
The existing statistical propagation models like Walfish-Ikegami can be further enhanced and used with high accuracy digital maps for the densely populated areas.
Also some new models developed for microcellular planning are available. Some of these models are able to calculate the effects of multiple reflections and diffractions from roofs.
The resolution of the maps can easily go down to 5-10 meter/pixel size and include new data layers like building heights, streets in vector format, etc. Nokia's network planning tool has been developed to fully supports the required simultaneous work with different map resolutions, data layers and propagation models.
Coverage planning will more and more rely on measurements not only for propagation model tuning but also for presenting the coverage areas as a mixture of predicted and measured data. The different types of NMS/X measurements, like dedicated field strength measurements or MS originated signal level and quality reports, can be performed and managed by the NPS/X.
Capacity Planning
Becoming the dominating factor for the network transition to hierarchical two-layer structure, the capacity growth must be followed very carefully. Due to small coverage areas of microcells their capacity will be normally provided by one or two TRXs.
However, with the NPS/X planning tool it is possible to use a traffic density map for automatic cell capacity calculation. In the future, the traffic density map could be updated by means of direct connection to the OMC traffic load statistics.
Frequency Planning
Frequency allocation is the most complicated task even in case of a conventional one-layer network. The microcellular layer adds extra complexity but does not require totally new approach - the heuristics can be used in a similar manner. When it comes to the frequency allocation there are two options: use two separate frequency groups for the two layers or to have a common pool of frequencies and assign them freely to macro and microcells. The second one offers better usage of the available frequencies but the first one makes the expansion and the operation of the network easier.
Parameter Planning and Network Optimization
The role of the parameter planning is quite important for a hierarchical network since it covers the interaction between the layers as well as the standard handover and power control issues. What is the support from the NPS/X? There are two main areas of interest, namely the NPS/X Simulator and the direct Link to the OMC. The network optimization therefore is based on measurements done with NMS/X and OMC, and analysis and simulations done with NPS/X. The existing BSS parameters are taken directly from the OMC into NPS/X and the resulting changes are sent back via the NPS/X-OMC interface.
New Transmission Possibilities are available for the A-bis interface
Because of the small cell size, the microcellular sites are located close to each other. There can be up to 20 sites in one square kilometer. This can increase the transmission cost tremendously. Therefore, the transmission must be taken into account when planning the network expansion. A cost-effective transmission method must be carefully selected. Flexible transmission options must be available.
To ensure an economical approach to build and expand the network, Nokia's BTSs support multi-drop chains and branched networks as standard. Fully integrated radio links give more flexibility for network planning, as well. Having built-in transmission equipment, standard repeater functionality with maximum 20 dB cable loss allowed enables economical network structure without extra equipment. Distance between BTSs can be approximately 1 km apart linked with coax cables or twisted pairs (in case of twisted pairs, up to 60% of telephone lines can be utilized) and can be extended by standard 2 Mbit/s line repeater. This is the PCM line approach.
With the help of 2 Mbit/s baseband modem which could be integrated at the base station site, BTSs can be linked ranging from 0.5 to 12 km with copper wires depending on the diameter and transmission speed. There are three options for data transmission speed available for this approach: 2 Mbit/s, 1Mbit/s and 512 kbit/s. The baseband modem utilization is a very cost effective approach to connect BTSs, and this approach has low requirements for the quality of copper wire. Compared to the PCM line approach, the baseband approach has the advantage of low noise (cross-talk) level if copper wires would be used in both cases.
Antenna location
With the new Nokia BTS equipment it is possible to place a base station and an antenna next to each other, which reduces the cost of antenna cabling, regardless of the location of the transmission equipment and power equipment. This is especially useful in microcellular networks, where the antenna locates low, for example on a wall of a building, whereas power and transmission equipment probably on the roof.
8.3 BSC features for Microcellular Network
As stated above, advanced handover algorithms are essential for a microcellular network. Besides conventional features, Nokia has developed several BSC features including handover supporting features, which are especially suitable in a microcellular network.
Combined umbrella and power budget handover
The umbrella handover is defined to control traffic between different layers. The umbrella handover is based on the field strength of the candidate cell and is totally independent on the radio properties of the serving cell. The traffic between the cells of the same layer is controlled with power budget handover.
Rapid Field Drop handover and Fast Moving Mobiles
In a rapid field drop situation, the normal handover procedure will fail to hand a call over to the target cell in a microcellular network since the radio field strength drops so rapidly that the MS does not have enough time to perform measurements and handovers. In the Rapid Field Drop handover this field drop can be detected from the n last uplink RX levels, which are lower than a predefined limit. A chained neighbor cell is considered as the target cell of the handover if its last averaged RX level exceeds a predefined limit defined to this neighbor cell.
The BSIC decoding and neighbor measurements together with the MS speed determine the required degree of overlapping of two neighboring cells. The time needed for BSIC decoding and handover process may be extended also by dividing the signal to several antennas or by using leaky cables or repeaters. See the examples below.
C2 Re-select Feature and Attractiveness of Cells
The C2 re-select feature controls cell re-selection of a MS in idle state by introducing a so-called penalty time. The penalty time makes the microcells more attractive to those slow moving MS whereas make macrocells more attractive to the fast moving MSs. Therefore, the fast moving MSs will be camped on the 'stable' macrocells and the slow moving or static MSs on microcells.
The Problem with Fast Moving MS
The fast moving mobiles are detected by calculating the measurement reports where the microcell is properly received. If the microcell is received well for long enough, the measuring mobile station is interpreted as slow moving or stationary and is permitted to attempt an umbrella handover.
Microcells are small cells in city offering more capacity to the high traffic density area. The coverage area is limited to 0,1 -1 km with antennas installed to below roof top. Coverage can be controlled also with transmitting power and antenna type (gain, radiation pattern).
Because surrounding buildings prevent propagation the interference area is also reduced. Thus the frequency reuse is more effective. Dedicated frequencies for microcells or mixed frequencies for both cells (also macrocells) can be allocated.
The radiowaves propagate mainly along the streets. Signal levels can vary rapidly in corners or in the side street. In some cases horizontal radiation pattern should be narrow and vertical one wide.
Walfish-Ikegami- propagation model has detailed information of buildings and street which is used for microcellular predictions.
Small and lightweight Mini-BTSs as well as small and unnoticeable antennas are recommended to use in microcells.
The indoor coverage is not easy to predict, since the building penetration loss shows relatively large standard deviation. It can be explained with the big variety of construction materials and floor differences. In 900 MHz band an average value of 15 dB and standard deviation of 15 dB can be used for planning purposes. For 1800 MHz the average value may be 20 dB. The reference values are based on some published results and the measurements done by NCS.
9 Indoor coverage planning
Indoor coverage is becoming increasingly important for extending voice and data communication services within the work place and residential homes. Hence, it does not come as a surprise when operators impose demanding specifications for indoor coverage.
In some cases, indoor coverage problems could be resolved by down tilting or re-orientating rooftop antennas towards the building concerned. Furthermore, with the deployment of microcells below the rooftop level, indoor coverage could be enhanced within the building. However, due to the complexity of indoor propagation and the attenuation caused by external building walls, signals from neighboring outdoor sites may not be able to provide sufficient indoor coverage. In such situations, rather than depending on outdoor sites to provide indoor coverage, antennas and BTSs must be installed inside the building itself. Such indoor enhancing solutions are normally referred to as Picocells.
During the planning stage, indoor coverage plans must be as accurate as possible. Ideally, test transmitters should be deployed to confirm the location of antennas, types of antennas, methods of indoor solution, and to optimize the indoor propagation models.
9.1 Indoor Environment and Propagation
Radio wave propagation in an indoor environment involves external and internal wall penetration, absorption of radio wave energy by furniture, as well as body losses due to the presence of people in the surrounding area. Furthermore, indoor propagation is subjected to fast multipath fading due to multiple path reflected signals, and also diffracted waves due to corners and furniture such as cabinets.
In order to differentiate propagation phenomena, building walls are normally categorized into: reinforced concrete used in modern buildings walls and ceiling; brick and concrete used in older buildings and also residential homes; and light concrete or plaster walls for internal building walls. Inside a building, propagation may be classified as either with line of sight (LOS) or obstructed.
Another effect which often occurs to indoor propagation is the so-called Breakpoint phenomena, where the signal strength tends to decay slower when closer to the transmitter and then much more rapidly when it is further away. Typically, the receive signal tends to decay at 20logd before the breakpoint, and beyond the point, the signal may decay at between 40logd to 80logd, implying a steeper decrease in signal strength.
9.2 Indoor Solutions
The following are various indoor solution which has been implemented by Nokia.
• Power Splitters
• Radiating Cables
• Passive Repeaters
• Active Repeaters (Under Study)
• Remote RF Head
• Micro BTS
• RF Distribution Hub and Optical Antenna (Under Study)
Power Splitter
Power splitter is an inexpensive method used specifically to divide energy from the BTS transmitter to several antennas. The following are some pictures of the 2-way, 3-way and 4-way power splitters.
Figure 1 Power Splitters
In most cases, lower levels of buildings present the most problems in terms of indoor coverage. In such a case, the BTS output power could be divided by using power splitters, and distributed to several antenna systems. It is an ideal solution to extend indoor coverage within a BTS cell site. However, particular attention should be paid when selecting the cell to divide as the outdoor signal will be degraded. Figure 3.2 depicts a typical application of antenna installation using splitters with outdoor and indoor antennas.
Fig. 2 Example of Splitter in a cell site
Radiating Cables
Radiating cables are wave guides with openings along their length allowing fractions of the transmitter energy to leak out continuously. Basically, radiating cables are just slots along coaxial cable allowing for a controlled fraction of RF energy to be radiated along the entire cable. Thus, they function like continuous antennas. The radiating cables is a good solution to coverage problem in tunnels and subways. Also, it could be used to provide indoor coverage in large multi-level shopping malls. Figure 3.3 depicts a picture of a typical radiating cable.
Figure 3 Radiating Cable
Figure 3.4 and Figure 3.5 illustrate two example applications of radiating cable. In the first application, radiating cables are used to provide coverage to a multi-level building. In the second application tunnel coverage is built by using radiating cables and BTS.
Fig. 4 Multi-level Building Fig. 5 Tunnel or Subway Coverage
Passive Repeater
In some cases, it may be difficult to provide indoor coverage to certain parts of the building even though it is located close to a BTS site. Examples of areas which may cause problems are basement and the center of the building. In such cases, passive repeaters could be a solution, whereby a good outdoor signal is repeated to the area concerned.
Fig. 6 Example Application of Passive Repeater in Medium Size Building
Figure 3.6 depicts the concept of passive repeaters, whereby a good outdoor signal from the closest BTS is passively repeated to the area concerned. This method is an inexpensive solution which requires only feeder cables and antennas. Additionally, this solution has an added advantage in that it is simple to realize.
However, because this method does not actually repeats the signal, several disadvantages are obvious. Firstly, it is dependent on the outdoor received level. Secondly, the repeated signal will be attenuated by the feeder loss. Also, all signals from different BTSs including the competitors network BTSs will be repeated. Hence, based on these arguments, a passive repeater solution is suitable only in applications where:
• there is a strong dominant outdoor server.
• the feeder run is short, and
• the area to be covered is small with little or no obstruction.
Active Repeaters
As mentioned in the previous section, the disadvantage of the passive repeater is that it depends on a strong signal and a short feeder run. Unfortunately, in some situation, it could be difficult to fulfill these requirements. For example, if the outdoor signal is only moderate and the feeder loss is quite substantial due to excessive cable length, an active repeater could be a practical choice. However, the repeater is only suitable to provide coverage for small to medium size area without the need for much capacity. If the indoor area concerned is large, such as large shopping malls, other indoor solutions must be considered. Figure 7 shows a picture of a typical repeater and its specification.
Figure 7 Repeater
Figure 8 illustrates an application example of a repeater system. Also shown are the EIRP of each indoor antenna based on typical feeder loss, antenna gain and repeater gain of 80 dBm.
Fig. 8 Example Application of Active Repeater & Splitter in Shopping Malls with Basement
The following are the advantages of active repeaters over the passive repeaters.
• Does not require high received signal from donor cell.
• Because the signal is actively repeated, this solution is more tolerant to feeder loss. Hence, longer feeder run is acceptable.
• A repeater may be channeled by using filters. This means that preferred channels may be selected for repeating. Also, channel filters could be used to prevent exploitation by a competitive network operator.
Remote RF Head
Remote RF Head is a technique whereby the RF part of the BTS is placed remotely in an area where coverage is required. It allows the separation of the RF unit from the main BTS using optical links of up to 500m. Figure 9 depicts an application of the remote RF head. The Remote RF Head offers reduced feeder loss, thus resulting in coverage extension. Therefore, this solution is ideal for providing indoor coverage in multi storeys building and large shopping malls.
Fig. 9 Example Application of Remote RF Head in Large Shopping Mall.
Although the remote RF head is an ideal solution to provide indoor coverage with little cable loss, it presents one important disadvantage. Currently, each RF head is capable of handling only one TRX, which means 7 traffic channels. Hence, in areas with high traffic density, more than one RF heads are required. Figure 10 depicts a Remote RF Head.
Figure 10 Remote RF Head
Mini BTS and Micro BTS
In some cases, due to the complexity of the building, signals from the repeater or the Remote RF Head may not be sufficient to provide for a good quality indoor coverage. For example, if the area concerned is large and located in multi-level shopping malls with high traffic density, then Remote RF Head and active repeater may not be suitable. Furthermore, if the outdoor signal is weak, then an active repeater solution may be impractical. Hence, in such situations, the only other option is to deploy an indoor BTS in the building.
Nokia offers a variety of base stations, ranging from 2nd generation indoor BTS to the 4th generation micro-BTS. For indoor solution, the 3rd generation Flexitalk mini-BTS and 4th generation PrimeSite are the most practical choice.
Figure 11 is a picture of the 3rd generation indoor Flexitalk BTS. This BTS has been optimized in size and cost and hence, it is easy to install and maintain. It provides for a maximum of 2 TRXs, with maximum output power of 18.7 W and 8.8 W for 1 and 2 TRXs respectively.
Figure 11 Nokia 3rd Generation Mini BTS Flexitalk
Figure 12 is a picture of Nokia PrimeSite 4th generation BTS. The compact and highly integrated design of this base station product is a perfect solution to any indoor coverage problems. It is designed for minimum cost and to reduce the cost of cell site ownership. It has dimension of 650mm * 385mm * 142mm, and weights only 25Kg. Nokia PrimeSite is optimized with one TRX and two fully integrated 90 deg horizontal beamwidth antenna for diversity. It can be expanded to a chain of interconnected microcells with capacity of 4*4*4 TRXs. Additionally, this product also employs downlink diversity to improve downlink quality.
Figure 12 Nokia PrimeSite
Figure 13 depicts the concept of mini and micro BTS in a multi-level shopping complex.
Figure 13 Example Application of Indoor BTS.
RF Distribution Hub and Optical Antennas
Figure 14 RF Distribution Hub and Optical Antenna
The fibre optic RF distribution system uses small optical antennas which are distributed strategically around the building to provide for a uniform in-building coverage. These fibre optic antennas are connected to the RF distribution hub by optical fibres. The RF distribution hub acts as an interface between the optical antennas and BTS system or the cellular repeater. By using optical fibbers as the carrier media, it possible to distribute the antennas throughout the building without concern to the distance between antennas and hub. Figure 14 is a picture of the distribution hub and the optical antenna.
As mentioned, this system can be interfaced with either repeater or the BTS. Typically, in situations where coverage is required without high traffic capacity, the repeater solution would probably be a better solution. Examples of application could be medium shopping malls and hotel lobbies. On the other hand, if the required coverage is large and the expected traffic is high, then the micro BTS connected to the RF distribution hub would be a more logical choice. Example of such applications would be airport terminals. Figure 3.16 depicts such an application.
Figure 16 Example Application of Distributed Optical Antennas.
10 Extension of Cell Coverage Area
The coverage area extension can be done with cellular repeaters and preamplifiers.
The cellular repeater amplifies the RF signal in both uplink and downlink directions, i.e. it is a device which compensates the propagation loss between the base station antenna and the mobile station antenna. The cellular repeater is connected between two antennas: the first antenna is pointed to the base station site and the second one (reradiating element) is pointed to the area to be covered.
Cellular repeaters are classified according to the total output power. Low power repeaters (total output power below 100 mW) can be used to extend the cellular coverage inside buildings and basements. High power repeaters (total output power above 1 W), can be used for example to extend the coverage area behind hills and inside long tunnels.
Radiating cable (leaky cable) can be used in tunnels as a reradiating element to provide homogeneous field strength inside the tunnel.
Mast Head Preamplifiers (MHPx) are installed at the base station antenna mast after the RX antenna to amplify the uplink signal. The preamplifier has a low noise figure and adjustable gain to compensate the RX antenna feeder attenuation. It can be very helpful when low-power handportables are used in the network.
10.1 Extended Cell (E-Cell)
Normally GSM Cell radius is max 35 km but with E-Cell concept it is possible to increase the cell radius to 70 km. Technically this is done with one BCCH and two- TRX solution. Whole cell is using same BCCH (N-TRX) on downlink direction but normal coverage area is served with different TRXs than extended (E-TRX) coverage area.
Timing of receiver of E-TRX is delayed so that it can serve area beyond 35 km. Timing of transmitters of both N-TRX and E-TRX is same. Time slot 0 of E-TRX is tuned to BCCH frequency in order to get random accesses from the extended area. Time slot 1 of N-TRX cannot be used because E-RACH is interfering it.
Handovers to e-cell and from e-cell are performed according to existing handover criterias. However in case of HO to e-cell the target area is predetermined. The BSC allocates recources according to specified adjacent cell parameter. Handovers between different areas of e-cell are performed according to timing advance information.
E-Cell solution is possible in 3rd and 4rd generation BTS.
11 Intelligent Underlay-Overlay
Intelligent underlay-overlay (IUO) is a solution to increase the intensity of the frequency reuse. The risen interference level caused by the tightened frequency reuse is managed by estimating the degree of interference. An intelligent handover control algorithm directs calls always to frequencies that are clean enough to sustain a good radio connection quality. The interference estimation is based on the downlink measurement results reported by mobile stations and various adjustable parameters.
The intelligent underlay-overlay is implemented solely in Base Station Controller (BSC) software and the introduction of IUO does not necessitate any changes in hardware configuration. The performance of the intelligent underlay-overlay procedure is measured by means of underlay-overlay statistics.
Figure 1. IUO Network configuration
The operating spectrum of the network is divided into regular frequencies and super-reuse frequencies. The regular layer or the overlay network uses regular frequencies to provide continuous radio coverage with overlapping cell areas required for safe handovers. The interference control mechanism is not applied in the regular layer and co-channel and adjacent channel interference should be considered in the frequency assignment.
The super layer or the underlay network uses super-reuse frequencies that are reused intensively to make the use of the available spectrum more efficient. The super-TRXs are configured exactly the same way as the regular-TRXs, using the same antennas and transmission power levels. Thus the coverage area of the super layer is the same as the coverage area of the regular layer. The suitable service area of the super layer is limited by the interference and the super-TRXs are intended to serve mobile stations that are close to the base station and other locations where the radio conditions are less vulnerable to the interference.
An ordinary IUO-cell has both types of TRXs, regular-TRXs and super-TRXs. The regular-TRXs allocated to the cell belong to a regular frequency group. A super-TRX allocated to the cell belongs to one of the sixteen super-reuse frequency groups. The super-TRXs allocated to the cell are divided into super-reuse groups according to the source of interference. Those super-TRXs that belong to the same super-reuse frequency group have the same sources of interference. The super-TRXs of the cell may belong to the same super-reuse frequency group or they may belong to different super-reuse frequency groups.
In the call set-up the traffic channel (TCH) is always allocated from a regular-TRX, because the C/I ratio for the call is not yet known. Thus the IUO-cell must have at least one regular-TRX, i.e. the BCCH-TRX. An exception is so called child cell, which will be explained soon.
The BSC monitors the downlink C/I ratio for every ongoing call. The call is always handed over from a regular-TRX to a super-TRX when the downlink C/I ratio of the super-TRX is good enough. If the C/I ratio of the super-TRX worsens below the acceptable value, the call is returned back to a regular-TRX. The handover between the layers is an intra-cell handover.
The C/I ratio is calculated by comparing the downlink signal level of the serving cell and the downlink signal level of those neighboring cells that use the same super-reuse frequencies as the serving cell.
A child cell is a microcell equipped with super-reuse frequencies. Unlike in ordinary IUO-cells, no regular-TRXs are needed in the child cells. This means that the call set-up is not allowed through the child cell and the cell must be set barred by means of a parameter. Because child cell is an independent cell, one of the super-TRXs is defined as BCCH-carrier, which is needed in every cell. The super-TRXs of the child cell may belong to the same super-reuse frequency group or they may belong to different super-reuse frequency groups.
The “umbrella” cell or an ordinary IUO-cell which serves the calls in case of bad C/I ratio in the child cell is called parent cell. The BSC monitors the downlink C/I ratio of a child cell for each ongoing call. The call is always handed over from a parent cell to a super-TRX of the child cell which is adjacent to the serving cell when the downlink C/I ratio of the super-TRX is good enough. If the C/I ratio of the super-TRX worsens below the acceptable value, the call is returned back to a regular-TRX in a parent cell adjacent to the child cell. The handover between the layers is an inter-cell handover.
The C/I ratio is calculated by comparing the downlink signal level of the serving cell and the downlink signal level of a reference cell having similar RF signal profile as the interfering cell. The use of reference cell is necessary because the interference measurement is impossible when the serving super-TRX and the BCCH-TRX of an interfering child cell are in the same channel. Thus, this may happen when the mobile station is in a child cell or in the super-TRX of an ordinary IUO-cell and there interfering child cells in the vicinity.
Figure 2. The measurement principle of the C/I ratio in IUO.
FREQUENCY PLANNING
1 Introduction
The main goal of the frequency planning task is to increase the efficiency of the spectrum usage, keeping the interference in the network below some predefined level. Therefore it is always related to interference predictions. There are two basic approaches to solve the frequency assignment problem:
* frequency reuse patterns
* automatic frequency alocation
Frequency allocation for the final network configuration is a demanding optimisation problem which requires efficient and systematic approach. NPS/X is utilising automatic frequency allocation algorithms for finding the optimum solutions. The frequency allocation is generally guided by the following information:
• channel requirement on per cell basis according to the capacity plan
• channel spacing limitations according to BTS specification
• quality of Service requirement which is converted to acceptable interference probability
• traffic density distribution over the service area
• performance of advanced system features (frequency hopping, IUO, etc.)
In practical cellular networks, the usage of regular reuse patterns, like 4x3, is seldom giving the best efficiency and quality in limited bandwidth operation. The NPS/X frequency allocation is based on cell-to-cell interference probability estimation according to the network topology, field strength distribution and traffic load. This results in customised frequency allocation patterns, providing the best spectrum utilisation efficiency with the given reference performance of the selected radio network elements.
Many articles and reports have been devoted to the subject showing the pros and the cons of the above approaches. We can find a lot of differences, but the most important is perhaps the time when the task is performed: all frequency reuse schemes are made before, while the heuristics are used after the real sites selection. Thus the main disadvantage of the first approach comes from its regularity: the terrain is flat, the sites are uniformly distributed, all the base stations in a given region have same parameters: powers, antennas, capacity, etc. With this initial assumptions the calculation of the expected interference is simple but unreliable.
The starting point of automatic frequency allocation is much better, since the exact site coordinates and BTS characteristics are available. Assuming usage of propagation model based on digital maps, we are able to obtain interference predictions very near to reality. However, the trade-off is frequency reuse patterns (if required) at the ND stage, and some frequency allocation when it comes to detailed network planning.
2 Interference Calculations
The reference interference ratio is defined in GSM Rec.05.05 as the interference ratio for which the required performance in terms of frame erasure, bit error rate or residual bit error rate is met. The reference interference ratios for BS and all types of MSs are the following:
* cochannel interference: C/Ic <= 9 dB
* first adjacent channel interference: C/Ia1 <= -9 dB
* second adjacent channel interference: C/Ia2 <= -41 dB
However, according to GSM Rec.03.30, the adjacent channel suppression for the first adjacent channel is large enough to allow usage of adjacent channels in adjacent cells. The second adjacent channel can be used in the same cell, due to the MS power control in the uplink and intracell hand-over. Using these simple constraints for adjacent channels assignment, we can skip the adjacent channel interference predictions.
2.1 Cochannel Interference
The carrier to interference (C/I) ratio at a given mobile receiver can be calculated as follows:
C/I = C/(I1+I2+...+IK)
where K is the number of cochannel interfering cells. For the regular grid case it is possible to simplify the calculations by using the popular path-loss expressions , see the picture.
The propagation loss slope b is different in different areas, but for the mobile systems it is normally between 3 and 5. The C/I can easily be expressed in terms of distances, for example in the hexagonal shaped cellular network with omni cells
C/I = R-b/(6*D-b)
is a feasible approximation. In this case R stands for the cell range and D is the frequency reuse distance.
If directional antennas are used, the number of principal interferers is two instead of six (omnidirectional case). The following estimation formula, as presented by Lee, calculates the C/I ratio in this case. The mobile station at position E experiences stronger interference from the BTS which is at distance of D from the mobile station.
C/I = R-b/((D+0.7R)-b + D-b)
Figure 2 Two interferers in the sectorized network (reuse factor 21).
2.2 Time Dispersion
Some interference effects may be caused from the reflected signals if received outside the equalizer window. This happens only when the difference between direct path and reflection path is larger than the equalizer window (about 4.5 km) and the reflected signal is strong enough. The reflection outside the equalizer window should be regarded as an independent cochannel interferer, therefore the same reference C/I <= 9 dB should be used.
2.3 Digital Maps Based Cochannel Interference
From the coverage areas calculated by the help of digital maps it is quite easy to obtain the expected interference areas. Since the frequency plan is still to be done, the multiple interference cannot be calculated. Thus the process works for every pair of BS, checking the ratio between the two signals pixel by pixel. The probability of future multiple interference can be reduced by adding some margin, say 6 dB to the reference interference ratio. If the percent of the interfered area is larger than a given predefined level (depending on the required service quality), the pair cannot operate in the same channel. The results are presented as a matrix with elements giving the minimum allowed channel difference (in this case only 0 and 1) for every pair of BSs.
3 Heuristic Algorithms
In general the Frequency Assignment Problem (FAP) for a cellular network can be formulated as follows:
Given N base stations having a requirement of Cn,n = 1, 2, ..., N channels each, total amount of M channels, and separation matrix. Let plan (frequency plan) be any distribution of frequencies, fulfilling exactly the requirements of BSs. Find a frequency plan conforming the constraints in the separation matrix by using minimum number of the available channels.
The problem is proven to be NP hard, meaning that the number of plans grows exponentially with the number of BSs, so heuristic algorithms should be used. The idea is to generate and check only subset of all possible plans, finding an acceptable rather than the optimal solution.
There exist many different approaches and algorithms, some of them implemented for computational use. However, we are going briefly to mention only the most popular.
Graph coloration approach is based on the analogy between FAP and graph coloration problem: To colour the vertices of a given graph with minimum number of colours without using the same colours for adjacent vertices. The vertices are taken to be BSs, the colours stand for channels (frequencies), while adjacent vertices mean that the corresponding pair of BSs cannot use the same channel.
Another possibility is to generate an acceptable plan by satisfying the requirements of only one BS at a time. Different rules can be applied to the decision making process: from "First BS - First Channels" to "Worst BS - Best Channels". The first rule does not require big computational effort, but very often it may not solve the problem. The second rule is usually based on time consuming investigations to offer a better choice of BS and channels. Some trade-offs are also possible.
All heuristics mentioned above are referred to as deterministic ones. There are some attempts to use non-deterministic approaches for FAP solving e.g. an analogy with the neural networks. The most important difference is that the separation matrix is not used anymore to allow or deny given channel difference for given pair of BSs. A so-called energy function is used instead, trying softly to minimize the "temperature" (interference probability) in the system.
4 Frequency Hopping
Frequency Hopping (FH) is changing the frequency of information signal according to a certain sequence. The transmission frequency may change at each time slot or burst and remains constant during the transmission of a burst.
FH can also decrease the overall C/I value in the network and thus improve the Quality Of Service (QOS).
4.1 Frequency Hopping Behaviour
As described earlier (coverage planning) the shadowing (log-normal fading) and Rayleigh- distributed fast fading can decrease the specch quality. Rayleigh fades are the sum of a lot of reflected and phaseshifted signals. These fades occuer at every half wavelength which is 204/6 ms duration at 3/100 km/h with 900 MHz.
The fading at different frequencies are not the same and become more and more independent when the difference in frequency increases. With frequencies spaced sufficiently apart (say 1 MHz), they can be considered completely independent (no correlation). Thus all the bursts containing the parts of one code word are then not damaged in the same way by Rayleigh fading.
When the MS moves of high speed, the difference between its positions during the reception of two successive bursts of the same channel (i.e., at least 4,615 ms) is sufficient to decorrelate Rayleigh fading variations on the signal. In this case FH does not help except if there is interference.
The worst case is when MS is stationary or moves at slow speeds because the interleaved coding does not bring any benefit to reception. In this case FH "simulates MS movement" and thus raises the reception quality. This phenomenon is called frequency diversity.
In the other hand frequency hopping averages the interference directed towards each base station. Instead of a continuous interferer there are several interferers that affect only a short time each and with different intensity. Methods like power control and DTX (Discontinuous transmission, see below) affect only a single interference source and the benefits can be distributed to the whole network by using FH. The gain which comes from interference averaging is called interference diversity.
The effective utilisation of frequency hopping is considered to give 6 dB gain. This is possible with min 4 carriers. With 3 carriers the estimated gain is 4 dB and with 2 carriers the gain can be only 2 dB. With interference limited network the gain can be 3 dB.
4.2 Baseband Hopping
Baseband hopping occurs between TRXs in BTS. The number of frequencies used in the hopping sequence is the same as the number of TRXs in the sector. Both random and cyclic hopping can be used.
The digital (baseband) and analogue (RF) parts of the TRX are separated from each other. The switching of TRXes is on a per timeslot basis and enables a particular TCH to hop from one carrier to another.
Figure. Baseband frequency hopping
4.3 Synthesized Hopping
Synthesized hopping is available in configurations which have at least 2 TRX per sector. It enables each TRX to change frequency on successive time slots, so that given carrier can hop quickly onto many different frequencies. The carrier on which the BCCH is transmitted must remain at fixed frequency to enable the MS to measure correct signal strength. Both random and cyclic hopping can be used.
Figure. Synthesised frequency hopping
4.4 Discontinuous Transmission (DTX)
The transmission is "disconnected" when no information flow happens in signal. This is done by lower speech encoding bit rate than when the user is effectively speaking. This low rate flow is sufficient to encode the background noise, which is generated for the listener to avoid him thinking that the connection is broken. The low rate endoding corresponds to a decreased effective radio transmission of one frame each 20 ms to one such frame each 480 ms. Typically transmission is effective 60 % of the time which decreases the interference.
In order to implement such a mechanism, the source must be able to indicate when transmission is required or not. In the case of speech, the coder must detect whether or not there is some vocal activity. This function is called Voice Activity Detection (VAD). At the reception side, the listener's ear must not be disturbed by the sudden disappearance of noise and the decoder must therefore be able to generate some "comfort noise" when no signal is received.
DTX is an option controlled by the operator, and which may be used independently in the MS to BTS and in BTS to MS.
PARAMETER PLANNING
1 GSM Radio Path
GSM is a digital system using Time Division Multiple Access (TDMA) frame structure. The TDMA frame has a duration of 4.615 ms and consists of 8 timeslots. There are two types of logical channels carried over the timeslots: Common Channels and Dedicated Channels.
1.1 Common Channels
The Common Channels are used for signaling and can be divided into Broadcast Channels (BCH), continuously sending information from BTS to MS, and Common Control Channels (CCCH).
The Broadcast Channels send information on the cell properties such as synchronization, frequency correction, used frequenciesand power levels, neighboring cells. There are three different broadcast channels: Synchronization Channel (SCH), Frequency Correction Channel (FCCH) and Broadcast Control Channel (BCCH).
The Common Control Channels are used when establishing a signaling connection between the MS and BTS. The Paging Channel (PCH) is used when BTS wants to contact the MS. The MS requests a signaling channel on a Random Access Channel (RACH). The signaling channel is allocated to the MS by using Access Grant Channel (AGCH).
1.2 Dedicated Channels
The Dedicated Channels are divided into Dedicated Control Channels and Traffic Channels. Call set-up signaling and location updating procedures are performed on Stand-alone Dedicated Control Channel (SDCCH). In case of a call set-up, the connection is transferred into a Traffic Channel (TCH).
Both SDCCH and TCH have a parallel Slow Associated Control Channel (SACCH) which is used for transfer of measurement results from MS to BTS and power control commands from BTS to MS. During the call the short messages are transmitted over SACCH channel, while the Fast Associated Control Channel (FACCH) is used to transmit the handover commands to the MS.
1.3 Usage of the Channels
The usage of channels may be described as follows:
* MS listens to the common channels
* BTS sends a paging command over PCH
* MS responds by sending a random access message over RACH
* BTS gives to MS the SDCCH number to be used for signaling over AGCH
* Call set-up signaling takes place on SDCCH
* BTS orders the traffic channel
* During the call SACCH and FACCH are used
1.4 Radio Path Measurements
The radio path measurements are used to keep the connection in good quality and therefore to trigger power changes and handovers if needed. Both MS and BTS measure signal level and quality (bit error ratio). In addition to that MS measures the signal levels of all adjacent BCCH frequencies even though it is able to report only six best measurements.
During one TDMA frame (4.615 ms) MS receives one signal sample from one BCCH frequency. Measurement data is averaged during SACCH multiframe period (480 ms) and the results are transmitted to the BTS in the next SACCH block. Since the MS receives about 102 field strength samples during one SACCH the number of samples to be averaged can easily be calculated. For example in case of 9 BCCH frequencies (9 cell reuse pattern) MS will have about 11 samples from each frequency while in case of 12 BCCH frequencies the number of the samples will be approximately 8.
2 Power Control and Handover
The BTS sends the raw measurement results received from the MS (downlink) and the results of its own measurements (uplink) to the BSC every SACCH multiframe period. The BSC does not support the measurement preprocessing in the BTS.
The BSC does the preprocessing of the measurement samples namely the bookkeeping and the averaging. The BSC is able to maintain a table of maximum 32 measurement results for up to 32 adjacent cells per call. After the averaging the BSC makes comparisons with the thresholds related to both Power Control (PC) and Hand-Over (HO) algorithms.
The BSC determines the RF output power of the MS and the BTS on the basis of the results received from the PC threshold comparison process.
The HO decision is based on signal strength (RXLEV), quality (RXQUAL) and distance measurements. Another possible criterion is the power budget (PBGT) or umbrella condition fulfillment from an adjacent cell. The HO command is given over FACCH, which uses TCH temporarily. Handovers can be done to TCH and SDCCH. The Intra-BTS handover can occur either to a timeslot on a new carrier or to a different timeslot on the same carrier. The intra-BSC handover is performed autonomously by the BSC. If there is an inter-BSC handover to be performed, the BSC sends the list of preferred cells to the MSC and MSC performs the handover according to that list.
2.1 Power Control Strategy and Parameters
The RF power control strategy employed by the BSC determines the RF power level that is signaled to the MS and the RF power levelused by the BTS. The RF power control process optimizes the RF power output from the MS and the BTS at the same time ensuring that the power levels are sufficient to keep adequate speech quality.
The RF power level (MS or BTS) is controlled by means of received level (RXLEV) and quality (RXQUAL). Both RXLEV and RXQUAL are considered separately for uplink (UL) and downlink (DL) and have several associated parameters:
* averaging (sliding) window size and weighting
* Px and Nx (Px averages out of Nx to fulfill the criteria)
* upper and lower thresholds
The averaging window size determines the number of the consecutive measurement samples to be taken into account in the averaging process. The averaging window acts as a sliding window since after each sample an average value is calculated. The number of the averages to be below/above the corresponding threshold is selected with the pair Nx and Px. The meaning is that at least Px out of Nx averages must fulfill the criterion. When the level and the quality are between the lower and upper thresholds the power is not regulated
All parameters controlling the power control process are defined on a cell by cell basis by means of O&M. By changing the values of the parameters it is possible to affect the RF power control in all stages of the process.
2.2 Handover Strategies and Parameters
The HO decision process may be triggered in different situations. Similarly to the PC it is controlled by the level (RXLEV) and quality (RXQUAL) in both UL and DL. In addition to these it depends on the distance and some periodic checks (PBGT, UMBRELLA). Only one type of periodic checks can be used per cell. The main principle when making HO caused by radio criteria is that the new server should be better than the current one.
The parameters, averaging and threshold comparison for level, quality and distance are similar to PC but only one threshold associated. The periodic checks occur every power budget (hoPeriodPBGT) or umbrella (hoPeriodUmbrella) period. In order to be performed the periodic checks require some data for the neighboring cells: the comparison process uses the calculated PBGT(n) or AV_RXLEV_NCELL(n) for the neighboring cells instead of fixed thresholds.
Like with the PC it is possible by changing the HO related cell parameters to affect the HO algorithm at all stages: preprocessing, threshold comparison, decision making.
2.3 HO and PC Interaction
Power control and handover processes run independently in parallel, but the PC is aware of the status of HO and vice versa. The BSC therefore does not try to adjust the power levels and perform a handover simultaneously. In the HO algorithm the current power levels of the BTS and the MS are taken into account in order to recognize the state when the PC can no longer maintain the call quality.
With a proper choice of the PC and HO parameters the BSC will keep the call quality by means of PC and will propose HO only when the MS actually reaches the border of the serving cell. If both HO and PC threshold conditions are fulfilled the HO has greater priority than the PC. In case of HO failure power increase can be used.
The priority order is as follows:
* HO quality uplink/downlink
* HO level uplink/downlink
* Distance
* Better cell (periodic checks for PBGT or Umbrella)
* PC Lower quality/level thresholds uplink/downlink
* PC Upper quality/level thresholds uplink/downlink
2.4 BSS Parameters
In the following figures some of the radio parameters are shown.It’s not the scope of this course to show all the parameters, but someexaamples of general parameters on how the structure of the network is defined (location area codes and BS identification), TDMA frame structure handover and power control.
NETWORK VERIFICATION AND OPTIMIZATION
This is the last step of the network planning procedure. It can start during the network trial period and continues after opening the commercial service and during the network expansion.
The aim of this process is to evaluate and maximize the quality of service in the network with the corresponding set of quality criteria.
1 Network Verification
The purpose of the Network Verification (NV) is to evaluate an independent and objective Quality Of Service (QOS) inside a given service area. This is done with Network Measurement System (NMS/X). Some OMC traffic measurements are done in parallel to provide a statistical data and to complete the network picture.
The network verification procedure consists of the following steps:
1. Planning of the measurement resources (including tools), reference network,
schedule, and test route(s)
2. Setting of the network performance objectives and quality criteria
3. Measurement execution and analysis of the statistical results
4. Reporting to the customer the results of analysis
5. Agreement on possible corrective actions if the set quality criteria is not met
The field verification takes place after successful completion of Site Acceptance. It should be repeated before and after any major network hardware/software changes to verify their affect on the network quality.
The service area, or the part of the network to be verified, is defined as a group of cells giving continuous coverage. It is always connected with a selection of Test Routes, since all verification and optimization activities are based on recurrent measurements over the same routes.
1.1 Network Quality Criteria
The quality objectives are specified according to the capacity requirements and customer's QOS strategy which are agreed with the customer.
Some examples are listed here.
Number of successful mobile originating (MO) call attempts
with normal cell releases: 90-95 %
Minimum overall downlink quality: value 0-5
in 90-95 %
Minimum overall downlink level: >37dBuV/m
on street level for 2 W mobile in 90-95 %
Minimum successful rate for hanovers (HO): 90-95 %
Maximum system response time: 0-7 s
(Time for TCH assigment) in 90-95 %
One basic requirement for Network Quality Survey is to monitor continuously QOS and NP behaviour, compare the network against other similar networks and present the results in such a way, that they are easy to understand for non technical parts of the operator’s organization as well. To be capable of doing that requires additional metrics that are sensitive enough and very easy to understand.
For this purpose there are two Nokia specific metrics. NQ&E (Network Quality and Efficiency) factor driven from NMS/X tries to capture QOS and NQ&E driven from OMC tries to capture NP in one figure. Both metrics are working in 0 to 100 scale.
1.2 Measurement Procedure
The field test measurements are made with Nokia's NMS/X Network Measurement System or a compatible system which contains a GSM/DCS-1800 mobile as measuring equipment and a connection from this mobile to a microcomputer. Optionally there can be integrated navigation equipment, e.g. GPS, for the collection of location information for network optimization purposes.
The test vehicle is driven along the pre-defined test routes while the NMS/X will generate mobile originating (MO) calls to another mobile or a terminal in the PSTN. A call that is successfully established is held for two minutes and then cleared. During the call the down link speech quality, serving cell down link level, and mobile output power level are measured. Statistical information of the previous measurements, system response times, and hand-overs is collected. The number of call attempts and the ratio between normal call releases and failures from all call attempts are recorded. The recommended minimum number of total call attempts is 500 to get statistically reliable results.
To be able to verify the effect of hardware/software changes of network elements on the network quality, the quality measures are performed by using the same reference network (and the same test routes).
During the measuring phase no hardware/software of network elements affecting the measurement results may be modified.
All the changes to the reference network are done in a controlled manner by Nokia together with the customer.
1.3 Analysis of the Results
The main result of the verification procedure is the statistical Quality Sheet generated by the NPS/X or similar system.
In addition to the statistical Quality Sheet, the network element availability statistics from the OMC also form part of the network verification results. These results are collected either on a monthly or weekly basis, and they concern the network elements as a whole or by unit basis. The statistics of unit restarts and the availability of transceiver units are reported.
The congestion level of each BTS is obtained from the OMC traffic statistics, and reported together with other Network Verification results as the Network Verification Report.
2 Network Optimization
Network Optimisation can be defined as a continuous process of improving overall network quality. Looking at network quality two different views should be considered. The customers (subscribers) view and the more comprehensive operators view. Figure 1. ‘Overall Network Quality’ is illustrating this.
Figure 1. Overall Network Quality.
Usually a subscriber is not interested in site leasing or maintenance costs. As long as his service is not affected things like spectrum efficiency and network traffic are of no interest to him. For the operator these figures are of fundamental importance.
Network Optimisation service and more general the Nokia Quality Cycle service package are designed to support the operator in the most efficient way to improve all different aspects of network quality. Nokia’s tools, experts with detailed system knowledge and the global network of experience provide the operator with the most sophisticated services.
2.1 BSS Default Parameter Assessment
Proper BSS default parameter settings are needed to ensure the best possible performance of the network. The parameter sets are based on experience from optimised networks.
2.2 Small Quality Cycle
The ‘Small Quality Cycle’ is, as the name indicates, the smallest possible quality monitoring and improving process. It is the combination of the Basic Configuration Analysis Module and the Basic Network Optimisation Module which are described in the following two chapters. Figure 2 ‘Small Quality Cycle’ may illustrate the relation between the different service modules.
Figure 2. Small Quality Cycle.
2.2.1 Basic Configuration Analysis Module
Network Configuration Analysis is the smallest possible service module of Network Optimisation. With this service the system configuration as existing in the real network (System Configuration ‘Network’) is compared against the system configuration as provided by network planning (System Configuration ‘NW Planning’). This task ensures consistency between different system configuration databases and therefore is the basis of all following tasks. Basic Configuration Analysis should be repeated on a regular basis. Nokia can support with improving or designing and implementing procedures for regular consistency checks.
2.2.2 Basic Network Optimisation Module
Field tests, OMC measurements and customer complaints are the three main sources to provide a detailed network quality picture (e.g. call and handover success rates, problems reported by customers and field test personnel). The network performance data analysis together with single quality improvement actions raise network quality on a case by case basis. These tasks are combined in the Basic Network Optimisation Module.
2.3 Full Optimisation Service
As Network Optimisation has to be seen as part of a bigger process which is embedded in the operators organisation Nokia is offering services and consultancy in the area of quality definition, monitoring and improvement. This service is called ‘Full Optimisation Service’ and contains the ‘Small Quality Cycle’ as described in the previous chapter (chapter 2.2). Figure 3 ‘Network Quality Cycle’ is showing the relation between the ‘Small Quality Cycle’ and the bigger ‘Network Quality Cycle’.
Figure 3. Network Quality Cycle.
As the ‘Small Quality Cycle’ is required to monitor and improve quality on a more or less ‘problem-by-problem’ basis a more global approach of quality monitoring and improvement is required.
Global quality reporting on regional and network level is required for management, marketing and network planning. Global quality monitoring is similar to quality monitoring as required for Basic Network Optimisation, but it is reporting on a more general level (field tests, OMC and customer complaints). Global quality reporting allows to monitor the impact of major changes in the network (e.g. frequency changes, BSS default parameter changes or massive traffic increase).
Quality definition and quality target setting are needed for agreements between different departments (e.g. planning and marketing). As a conclusion from global quality reporting and experience from the ‘Small Quality Cycle’ work general improvement actions (e.g. testing new system features) and general corrective actions (e.g. major frequency change, capacity extension, introduction of microcells or IUO) might follow.
In all phases of the Network Quality Cycle Nokia is supporting with consultancy, tools and training.
2.4 Optimisation tools
Nokia is providing a variety of tools for network optimisation. NMS/X, Nokia’s Network Measurement System for GSM/DCS and NMT network quality survey, network tracing and for multichannel field strength measurements covers all demands for field measurements. A portable version allows indoor measurements.
NPS/X, Nokia’s Network Planning System, is also a powerful tool for network optimisation. It’s features for down- and uploading data from the OMC, the GSM/DCS simulator, the link to the measurement system NMS/X and many other features provide the optimisation personnel with an advanced tool.
NPLOVE
RADIO NETWORK PLANNING
NETWORK PLANNING TRAINING PROGRAM
1 Network Planning Overview Course
Training group
Number of participants
Object
Prerequisite knowledge
Duration
Form
Subject
Planning principles and theory
Practical tasks Persons dealing with the network planning
3 - 20
To provide the trainees with a general knowledge of GSM/PCN network planning
SYSTRA and general knowledge of Radio Systems and Radio-wave Propagation
1 day
Lectures
Duration (h)
5
1
CONTENTS
GSM/PCN RADIO NETWORK PLANNING OVERVIEW
1 Radio Network Planning of GSM/PCN Cellular System
2 Radio Network Planning Procedure
3 Radio Network Planning Tools
3.1 NPS/X
3.2 NMS/X
NETWORK DIMENSIONING
1 Network Dimensioning Input
2 Capacity Calculations
3 Frequency Reuse Schemes
4 Power Budget Calculations
5 Cell Size Calculation
6 Antennas
7 Commercial Aspects and Business Planning
8 Network Layout
9 Exercise
COVERAGE PLANNING AND SITE SELECTION
1 Introduction
2 Site Selection
3 Location Probability
4 Propagation and Propagation Models
5 Coverage Predictions
5.1 Digital Maps
5.2 Point to Point and Cell Coverage
6 Field Strength Measurements
7 Propagation Model Tuning
8 Microcellular planning
8.1 Microcellular Network Applications
8.2 The two-layer Microcellular network Planning Process
8.3 BSC Features for Microcellular Network
9 Indoor Coverage Planning
9.1 Indoor Environment and Propagation
9.2 Indoor Solutions
10 Extension of Cell Coverage Area
10.1 Extended cell (E-Cell)
11 Intelligent Underlay-Overlay
FREQUENCY PLANNING
1 Introduction
2 Interference Calculations
2.1 Co-channel interference
2.2 Time Dispersion
2.3 Digital maps based co-channel interference
3 Heuristic Algorithms
4 Frequency Hopping
4.1 Frequency Hopping Behaviour
4.2 Baseband Hopping
4.3 Synthesized Hopping
4.4 Discontinuous Transmission (DTX)
BSS PARAMETER PLANNING
1 GSM Radio Path
1.1 Common Channels
1.2 Dedicated Channels
1.3 Usage of the Channels
1.4 Radio Path Measurements
2 Power Control and Handover
2.1 Power Control Strategy and Parameters
2.2 Handover Strategies and Parameters
2.3 HO and PC Interaction
2.4 BSS Parameters
NETWORK VERIFICATION AND OPTIMIZATION
1 Network Verfication
1.1 Network Quality Criteria
1.2 Measurement Procedure
1.3 Analysis of the Results
2 Network Optimization
2.1 BSS Default Parameter Assessment
2.2 Small Quality Cycle
2.2.1 Basic Configuration Analysis
2.2.2 Basic Network Optimisation
2.3 Full Optimisation Service
2.4 Optimisation Tools
GSM/PCN RADIO NETWORK PLANNING OVERVIEW
1 Radio Network Planning of GSM/PCN Cellular System
Network planning is a process where the planning engineers have the following main targets:
* to achieve the required radio coverage with maximum time and location probability (more than 90%)
* to maximize the network capacity (Erl/km2) with limited frequency band (MHz) by reusing same frequencies
* to reach good quality of service (QOS) with a minimum level of interference
* to minimize the number of network elements (MSC, BSC, BTS, ...) and therefore the cost of the network infrastructure
The described network planning process is an optimization process which needs information about network elements, system properties (GSM/PCN), planning environment, topography of the service area, existing facilities of the operator, distribution of the subscribers, and estimated subscriber future growth.
In rural areas the aim is to get large coverage with high transmission power and high antenna height. Omnidirectional and directional antennas are used, the latter ones for two-sector cells along the roads.
In city areas with high traffic density the aim is to get high capacity with small or micro cell structure by using low transmission power and low antenna height. Directional antennas are used and most of the sites are sectorized.
2 Radio Network Planning Procedure
The planning work starts by collecting all the relevant information. The basis is the data obtained from the operator, but in most cases it is not sufficient, so the network planner must use topographical maps and statistical books.
Network dimensioning is done so that the coverage and capacity requirement based on subscriber growth forecast are fulfilled. Once the number of network elements and commercial aspects are available the business plan can be made.
The selection of base station site candidates is based on the terrain topography and the existing facilities at those sites. This means that the coverage area of an existing site can be limited due to some terrain fluctuations or other obstacles and still to be preferable, when the cost of establishing a new site is taken into account.
Visiting the sites gives accurate information and is done when making the final decisions on site selection. This task is called field or site survey. The radio wave propagation environment is studied, and field strength measurements can be done to clarify the actual coverage or radio interference. The existing antenna mast (if any), space for antennas, space for equipment and expandability, air conditioning, power source and grounding, transmission lines and capacity are studied during the survey.
A computer aided design system is used for coverage predictions, interference analysis and frequency allocation. A measurement system is used for coverage and interference measurements and for predictions verification. Nokia Cellular Systems (NCS) has developed own tools for network planning and measurement:
* NPS/X Network Planning System
* NMS/X Network Measurement System
The network planning engineers in NCS have extensive experience in designing the network structure. The latest technical properties of the network elements are taken from the manufacturers - a good knowledge of this is needed. The transmission planning is an optimization task related to network planning, and the results constitute considerable savings for the operator.
The last activity of network planning is the base station sub-system (BSS) parameter planning and radio network verification and optimization. This is based on experience gained during the trial period of the network. The successful hand-overs as well as the hand-over and power control algorithms will greatly affect the speech quality of the network.
The general results and documents from network planning are:
* List of sites and network elements
* Coverage predictions and measurements
* Frequency plan and interference analysis
* Capacity calculations
* BSS parameter plan
* Transmission network plan
During the network existence the knowledge about its operation and subscriber behaviour will improve. The network will also expand in terms of capacity and coverage because of the continuous subscriber growth. Therefore the network planning is also a continuous process which tries to minimize the modifications to the existing network while preparing extension plans. Testing and tuning is also needed at each new stage of the network to ensure good quality of service.
3 Radio Network Planning Tools
3.1 NPS/X
The Network Planning System NPS/X is an integrated software package for cellular network design. It provides interactive and easy-to-use tools for network planning and documentation from rough sketches to accurate designs. NPS/X runs on a Sun(TM) or Hewlett-Packard(TM) workstations, using all the benefits of UNIX networking capabilities. The use of pop-up menus and input/output panels with action buttons make NPS/X easy to work with.
Cellular planning with NPS/X is based on utilization of digitized map and measurement results. The design database includes the parameters of the base sta¬tions, antennas, propagation models and system parameters.
Coverage predictions can be based on different propagation models such as Okumura-Hata or Walfish-Ikegami. The model parameters can be modified and tuned for a particular propagation environ¬ment.
The basic package includes:
* coverage area calculation
* composite coverage area and dominance
* point-to-point calculation, BS->MS
* interference area calculation
Options:
* automatic frequency allocation
* digitizing and editing system for digitized maps
* measurement system interface (to NMS/X) and route calculation
* utilization of traffic density map
* micro cell modelling
* cellular simulation
3.2 NMS/X
The Network Measuring System NMS/X is a field strength measuring system with an integrated navigation equipment. The NMS/X consists of a navigation system, a field strength receiver or a mobile and a portable computer which collects geographical coordinates and field strength values. The system can be easily installed in a measuring vehicle.
Measurement results can be further processed in two ways: field strength curves with location marks can be printed out with a separate printing program or results can be transferred in diskettes to NPS/X (Network Planning System) for analysis.
NETWORK DIMENSIONING
Network Dimensioning (ND) is usually the first task to start the planning of a given cellular network. The main result is an estimation of the equipment necessary to meet the following requirements:
* capacity
* coverage
* quality
ND cannot replace the detailed network planning and vice versa. It gives an overall picture of the network and is used as a base for all further planning activities.
1 Network Dimensioning Input
The requirements mentioned above imply the input information for ND task as follows:
The input data is normally supplied by the operator, but use of defaults is also possible. However, the marked items are the absolute minimum to be given in advance.
The technical parameters and characteristics of the equipment to be used are another very important part of the input. This includes the basic network modules (MSC, BSC, BTS) as well as some additional elements (antennas, cables ...).
2 Capacity Calculations
The capacity of a given network is measured in terms of the subscribers or the traffic load that it can handle. The former requires knowledge of subscriber calling habits (average traffic per subscriber), while the latter is more general. The network capacity calculation can be performed in several simple steps:
* Find the maximum number of carriers per cell that can be reached for the different regions, based on the frequency reuse patterns and the available spectrum.
* Given the blocking probability and the number of carriers, calculate the capacity of a given cell.
* Find the system capacity as a sum of all cell capacities.
ERLANG B TABLE
Chs 1% 2% 3% 5% Chs 1% 2% 3% 5%
1 0.01 0.02 0.03 0.05 21 12.80 14.00 14.90 16.20
2 0.15 0.22 0.28 0.38 22 13.70 14.90 15.80 17.10
3 0.46 0.60 0.72 0.90 23 14.50 15.80 16.70 18.10
4 0.87 1.09 1.26 1.52 24 15.30 16.60 17.60 19.00
5 1.36 1.66 1.88 2.22 25 16.10 17.50 18.50 20.00
6 1.91 2.28 2.54 2.96 26 17.00 18.40 19.40 20.90
7 2.50 2.94 3.25 3.75 27 17.80 19.30 20.30 21.90
8 3.13 3.63 3.99 4.54 28 18.60 20.20 21.20 22.90
9 3.78 4.34 4.75 5.37 29 19.50 21.00 22.10 23.80
10 4.46 5.08 5.53 6.22 30 20.30 21.90 23.10 24.80
11 5.16 5.84 6.33 7.08 31 21.20 22.80 24.00 25.80
12 5.88 6.61 7.14 7.95 32 22.00 23.70 24.90 26.70
13 6.61 7.40 7.97 8.83 33 22.90 24.60 25.80 27.70
14 7.35 8.20 8.80 9.73 34 23.80 25.50 26.80 28.70
15 8.11 9.01 9.65 10.60 35 24.60 26.40 27.70 29.70
16 8.88 9.83 10.50 11.50 36 25.50 27.30 28.60 30.70
17 9.65 10.70 11.40 12.50 37 26.40 28.30 29.60 31.60
18 10.40 11.50 12.20 13.40 38 27.30 29.20 30.50 32.60
19 11.20 12.30 13.10 14.30 39 28.10 30.10 31.50 33.60
20 12.00 13.20 14.00 15.20 40 29.00 31.00 32.40 34.60
To do the second task we need the well-known Erlang B tables or formulas and the number of traffic channels for different number of carriers. The result we get is the traffic capacity in Erlangs, which can easily be transferred into the number of subscribers.
3 Frequency Reuse Schemes
A cellular network can easily be drawn as a combination of hexagons or circles by the help of regular grids. The hexagonal world is quite different from the real one, but is still the best approximation at the ND stage. One of the benefits is the possibility to try different frequency reuse patterns (clusters) and calculate the expected C/I ratio. The goal is to assign a frequency reuse number (cluster size) to any of network regions or area types. It is clear that the high density regions (big cities) are the most problematic parts of the network.
For the city areas in a GSM network, where sectorization is expected, the optimistic estimate of the cluster size is 9 (3x3) while the pessimistic estimate is 12 (4x3).
4 Power Budget Calculations
To guarantee a good quality in both directions (uplink and downlink) the powers of BTS and MS should be in balance at the edge of the cell. The main idea behind the power budget calculations is to receive the maximum output power level of BTS transmitter as a function of BTS and MS sensitivity levels, MS output power, antenna gain (RX and TX), diversity reception, cable loss, combiner loss, etc.
The Power budget calculation provides following useful results:
BTS transmitted power. BTS transmitted power is adjusted to provide a balanced radio link (i.e. Uplink and Downlink radio link performance is the same) for given BTS and MS receiver performance, MS transmitter performance, antenna and feeder cable characteristics.
Isotropic path loss. This is the maximum path loss between BTS and MS according to given radio system performance requirements.
Coverage threshold. Downlink signal strength at coverage area border for given location probability. Coverage threshold value can be used in coverage prediction with NPS/X (Network Planning System).
Cell range for Indoor and Outdoor coverage. This is an rough indication about cell range in different area types and can be used for network dimensioning. It can also be used for comparing the effect of different equipment specifications and antenna heights for the cell range.
The following table explains the meaning of each of the parameters used in the Power Budget calculation.
Parameter Description Reference
MS CLASS Mobile power class GSM 05.05
RX RF-input sensitivity For MS this is updated according to given MS class. For BTS this can be adjusted according to the performance of Nokia BTS type in question. GSM 05.05
Cable loss + connector Antenna feeder cable and connector losses for BTS site and for car mounted MS. For handheld MS this is 0 dB. In dimensioning some default value is selected for BTS site, e.g. 3 dB
RX antenna gain Gain (dBi) of used BTS and MS antenna types. For handheld MS, default is 0 dB. Antenna product specification
Diversity gain RX diversity gain. For BTS, the practical range is 0...5 dB depending on environment and antenna installation (separation). When BTS RX diversity is used, the default value is 4 dB for urban areas.
Isotropic power Minimum signal strength (dBm) in the receiving end calculated from the above factors. GSM 03.03
Field strength Minimum field strength requirement (dBV/m) converted from isotropic power.
TX RF output peak power For MS this comes from selected MS class. For BTS this is calculated according to balanced radiolink requirement. GSM 05.05
Isolator + combiner + filter Attenuation of the BTS RF components between TX unit and antenna feeder connector of BTS. The exact value depends on BTS model and configuration, typical values are 3,5 .... 5 dB. BTS product specification
RF peak power (Combiner) output Transmit power at BTS/MS antenna connector
TX antenna gain According to selected RX antenna gain
Peak EIRP Effective Isotropic Radiated Power level
Isotropic path loss EIRP - minimum RX field strength
Standard deviation Standard deviation of the signal strength (outdoors). Typical value is 6 ... 8 dB depending on the environment.
BPL average Average building penetration loss in the target area (for indoor coverage calculation), typically between 8 ... 18 dB.
BPL deviation Signal deviation indoors (for indoor coverage calculation), typically 8 ... 12 dB.
Area type correction Area type correction factors for the Okumura-Hata model
5 Cell Size Calculations
The calculation of the cell radius at the ND stage is usually based on simple and easy to use propagation loss formulas: Okumura-Hata, Juul-Nyholm, Walfish-Ikegami (city areas). Since some of the steps are similar, both radio link budget and cell size calculations can be done simultaneously, see the power budget and cell sizes table.
There are many parameters affecting the cell size such as BTS power, antenna heights (BS and MS), and BTS TX antenna gain, but the area type (dense urban, urban, suburban, rural, forest, etc.) is of great importance. The expected outcome are some average cell sizes for all the different area types. It is also possible to obtain specific cell sizes for the most often used site configurations: omni, two-sectors (road) and three-sectors (city).
6 Antennas
The antenna is an element that radiates an electric signal into the air. Typically antennas are characterized by their gain, beamwidth and band of operation.
To have a reference an isotropic antenna is used, which has 0 dBi gain. This is a theoretical antenna that radiates the same power in all 3D directions. The gain gives the maximum power trasmittiter in one (directional antennas) or several (omnidirectional) directions.
The radiation pattern shows how the power is transmitted into the air in the differnt directions. Because it’s 3 dimensional 2 diagrams are given, the horizontal ( like a top view of a building ) and the vertical ( side view ).
The beamwidth is defined by the points of 3 dB atenuation compare to the maximum.
Usually the lower the frequency the bigger the antennas to achieve the same gain and the most difficult to make high gain antennas.
In the 900 Mhz (GSM) and 1800 (DCS) Mhz bands the antenna gains vary between 10 dBi (omni) and 18 dBi (directional)
7 Commercial Aspects and Business Plans
The final step in the ND task is to add the prices of the equipment and links and to calculate the overall network cost for a given period of time. This is important not only for the operator but also for the network planner. Adding the cost of transmission links may lead to some changes in the network structure, especially in the number of BSCs. The optimization of the network structure is now based on cost function minimisation.
It is possible to combine capacity, coverage and cost calculations in so-called business network plan application. In this way the ND procedure can start with cell sizes and using the results go directly to the business network plan.
8 Network Layout
The output from the ND task can differ from case to case but some of the items are common. These are average cell sizes, frequency reuse schemes, network evolution figures (area, subscribers, equipment), and sketches of the network or parts of it.
9 Exercise
1. Fill the following power budget calculations
Omni Directional
GENERAL INFO
Frequency (MHz): 1800 System:
RECEIVING END: BS MS
RX RF-input sensitivity dBm -104.00 -100.00 A
Cable loss + connector dB 4.00 C
Rx antenna gain dBi 12.00 0.00 D
Diversity gain dB 0.00 E
Isotropic power dBm F=A+C-D-E
TRANSMITTING END: MS BS
TX RF output peak power W 1.00
(mean power over RF cycle) dBm 30.00 K
Isolator + combiner + filter dB 4.0 L
RF-peak power, combiner output dBm M=K-L
Cable loss + connector dB N
TX-antenna gain dBi O
Peak EIRP W
(EIRP = ERP + 2dB) dBm P=M-N+O
Isotropic path loss dB Q=P-F
Directional
GENERAL INFO
Frequency (MHz): 1800 System:
RECEIVING END: BS MS
RX RF-input sensitivity dBm -104.00 -100.00 A
Cable loss + connector dB C
Rx antenna gain dBi 17.00 0.00 D
Diversity gain dB E
Isotropic power dBm F=A+C-D-E
TRANSMITTING END: MS BS
TX RF output peak power W 1.00
(mean power over RF cycle) dBm K
Isolator + combiner + filter dB 4.00 L
RF-peak power, combiner output dBm M=K-L
Cable loss + connector dB N
TX-antenna gain dBi O
Peak EIRP W
(EIRP = ERP + 2dB) dBm P=M-N+O
Isotropic path loss dB Q=P-F
2. Calculate the cell range and the number of needed bases stations for coverage:
Cell Range
Parameters
MS Antenna Height (m) 1.5
Frequency (MHz) 1800
Area (Sq.km) 1000
Road Lenght (km) 500
Urban S_Urban Rural(sec) Rural(omni) Roads
BTS Antenna Height 35 30 40 40 40
Correction Factor 0 -5 -12 -12 -12
Isotropic Path Loss
Cell Range omni(km)
Cell Range dir (km)
Cell Area (sq.km)
Some hints!
Path loss Range
140 2.40
141 2.56
142 2.74
150 1.97 2.55 4.68 4.68
151 2.11 2.72 5.00 5.00
152 2.25 2.91 5.35 5.35
153 2.40 3.10 5.72 5.72
BTS Distribution Urban S_Urban Rural Roads
BTS#0 0 % 0 % 20 % 0 %
BTS#D#00 0 % 0 % 0 % 100 %
BTS#D#000 100 % 100 % 80 % 0 %
Area Distribution 15 % 35 % 50 % 100 %
Sites Needed
3. Capacity calculation
Traffic
In air interface Erlang B table can be used
Traffic
TRXs TCHs CCs 1 % 2 % 5 %
1 7 1 2.50 2.94 3.25
2 15 1 8.11 9.01 9.65
3 22 2 13.70 14.90 15.80
4 30 2 20.30 21.90 23.10
5 38 2 27.30 29.20 30.50
6 45 3 33.43 35.60 39.55
7 53 3 40.60 43.06 47.53
8 61 3 47.86 50.59 55.57
9 69 3 55.19 58.18 63.65
10 76 4 61.65 64.85 70.75
11 84 4 69.08 72.52 78.89
12 92 4 76.55 80.23 87.05
Subscribers (000) 100000
Traffic per Subs 20 mErl
Blocking 2 %
Channel Spacing 200 kHz
Frequency Reuse 12
Band Width 4.8 MHz
Max. Carriers/Sector 2
Omni 2-sector 3-sector
Terrain Type Subs Dist. # of Subs Traffic # of BTSs # of BTSs # of BTSs
Urban 50 % 50000 1000
S_Urban 35 % 35000 700
Rural 15 % 15000 300
Total (Area) 100 % 100000 2000
Total (Roads): 100 % 10000 200
4. Taking into account the 2 previous resluts estimate the total amount of equipment need:
SUMMARY
Urban S_Urban Rural Roads Total
Sites, max(coverage, capacity)
Sectors
TRXs
Traffic
Traffic/Site
TRXs/Site
TRXs/Sector
Number of BSCs
BSC#032
BSC#064
BSC#128
MSC#050
MSC#100
COVERAGE PLANNING AND SITE SELECTION
1 Introduction
The objective of coverage planning phase in coverage limited network areas is to find a minimum amount of cell sites with optimal locations for producing the required radio coverage for the target area. This phase includes the selection of practical site candidates being an iterative process until the final site locations are frozen and initial coverage related cell parameters selected.
Coverage planning is normally performed with prediction modules on digital map databases. The task is guided with the requirements coming from Purchaser and from System Configuration and Dimensioning. The basic input information for coverage planning includes:
• Coverage regions
• Coverage threshold values on per region (Outdoor, In-car,Indoor)
• Antenna (tower) height limitations
• Preferred antenna line system specifications
• Preferred BTS specification
Activities such as propagation modelling, field strength predictions and measurements are usually referred to as coverage planning. The coverage is statistical in nature and always related to a given level of field strength, location and time probability. For the mobile radio the dependence on time can be neglected since the standard deviation of the field strength time distribution is relatively small. The radio path description includes fast (Rayleigh) fading and slow (log-normal) fading. The former is caused by multipath propagation and is typical for land mobile service. The slow fading, so-called "shadowing" is caused by terrain fluctuations and obstacles (buildings).
2 Site Selection
Coverage planning and site selection are performed in parallel with the site acquisition in interactive mode. Both network planning team and site acquisition team should have well defined responsibilities and means to communicate. This chapter covers the case where Nokia does the network planning and the Purchaser takes care of the site acquisition, however the exact procedure and interface are subject of agreement between the two parties.
An initial set of site candidates, if available, is provided . The data module for each site candidate should contain at least the coordinates and the antenna height range.
Coverage analysis and site surveys is performed for perspective candidates. The initial candidates are separated in groups depending on the analysis results, for example: Accepted, Possible, Rejected. Based on the coverage and capacity needs which are still unsatisfied, a list of wanted sites including coordinates and antenna height is produced.
3 Location Probability
To "have a coverage" in a given cell means that the field strength (predicted or measured) exceeds E dB in more than L% of the locations, where E and L are predefined. For the cellular networks L=90% is a widely-used choice, while the field strength threshold E is a function of parameters like area type, mobile sensitivity, mobile antenna gain. The location probability L should be related to the cell area (La), while from the measurements or field strength predictions we can obtain a location probability related to the distance (Ld). In order to transfer Ld to La an integration over the cell area must be done : for example area location probability La=90% corresponds to about Ld=75% location probability at the edge of the cell (distance equal to the cell range). Therefore, the field strength planning threshold value should include a correction for the Ld transition from 50% to 75%, referred to as a slow fading margin, see Power Budget and Cell Sizes tables. Using 7 dB standard deviation of the field strength distribution, the slow fading margin is about 4.5 dB.
Since the location probability for the cellular radio is always area related, the notation L% will be assumed equivalent to La% unless specially noted otherwise.
To "have a coverage" in a given cell means that the field strength (predicted or measured) exceeds E dB in more than L% of the locations, where E and L are predefined. For the cellular networks L=90% is a widely-used choice, while the field strength threshold E is a function of parameters like area type, mobile sensitivity, mobile antenna gain. The location probability L should be related to the cell area (La), while from the measurements or field strength predictions we can obtain a location probability related to the distance (Ld). In order to transfer Ld to La an integration over the cell area must be done : for example area location probability La=90% corresponds to about Ld=75% location probability at the edge of the cell (distance equal to the cell range). Therefore, the field strength planning threshold value should include a correction for the Ld transition from 50% to 75%, referred to as a slow fading margin, see Power Budget and Cell Sizes tables. Using 7 dB standard deviation of the field strength distribution, the slow fading margin is about 4.5 dB.
Since the location probability for the cellular radio is always area related, the notation L% will be assumed equivalent to La% unless specially noted otherwise.
4 Propagation and Propagation Models
Propagation in land mobile services at frequencies from 300 to 1800 MHz is affected in varying degrees by topography, morphography, ground constants and atmospheric conditions. A very common way of propagation loss presentation is the usage of so-called propagation curves, normally derived from measurement data . This approach is not very suitable for computation, therefore some approximation formulas have been introduced: Hata , Juul-Nyholm , Walfish-Ikegami .
The propagation models normally include a basic propagation loss and different correction factors, see Hata's formula for urban areas (small or medium city) as an example.
5 Coverage Predictions
We have already discussed the possibilities for rough coverage calculations, based only on propagation curves or formulas. These average values are not enough for the detailed network planning, therefore many computer-aided tools based on digital maps usage have been developed to improve the quality of the predictions.
5.1 Digital Maps
There are different types of information that can be digitized and used for the coverage predictions. The most important from the network planning point of view are topography (terrain heights), morphography (area types), roads, and traffic density. There exist also different ways of data presentation, but the raster format seems to be the most popular. The raster unit (pixel) can be rectangle or square, having size in the 50m to 500m range for the cellular radio.
For the micro cell modelling, which is required in a dense urban environment, more information and higher resolution maps should be used. Information about the buildings and streets is essential, so the pixel size from 5m to 25m is reasonable. The streets can be stored and used in vector format.
5.2 Point to Point and Cell Coverage
Using a given digital map it is not difficult to obtain the path profile between any two points, say BS and MS. Furthermore the profile can be related to the corresponding area types, thus making possible the calculation of specific propagation loss. Normally different corrections, such as the diffraction loss or mixed land-sea path correction are added to the basic propagation loss.
The result of such point to point calculations can be used for cell coverage prediction. There are two basic approaches:
* radial calculations
* pixel by pixel calculations
The latter one gives better possibilities for the interference predictions, so the results should be transferred to the raster format even if the radial approach is used.
6 Field Strength Measurements
The field strength measurements are needed for determination of coverage areas as well as for tuning the propagation model of a network planning system.
In case of measurements before base station installation the site should be equipped with a test transmitter. Possible test transmitter configurations are mobile station, base station channel unit and signal generator with power amplifier.
The selection of routes to be measured depends on the purpose of the measurements. The most critical routes are typically located in urban or hilly areas, where it is difficult to predict the field strength value with high accuracy. For indoor coverage determination a portable measurement system have to be used.
During the field strength measurement the measuring system normally takes samples from the signal received by the antenna. The field strength samples are recorded by a control computer with time, distance or location marks. Using the field strength samples it is possible to calculate some average values . The recommendation for the averaging interval is 40 wavelengths, corresponding to about 13m at 900 MHz and 7m at 1.8 GHz. Furthermore, it is possible to make a detailed statistical analysis including mean value, standard deviation and different percentile value calculation.
7 Propagation Model Tuning
Practice has shown that the propagation models are not universal. The predictions must be verified by measurements and the models tuned accordingly. The model testing and tuning is a very sophisticated and challenging task, which requires detailed knowledge of the propagation nature. It should be done for every area type in a given country or region before the detailed network planning is started.
8 Microcellular Planning
Because of high subscriber growth rates and sometimes very limited frequency resources, the capacity issue is becoming increasingly important especially in the crowded urban areas and in markets where expanded coverage is needed.
For the cellular industry to sustain growth and to meet the demand for higher capacity, the cellular coverage must be continuously improved and the network capacity must be increased. One method of increasing the capacity of a cellular system is applying Microcellular technology. Microcells are especially suitable in urban areas and other traffic "hot spots". In addition to offering high traffic capacity in the crowded spots, microcells will in many cases improve the coverage, for example, by covering "dead spots" of the macrocellular network.
There are a few common characteristics for microcellular networks. Typically the cell range is less than one kilometer, often about 300 meters. The coverage of microcells is very limited and highly guided by surrounding buildings. The transmission power required is low, in range of 10 to 100 mW. The antennae are normally placed below the average roof level so that the radio wave propagation will be limited along the streets.
Microcells are typically used in high traffic density spots, in urban areas such as shopping centers and downtown areas, and in places that macrocells cannot cover, such as tunnels.
8.1 Microcellular network applications
One of the main applications for microcellular network is to use it for capacity fill-in purposes in city and high traffic density areas. The small cell size ensures enough capacity needed in high traffic density spots such as shopping centers and office areas. A small cell needs only 1-2 TRXs to fulfill the capacity requirement of its coverage area.
Theoretically, microcells achieved by macrocell splitting provide high capacity. Building of continuous coverage by using single micro layer is however difficult. Larger overlapping areas and thus higher transmission powers or higher antennas are usually needed. This means often reduction in the efficiency of frequency reuse. Besides a tight single layer network may cause troubles for fast moving mobiles.
A two-layer network gives means to build the network in flexible and efficient way. Umbrella cells provide the backbone of GSM service and general coverage, and microcells on the underlay network supply the high traffic capacity where needed. Two-layer solution also offers the operator a possibility to segment subscribers according to the service offered to them, and thus charge subscribers differently. The service offered to the mass market could be the right to use the microcellular network within its coverage. By paying a separate fee a subscriber could be entitled to use the overlay network as well, and extend his or her coverage area.
The network elements should support a microcellular network which can be implemented under the existing GSM network and can be extended according to the capacity needed. The infrastructure implemented should give an answer at least to the following aspects:
- What happens, when a fast moving mobile, connected to a macrocell, drives through a microcell area?
- How to prevent call drops when subscribers move from indoors to outdoors or outdoors to indoors?
- How to balance the load between cell layers to ensure available macrocell capacity?
Sophisticated handover algorithms are essential for radio resource management in a microcellular network. Fast moving mobiles in a microcellular network set special requirements for BSC handover features.
In order to segment subscribers to e.g. business and private users, some new features for MSC are needed.
8.2 The two-layer Microcellular Network planning process
Implementation of a microcellular network sets new challenging requirements for the network planning. In order to build an efficient two-layer network structure, the capacity, coverage and the used frequencies of the existing network must be taken into account together with the problematic areas calling for microcells. Moreover, special care should be paid to parameter planning in order to avoid trouble when subscribers move from one cell to another - especially when the movement is out of the coverage of a microcell: the transition should be smooth, without service breaks and dramatic changes.
Coverage Planning
The existing statistical propagation models like Walfish-Ikegami can be further enhanced and used with high accuracy digital maps for the densely populated areas.
Also some new models developed for microcellular planning are available. Some of these models are able to calculate the effects of multiple reflections and diffractions from roofs.
The resolution of the maps can easily go down to 5-10 meter/pixel size and include new data layers like building heights, streets in vector format, etc. Nokia's network planning tool has been developed to fully supports the required simultaneous work with different map resolutions, data layers and propagation models.
Coverage planning will more and more rely on measurements not only for propagation model tuning but also for presenting the coverage areas as a mixture of predicted and measured data. The different types of NMS/X measurements, like dedicated field strength measurements or MS originated signal level and quality reports, can be performed and managed by the NPS/X.
Capacity Planning
Becoming the dominating factor for the network transition to hierarchical two-layer structure, the capacity growth must be followed very carefully. Due to small coverage areas of microcells their capacity will be normally provided by one or two TRXs.
However, with the NPS/X planning tool it is possible to use a traffic density map for automatic cell capacity calculation. In the future, the traffic density map could be updated by means of direct connection to the OMC traffic load statistics.
Frequency Planning
Frequency allocation is the most complicated task even in case of a conventional one-layer network. The microcellular layer adds extra complexity but does not require totally new approach - the heuristics can be used in a similar manner. When it comes to the frequency allocation there are two options: use two separate frequency groups for the two layers or to have a common pool of frequencies and assign them freely to macro and microcells. The second one offers better usage of the available frequencies but the first one makes the expansion and the operation of the network easier.
Parameter Planning and Network Optimization
The role of the parameter planning is quite important for a hierarchical network since it covers the interaction between the layers as well as the standard handover and power control issues. What is the support from the NPS/X? There are two main areas of interest, namely the NPS/X Simulator and the direct Link to the OMC. The network optimization therefore is based on measurements done with NMS/X and OMC, and analysis and simulations done with NPS/X. The existing BSS parameters are taken directly from the OMC into NPS/X and the resulting changes are sent back via the NPS/X-OMC interface.
New Transmission Possibilities are available for the A-bis interface
Because of the small cell size, the microcellular sites are located close to each other. There can be up to 20 sites in one square kilometer. This can increase the transmission cost tremendously. Therefore, the transmission must be taken into account when planning the network expansion. A cost-effective transmission method must be carefully selected. Flexible transmission options must be available.
To ensure an economical approach to build and expand the network, Nokia's BTSs support multi-drop chains and branched networks as standard. Fully integrated radio links give more flexibility for network planning, as well. Having built-in transmission equipment, standard repeater functionality with maximum 20 dB cable loss allowed enables economical network structure without extra equipment. Distance between BTSs can be approximately 1 km apart linked with coax cables or twisted pairs (in case of twisted pairs, up to 60% of telephone lines can be utilized) and can be extended by standard 2 Mbit/s line repeater. This is the PCM line approach.
With the help of 2 Mbit/s baseband modem which could be integrated at the base station site, BTSs can be linked ranging from 0.5 to 12 km with copper wires depending on the diameter and transmission speed. There are three options for data transmission speed available for this approach: 2 Mbit/s, 1Mbit/s and 512 kbit/s. The baseband modem utilization is a very cost effective approach to connect BTSs, and this approach has low requirements for the quality of copper wire. Compared to the PCM line approach, the baseband approach has the advantage of low noise (cross-talk) level if copper wires would be used in both cases.
Antenna location
With the new Nokia BTS equipment it is possible to place a base station and an antenna next to each other, which reduces the cost of antenna cabling, regardless of the location of the transmission equipment and power equipment. This is especially useful in microcellular networks, where the antenna locates low, for example on a wall of a building, whereas power and transmission equipment probably on the roof.
8.3 BSC features for Microcellular Network
As stated above, advanced handover algorithms are essential for a microcellular network. Besides conventional features, Nokia has developed several BSC features including handover supporting features, which are especially suitable in a microcellular network.
Combined umbrella and power budget handover
The umbrella handover is defined to control traffic between different layers. The umbrella handover is based on the field strength of the candidate cell and is totally independent on the radio properties of the serving cell. The traffic between the cells of the same layer is controlled with power budget handover.
Rapid Field Drop handover and Fast Moving Mobiles
In a rapid field drop situation, the normal handover procedure will fail to hand a call over to the target cell in a microcellular network since the radio field strength drops so rapidly that the MS does not have enough time to perform measurements and handovers. In the Rapid Field Drop handover this field drop can be detected from the n last uplink RX levels, which are lower than a predefined limit. A chained neighbor cell is considered as the target cell of the handover if its last averaged RX level exceeds a predefined limit defined to this neighbor cell.
The BSIC decoding and neighbor measurements together with the MS speed determine the required degree of overlapping of two neighboring cells. The time needed for BSIC decoding and handover process may be extended also by dividing the signal to several antennas or by using leaky cables or repeaters. See the examples below.
C2 Re-select Feature and Attractiveness of Cells
The C2 re-select feature controls cell re-selection of a MS in idle state by introducing a so-called penalty time. The penalty time makes the microcells more attractive to those slow moving MS whereas make macrocells more attractive to the fast moving MSs. Therefore, the fast moving MSs will be camped on the 'stable' macrocells and the slow moving or static MSs on microcells.
The Problem with Fast Moving MS
The fast moving mobiles are detected by calculating the measurement reports where the microcell is properly received. If the microcell is received well for long enough, the measuring mobile station is interpreted as slow moving or stationary and is permitted to attempt an umbrella handover.
Microcells are small cells in city offering more capacity to the high traffic density area. The coverage area is limited to 0,1 -1 km with antennas installed to below roof top. Coverage can be controlled also with transmitting power and antenna type (gain, radiation pattern).
Because surrounding buildings prevent propagation the interference area is also reduced. Thus the frequency reuse is more effective. Dedicated frequencies for microcells or mixed frequencies for both cells (also macrocells) can be allocated.
The radiowaves propagate mainly along the streets. Signal levels can vary rapidly in corners or in the side street. In some cases horizontal radiation pattern should be narrow and vertical one wide.
Walfish-Ikegami- propagation model has detailed information of buildings and street which is used for microcellular predictions.
Small and lightweight Mini-BTSs as well as small and unnoticeable antennas are recommended to use in microcells.
The indoor coverage is not easy to predict, since the building penetration loss shows relatively large standard deviation. It can be explained with the big variety of construction materials and floor differences. In 900 MHz band an average value of 15 dB and standard deviation of 15 dB can be used for planning purposes. For 1800 MHz the average value may be 20 dB. The reference values are based on some published results and the measurements done by NCS.
9 Indoor coverage planning
Indoor coverage is becoming increasingly important for extending voice and data communication services within the work place and residential homes. Hence, it does not come as a surprise when operators impose demanding specifications for indoor coverage.
In some cases, indoor coverage problems could be resolved by down tilting or re-orientating rooftop antennas towards the building concerned. Furthermore, with the deployment of microcells below the rooftop level, indoor coverage could be enhanced within the building. However, due to the complexity of indoor propagation and the attenuation caused by external building walls, signals from neighboring outdoor sites may not be able to provide sufficient indoor coverage. In such situations, rather than depending on outdoor sites to provide indoor coverage, antennas and BTSs must be installed inside the building itself. Such indoor enhancing solutions are normally referred to as Picocells.
During the planning stage, indoor coverage plans must be as accurate as possible. Ideally, test transmitters should be deployed to confirm the location of antennas, types of antennas, methods of indoor solution, and to optimize the indoor propagation models.
9.1 Indoor Environment and Propagation
Radio wave propagation in an indoor environment involves external and internal wall penetration, absorption of radio wave energy by furniture, as well as body losses due to the presence of people in the surrounding area. Furthermore, indoor propagation is subjected to fast multipath fading due to multiple path reflected signals, and also diffracted waves due to corners and furniture such as cabinets.
In order to differentiate propagation phenomena, building walls are normally categorized into: reinforced concrete used in modern buildings walls and ceiling; brick and concrete used in older buildings and also residential homes; and light concrete or plaster walls for internal building walls. Inside a building, propagation may be classified as either with line of sight (LOS) or obstructed.
Another effect which often occurs to indoor propagation is the so-called Breakpoint phenomena, where the signal strength tends to decay slower when closer to the transmitter and then much more rapidly when it is further away. Typically, the receive signal tends to decay at 20logd before the breakpoint, and beyond the point, the signal may decay at between 40logd to 80logd, implying a steeper decrease in signal strength.
9.2 Indoor Solutions
The following are various indoor solution which has been implemented by Nokia.
• Power Splitters
• Radiating Cables
• Passive Repeaters
• Active Repeaters (Under Study)
• Remote RF Head
• Micro BTS
• RF Distribution Hub and Optical Antenna (Under Study)
Power Splitter
Power splitter is an inexpensive method used specifically to divide energy from the BTS transmitter to several antennas. The following are some pictures of the 2-way, 3-way and 4-way power splitters.
Figure 1 Power Splitters
In most cases, lower levels of buildings present the most problems in terms of indoor coverage. In such a case, the BTS output power could be divided by using power splitters, and distributed to several antenna systems. It is an ideal solution to extend indoor coverage within a BTS cell site. However, particular attention should be paid when selecting the cell to divide as the outdoor signal will be degraded. Figure 3.2 depicts a typical application of antenna installation using splitters with outdoor and indoor antennas.
Fig. 2 Example of Splitter in a cell site
Radiating Cables
Radiating cables are wave guides with openings along their length allowing fractions of the transmitter energy to leak out continuously. Basically, radiating cables are just slots along coaxial cable allowing for a controlled fraction of RF energy to be radiated along the entire cable. Thus, they function like continuous antennas. The radiating cables is a good solution to coverage problem in tunnels and subways. Also, it could be used to provide indoor coverage in large multi-level shopping malls. Figure 3.3 depicts a picture of a typical radiating cable.
Figure 3 Radiating Cable
Figure 3.4 and Figure 3.5 illustrate two example applications of radiating cable. In the first application, radiating cables are used to provide coverage to a multi-level building. In the second application tunnel coverage is built by using radiating cables and BTS.
Fig. 4 Multi-level Building Fig. 5 Tunnel or Subway Coverage
Passive Repeater
In some cases, it may be difficult to provide indoor coverage to certain parts of the building even though it is located close to a BTS site. Examples of areas which may cause problems are basement and the center of the building. In such cases, passive repeaters could be a solution, whereby a good outdoor signal is repeated to the area concerned.
Fig. 6 Example Application of Passive Repeater in Medium Size Building
Figure 3.6 depicts the concept of passive repeaters, whereby a good outdoor signal from the closest BTS is passively repeated to the area concerned. This method is an inexpensive solution which requires only feeder cables and antennas. Additionally, this solution has an added advantage in that it is simple to realize.
However, because this method does not actually repeats the signal, several disadvantages are obvious. Firstly, it is dependent on the outdoor received level. Secondly, the repeated signal will be attenuated by the feeder loss. Also, all signals from different BTSs including the competitors network BTSs will be repeated. Hence, based on these arguments, a passive repeater solution is suitable only in applications where:
• there is a strong dominant outdoor server.
• the feeder run is short, and
• the area to be covered is small with little or no obstruction.
Active Repeaters
As mentioned in the previous section, the disadvantage of the passive repeater is that it depends on a strong signal and a short feeder run. Unfortunately, in some situation, it could be difficult to fulfill these requirements. For example, if the outdoor signal is only moderate and the feeder loss is quite substantial due to excessive cable length, an active repeater could be a practical choice. However, the repeater is only suitable to provide coverage for small to medium size area without the need for much capacity. If the indoor area concerned is large, such as large shopping malls, other indoor solutions must be considered. Figure 7 shows a picture of a typical repeater and its specification.
Figure 7 Repeater
Figure 8 illustrates an application example of a repeater system. Also shown are the EIRP of each indoor antenna based on typical feeder loss, antenna gain and repeater gain of 80 dBm.
Fig. 8 Example Application of Active Repeater & Splitter in Shopping Malls with Basement
The following are the advantages of active repeaters over the passive repeaters.
• Does not require high received signal from donor cell.
• Because the signal is actively repeated, this solution is more tolerant to feeder loss. Hence, longer feeder run is acceptable.
• A repeater may be channeled by using filters. This means that preferred channels may be selected for repeating. Also, channel filters could be used to prevent exploitation by a competitive network operator.
Remote RF Head
Remote RF Head is a technique whereby the RF part of the BTS is placed remotely in an area where coverage is required. It allows the separation of the RF unit from the main BTS using optical links of up to 500m. Figure 9 depicts an application of the remote RF head. The Remote RF Head offers reduced feeder loss, thus resulting in coverage extension. Therefore, this solution is ideal for providing indoor coverage in multi storeys building and large shopping malls.
Fig. 9 Example Application of Remote RF Head in Large Shopping Mall.
Although the remote RF head is an ideal solution to provide indoor coverage with little cable loss, it presents one important disadvantage. Currently, each RF head is capable of handling only one TRX, which means 7 traffic channels. Hence, in areas with high traffic density, more than one RF heads are required. Figure 10 depicts a Remote RF Head.
Figure 10 Remote RF Head
Mini BTS and Micro BTS
In some cases, due to the complexity of the building, signals from the repeater or the Remote RF Head may not be sufficient to provide for a good quality indoor coverage. For example, if the area concerned is large and located in multi-level shopping malls with high traffic density, then Remote RF Head and active repeater may not be suitable. Furthermore, if the outdoor signal is weak, then an active repeater solution may be impractical. Hence, in such situations, the only other option is to deploy an indoor BTS in the building.
Nokia offers a variety of base stations, ranging from 2nd generation indoor BTS to the 4th generation micro-BTS. For indoor solution, the 3rd generation Flexitalk mini-BTS and 4th generation PrimeSite are the most practical choice.
Figure 11 is a picture of the 3rd generation indoor Flexitalk BTS. This BTS has been optimized in size and cost and hence, it is easy to install and maintain. It provides for a maximum of 2 TRXs, with maximum output power of 18.7 W and 8.8 W for 1 and 2 TRXs respectively.
Figure 11 Nokia 3rd Generation Mini BTS Flexitalk
Figure 12 is a picture of Nokia PrimeSite 4th generation BTS. The compact and highly integrated design of this base station product is a perfect solution to any indoor coverage problems. It is designed for minimum cost and to reduce the cost of cell site ownership. It has dimension of 650mm * 385mm * 142mm, and weights only 25Kg. Nokia PrimeSite is optimized with one TRX and two fully integrated 90 deg horizontal beamwidth antenna for diversity. It can be expanded to a chain of interconnected microcells with capacity of 4*4*4 TRXs. Additionally, this product also employs downlink diversity to improve downlink quality.
Figure 12 Nokia PrimeSite
Figure 13 depicts the concept of mini and micro BTS in a multi-level shopping complex.
Figure 13 Example Application of Indoor BTS.
RF Distribution Hub and Optical Antennas
Figure 14 RF Distribution Hub and Optical Antenna
The fibre optic RF distribution system uses small optical antennas which are distributed strategically around the building to provide for a uniform in-building coverage. These fibre optic antennas are connected to the RF distribution hub by optical fibres. The RF distribution hub acts as an interface between the optical antennas and BTS system or the cellular repeater. By using optical fibbers as the carrier media, it possible to distribute the antennas throughout the building without concern to the distance between antennas and hub. Figure 14 is a picture of the distribution hub and the optical antenna.
As mentioned, this system can be interfaced with either repeater or the BTS. Typically, in situations where coverage is required without high traffic capacity, the repeater solution would probably be a better solution. Examples of application could be medium shopping malls and hotel lobbies. On the other hand, if the required coverage is large and the expected traffic is high, then the micro BTS connected to the RF distribution hub would be a more logical choice. Example of such applications would be airport terminals. Figure 3.16 depicts such an application.
Figure 16 Example Application of Distributed Optical Antennas.
10 Extension of Cell Coverage Area
The coverage area extension can be done with cellular repeaters and preamplifiers.
The cellular repeater amplifies the RF signal in both uplink and downlink directions, i.e. it is a device which compensates the propagation loss between the base station antenna and the mobile station antenna. The cellular repeater is connected between two antennas: the first antenna is pointed to the base station site and the second one (reradiating element) is pointed to the area to be covered.
Cellular repeaters are classified according to the total output power. Low power repeaters (total output power below 100 mW) can be used to extend the cellular coverage inside buildings and basements. High power repeaters (total output power above 1 W), can be used for example to extend the coverage area behind hills and inside long tunnels.
Radiating cable (leaky cable) can be used in tunnels as a reradiating element to provide homogeneous field strength inside the tunnel.
Mast Head Preamplifiers (MHPx) are installed at the base station antenna mast after the RX antenna to amplify the uplink signal. The preamplifier has a low noise figure and adjustable gain to compensate the RX antenna feeder attenuation. It can be very helpful when low-power handportables are used in the network.
10.1 Extended Cell (E-Cell)
Normally GSM Cell radius is max 35 km but with E-Cell concept it is possible to increase the cell radius to 70 km. Technically this is done with one BCCH and two- TRX solution. Whole cell is using same BCCH (N-TRX) on downlink direction but normal coverage area is served with different TRXs than extended (E-TRX) coverage area.
Timing of receiver of E-TRX is delayed so that it can serve area beyond 35 km. Timing of transmitters of both N-TRX and E-TRX is same. Time slot 0 of E-TRX is tuned to BCCH frequency in order to get random accesses from the extended area. Time slot 1 of N-TRX cannot be used because E-RACH is interfering it.
Handovers to e-cell and from e-cell are performed according to existing handover criterias. However in case of HO to e-cell the target area is predetermined. The BSC allocates recources according to specified adjacent cell parameter. Handovers between different areas of e-cell are performed according to timing advance information.
E-Cell solution is possible in 3rd and 4rd generation BTS.
11 Intelligent Underlay-Overlay
Intelligent underlay-overlay (IUO) is a solution to increase the intensity of the frequency reuse. The risen interference level caused by the tightened frequency reuse is managed by estimating the degree of interference. An intelligent handover control algorithm directs calls always to frequencies that are clean enough to sustain a good radio connection quality. The interference estimation is based on the downlink measurement results reported by mobile stations and various adjustable parameters.
The intelligent underlay-overlay is implemented solely in Base Station Controller (BSC) software and the introduction of IUO does not necessitate any changes in hardware configuration. The performance of the intelligent underlay-overlay procedure is measured by means of underlay-overlay statistics.
Figure 1. IUO Network configuration
The operating spectrum of the network is divided into regular frequencies and super-reuse frequencies. The regular layer or the overlay network uses regular frequencies to provide continuous radio coverage with overlapping cell areas required for safe handovers. The interference control mechanism is not applied in the regular layer and co-channel and adjacent channel interference should be considered in the frequency assignment.
The super layer or the underlay network uses super-reuse frequencies that are reused intensively to make the use of the available spectrum more efficient. The super-TRXs are configured exactly the same way as the regular-TRXs, using the same antennas and transmission power levels. Thus the coverage area of the super layer is the same as the coverage area of the regular layer. The suitable service area of the super layer is limited by the interference and the super-TRXs are intended to serve mobile stations that are close to the base station and other locations where the radio conditions are less vulnerable to the interference.
An ordinary IUO-cell has both types of TRXs, regular-TRXs and super-TRXs. The regular-TRXs allocated to the cell belong to a regular frequency group. A super-TRX allocated to the cell belongs to one of the sixteen super-reuse frequency groups. The super-TRXs allocated to the cell are divided into super-reuse groups according to the source of interference. Those super-TRXs that belong to the same super-reuse frequency group have the same sources of interference. The super-TRXs of the cell may belong to the same super-reuse frequency group or they may belong to different super-reuse frequency groups.
In the call set-up the traffic channel (TCH) is always allocated from a regular-TRX, because the C/I ratio for the call is not yet known. Thus the IUO-cell must have at least one regular-TRX, i.e. the BCCH-TRX. An exception is so called child cell, which will be explained soon.
The BSC monitors the downlink C/I ratio for every ongoing call. The call is always handed over from a regular-TRX to a super-TRX when the downlink C/I ratio of the super-TRX is good enough. If the C/I ratio of the super-TRX worsens below the acceptable value, the call is returned back to a regular-TRX. The handover between the layers is an intra-cell handover.
The C/I ratio is calculated by comparing the downlink signal level of the serving cell and the downlink signal level of those neighboring cells that use the same super-reuse frequencies as the serving cell.
A child cell is a microcell equipped with super-reuse frequencies. Unlike in ordinary IUO-cells, no regular-TRXs are needed in the child cells. This means that the call set-up is not allowed through the child cell and the cell must be set barred by means of a parameter. Because child cell is an independent cell, one of the super-TRXs is defined as BCCH-carrier, which is needed in every cell. The super-TRXs of the child cell may belong to the same super-reuse frequency group or they may belong to different super-reuse frequency groups.
The “umbrella” cell or an ordinary IUO-cell which serves the calls in case of bad C/I ratio in the child cell is called parent cell. The BSC monitors the downlink C/I ratio of a child cell for each ongoing call. The call is always handed over from a parent cell to a super-TRX of the child cell which is adjacent to the serving cell when the downlink C/I ratio of the super-TRX is good enough. If the C/I ratio of the super-TRX worsens below the acceptable value, the call is returned back to a regular-TRX in a parent cell adjacent to the child cell. The handover between the layers is an inter-cell handover.
The C/I ratio is calculated by comparing the downlink signal level of the serving cell and the downlink signal level of a reference cell having similar RF signal profile as the interfering cell. The use of reference cell is necessary because the interference measurement is impossible when the serving super-TRX and the BCCH-TRX of an interfering child cell are in the same channel. Thus, this may happen when the mobile station is in a child cell or in the super-TRX of an ordinary IUO-cell and there interfering child cells in the vicinity.
Figure 2. The measurement principle of the C/I ratio in IUO.
FREQUENCY PLANNING
1 Introduction
The main goal of the frequency planning task is to increase the efficiency of the spectrum usage, keeping the interference in the network below some predefined level. Therefore it is always related to interference predictions. There are two basic approaches to solve the frequency assignment problem:
* frequency reuse patterns
* automatic frequency alocation
Frequency allocation for the final network configuration is a demanding optimisation problem which requires efficient and systematic approach. NPS/X is utilising automatic frequency allocation algorithms for finding the optimum solutions. The frequency allocation is generally guided by the following information:
• channel requirement on per cell basis according to the capacity plan
• channel spacing limitations according to BTS specification
• quality of Service requirement which is converted to acceptable interference probability
• traffic density distribution over the service area
• performance of advanced system features (frequency hopping, IUO, etc.)
In practical cellular networks, the usage of regular reuse patterns, like 4x3, is seldom giving the best efficiency and quality in limited bandwidth operation. The NPS/X frequency allocation is based on cell-to-cell interference probability estimation according to the network topology, field strength distribution and traffic load. This results in customised frequency allocation patterns, providing the best spectrum utilisation efficiency with the given reference performance of the selected radio network elements.
Many articles and reports have been devoted to the subject showing the pros and the cons of the above approaches. We can find a lot of differences, but the most important is perhaps the time when the task is performed: all frequency reuse schemes are made before, while the heuristics are used after the real sites selection. Thus the main disadvantage of the first approach comes from its regularity: the terrain is flat, the sites are uniformly distributed, all the base stations in a given region have same parameters: powers, antennas, capacity, etc. With this initial assumptions the calculation of the expected interference is simple but unreliable.
The starting point of automatic frequency allocation is much better, since the exact site coordinates and BTS characteristics are available. Assuming usage of propagation model based on digital maps, we are able to obtain interference predictions very near to reality. However, the trade-off is frequency reuse patterns (if required) at the ND stage, and some frequency allocation when it comes to detailed network planning.
2 Interference Calculations
The reference interference ratio is defined in GSM Rec.05.05 as the interference ratio for which the required performance in terms of frame erasure, bit error rate or residual bit error rate is met. The reference interference ratios for BS and all types of MSs are the following:
* cochannel interference: C/Ic <= 9 dB
* first adjacent channel interference: C/Ia1 <= -9 dB
* second adjacent channel interference: C/Ia2 <= -41 dB
However, according to GSM Rec.03.30, the adjacent channel suppression for the first adjacent channel is large enough to allow usage of adjacent channels in adjacent cells. The second adjacent channel can be used in the same cell, due to the MS power control in the uplink and intracell hand-over. Using these simple constraints for adjacent channels assignment, we can skip the adjacent channel interference predictions.
2.1 Cochannel Interference
The carrier to interference (C/I) ratio at a given mobile receiver can be calculated as follows:
C/I = C/(I1+I2+...+IK)
where K is the number of cochannel interfering cells. For the regular grid case it is possible to simplify the calculations by using the popular path-loss expressions , see the picture.
The propagation loss slope b is different in different areas, but for the mobile systems it is normally between 3 and 5. The C/I can easily be expressed in terms of distances, for example in the hexagonal shaped cellular network with omni cells
C/I = R-b/(6*D-b)
is a feasible approximation. In this case R stands for the cell range and D is the frequency reuse distance.
If directional antennas are used, the number of principal interferers is two instead of six (omnidirectional case). The following estimation formula, as presented by Lee, calculates the C/I ratio in this case. The mobile station at position E experiences stronger interference from the BTS which is at distance of D from the mobile station.
C/I = R-b/((D+0.7R)-b + D-b)
Figure 2 Two interferers in the sectorized network (reuse factor 21).
2.2 Time Dispersion
Some interference effects may be caused from the reflected signals if received outside the equalizer window. This happens only when the difference between direct path and reflection path is larger than the equalizer window (about 4.5 km) and the reflected signal is strong enough. The reflection outside the equalizer window should be regarded as an independent cochannel interferer, therefore the same reference C/I <= 9 dB should be used.
2.3 Digital Maps Based Cochannel Interference
From the coverage areas calculated by the help of digital maps it is quite easy to obtain the expected interference areas. Since the frequency plan is still to be done, the multiple interference cannot be calculated. Thus the process works for every pair of BS, checking the ratio between the two signals pixel by pixel. The probability of future multiple interference can be reduced by adding some margin, say 6 dB to the reference interference ratio. If the percent of the interfered area is larger than a given predefined level (depending on the required service quality), the pair cannot operate in the same channel. The results are presented as a matrix with elements giving the minimum allowed channel difference (in this case only 0 and 1) for every pair of BSs.
3 Heuristic Algorithms
In general the Frequency Assignment Problem (FAP) for a cellular network can be formulated as follows:
Given N base stations having a requirement of Cn,n = 1, 2, ..., N channels each, total amount of M channels, and separation matrix. Let plan (frequency plan) be any distribution of frequencies, fulfilling exactly the requirements of BSs. Find a frequency plan conforming the constraints in the separation matrix by using minimum number of the available channels.
The problem is proven to be NP hard, meaning that the number of plans grows exponentially with the number of BSs, so heuristic algorithms should be used. The idea is to generate and check only subset of all possible plans, finding an acceptable rather than the optimal solution.
There exist many different approaches and algorithms, some of them implemented for computational use. However, we are going briefly to mention only the most popular.
Graph coloration approach is based on the analogy between FAP and graph coloration problem: To colour the vertices of a given graph with minimum number of colours without using the same colours for adjacent vertices. The vertices are taken to be BSs, the colours stand for channels (frequencies), while adjacent vertices mean that the corresponding pair of BSs cannot use the same channel.
Another possibility is to generate an acceptable plan by satisfying the requirements of only one BS at a time. Different rules can be applied to the decision making process: from "First BS - First Channels" to "Worst BS - Best Channels". The first rule does not require big computational effort, but very often it may not solve the problem. The second rule is usually based on time consuming investigations to offer a better choice of BS and channels. Some trade-offs are also possible.
All heuristics mentioned above are referred to as deterministic ones. There are some attempts to use non-deterministic approaches for FAP solving e.g. an analogy with the neural networks. The most important difference is that the separation matrix is not used anymore to allow or deny given channel difference for given pair of BSs. A so-called energy function is used instead, trying softly to minimize the "temperature" (interference probability) in the system.
4 Frequency Hopping
Frequency Hopping (FH) is changing the frequency of information signal according to a certain sequence. The transmission frequency may change at each time slot or burst and remains constant during the transmission of a burst.
FH can also decrease the overall C/I value in the network and thus improve the Quality Of Service (QOS).
4.1 Frequency Hopping Behaviour
As described earlier (coverage planning) the shadowing (log-normal fading) and Rayleigh- distributed fast fading can decrease the specch quality. Rayleigh fades are the sum of a lot of reflected and phaseshifted signals. These fades occuer at every half wavelength which is 204/6 ms duration at 3/100 km/h with 900 MHz.
The fading at different frequencies are not the same and become more and more independent when the difference in frequency increases. With frequencies spaced sufficiently apart (say 1 MHz), they can be considered completely independent (no correlation). Thus all the bursts containing the parts of one code word are then not damaged in the same way by Rayleigh fading.
When the MS moves of high speed, the difference between its positions during the reception of two successive bursts of the same channel (i.e., at least 4,615 ms) is sufficient to decorrelate Rayleigh fading variations on the signal. In this case FH does not help except if there is interference.
The worst case is when MS is stationary or moves at slow speeds because the interleaved coding does not bring any benefit to reception. In this case FH "simulates MS movement" and thus raises the reception quality. This phenomenon is called frequency diversity.
In the other hand frequency hopping averages the interference directed towards each base station. Instead of a continuous interferer there are several interferers that affect only a short time each and with different intensity. Methods like power control and DTX (Discontinuous transmission, see below) affect only a single interference source and the benefits can be distributed to the whole network by using FH. The gain which comes from interference averaging is called interference diversity.
The effective utilisation of frequency hopping is considered to give 6 dB gain. This is possible with min 4 carriers. With 3 carriers the estimated gain is 4 dB and with 2 carriers the gain can be only 2 dB. With interference limited network the gain can be 3 dB.
4.2 Baseband Hopping
Baseband hopping occurs between TRXs in BTS. The number of frequencies used in the hopping sequence is the same as the number of TRXs in the sector. Both random and cyclic hopping can be used.
The digital (baseband) and analogue (RF) parts of the TRX are separated from each other. The switching of TRXes is on a per timeslot basis and enables a particular TCH to hop from one carrier to another.
Figure. Baseband frequency hopping
4.3 Synthesized Hopping
Synthesized hopping is available in configurations which have at least 2 TRX per sector. It enables each TRX to change frequency on successive time slots, so that given carrier can hop quickly onto many different frequencies. The carrier on which the BCCH is transmitted must remain at fixed frequency to enable the MS to measure correct signal strength. Both random and cyclic hopping can be used.
Figure. Synthesised frequency hopping
4.4 Discontinuous Transmission (DTX)
The transmission is "disconnected" when no information flow happens in signal. This is done by lower speech encoding bit rate than when the user is effectively speaking. This low rate flow is sufficient to encode the background noise, which is generated for the listener to avoid him thinking that the connection is broken. The low rate endoding corresponds to a decreased effective radio transmission of one frame each 20 ms to one such frame each 480 ms. Typically transmission is effective 60 % of the time which decreases the interference.
In order to implement such a mechanism, the source must be able to indicate when transmission is required or not. In the case of speech, the coder must detect whether or not there is some vocal activity. This function is called Voice Activity Detection (VAD). At the reception side, the listener's ear must not be disturbed by the sudden disappearance of noise and the decoder must therefore be able to generate some "comfort noise" when no signal is received.
DTX is an option controlled by the operator, and which may be used independently in the MS to BTS and in BTS to MS.
PARAMETER PLANNING
1 GSM Radio Path
GSM is a digital system using Time Division Multiple Access (TDMA) frame structure. The TDMA frame has a duration of 4.615 ms and consists of 8 timeslots. There are two types of logical channels carried over the timeslots: Common Channels and Dedicated Channels.
1.1 Common Channels
The Common Channels are used for signaling and can be divided into Broadcast Channels (BCH), continuously sending information from BTS to MS, and Common Control Channels (CCCH).
The Broadcast Channels send information on the cell properties such as synchronization, frequency correction, used frequenciesand power levels, neighboring cells. There are three different broadcast channels: Synchronization Channel (SCH), Frequency Correction Channel (FCCH) and Broadcast Control Channel (BCCH).
The Common Control Channels are used when establishing a signaling connection between the MS and BTS. The Paging Channel (PCH) is used when BTS wants to contact the MS. The MS requests a signaling channel on a Random Access Channel (RACH). The signaling channel is allocated to the MS by using Access Grant Channel (AGCH).
1.2 Dedicated Channels
The Dedicated Channels are divided into Dedicated Control Channels and Traffic Channels. Call set-up signaling and location updating procedures are performed on Stand-alone Dedicated Control Channel (SDCCH). In case of a call set-up, the connection is transferred into a Traffic Channel (TCH).
Both SDCCH and TCH have a parallel Slow Associated Control Channel (SACCH) which is used for transfer of measurement results from MS to BTS and power control commands from BTS to MS. During the call the short messages are transmitted over SACCH channel, while the Fast Associated Control Channel (FACCH) is used to transmit the handover commands to the MS.
1.3 Usage of the Channels
The usage of channels may be described as follows:
* MS listens to the common channels
* BTS sends a paging command over PCH
* MS responds by sending a random access message over RACH
* BTS gives to MS the SDCCH number to be used for signaling over AGCH
* Call set-up signaling takes place on SDCCH
* BTS orders the traffic channel
* During the call SACCH and FACCH are used
1.4 Radio Path Measurements
The radio path measurements are used to keep the connection in good quality and therefore to trigger power changes and handovers if needed. Both MS and BTS measure signal level and quality (bit error ratio). In addition to that MS measures the signal levels of all adjacent BCCH frequencies even though it is able to report only six best measurements.
During one TDMA frame (4.615 ms) MS receives one signal sample from one BCCH frequency. Measurement data is averaged during SACCH multiframe period (480 ms) and the results are transmitted to the BTS in the next SACCH block. Since the MS receives about 102 field strength samples during one SACCH the number of samples to be averaged can easily be calculated. For example in case of 9 BCCH frequencies (9 cell reuse pattern) MS will have about 11 samples from each frequency while in case of 12 BCCH frequencies the number of the samples will be approximately 8.
2 Power Control and Handover
The BTS sends the raw measurement results received from the MS (downlink) and the results of its own measurements (uplink) to the BSC every SACCH multiframe period. The BSC does not support the measurement preprocessing in the BTS.
The BSC does the preprocessing of the measurement samples namely the bookkeeping and the averaging. The BSC is able to maintain a table of maximum 32 measurement results for up to 32 adjacent cells per call. After the averaging the BSC makes comparisons with the thresholds related to both Power Control (PC) and Hand-Over (HO) algorithms.
The BSC determines the RF output power of the MS and the BTS on the basis of the results received from the PC threshold comparison process.
The HO decision is based on signal strength (RXLEV), quality (RXQUAL) and distance measurements. Another possible criterion is the power budget (PBGT) or umbrella condition fulfillment from an adjacent cell. The HO command is given over FACCH, which uses TCH temporarily. Handovers can be done to TCH and SDCCH. The Intra-BTS handover can occur either to a timeslot on a new carrier or to a different timeslot on the same carrier. The intra-BSC handover is performed autonomously by the BSC. If there is an inter-BSC handover to be performed, the BSC sends the list of preferred cells to the MSC and MSC performs the handover according to that list.
2.1 Power Control Strategy and Parameters
The RF power control strategy employed by the BSC determines the RF power level that is signaled to the MS and the RF power levelused by the BTS. The RF power control process optimizes the RF power output from the MS and the BTS at the same time ensuring that the power levels are sufficient to keep adequate speech quality.
The RF power level (MS or BTS) is controlled by means of received level (RXLEV) and quality (RXQUAL). Both RXLEV and RXQUAL are considered separately for uplink (UL) and downlink (DL) and have several associated parameters:
* averaging (sliding) window size and weighting
* Px and Nx (Px averages out of Nx to fulfill the criteria)
* upper and lower thresholds
The averaging window size determines the number of the consecutive measurement samples to be taken into account in the averaging process. The averaging window acts as a sliding window since after each sample an average value is calculated. The number of the averages to be below/above the corresponding threshold is selected with the pair Nx and Px. The meaning is that at least Px out of Nx averages must fulfill the criterion. When the level and the quality are between the lower and upper thresholds the power is not regulated
All parameters controlling the power control process are defined on a cell by cell basis by means of O&M. By changing the values of the parameters it is possible to affect the RF power control in all stages of the process.
2.2 Handover Strategies and Parameters
The HO decision process may be triggered in different situations. Similarly to the PC it is controlled by the level (RXLEV) and quality (RXQUAL) in both UL and DL. In addition to these it depends on the distance and some periodic checks (PBGT, UMBRELLA). Only one type of periodic checks can be used per cell. The main principle when making HO caused by radio criteria is that the new server should be better than the current one.
The parameters, averaging and threshold comparison for level, quality and distance are similar to PC but only one threshold associated. The periodic checks occur every power budget (hoPeriodPBGT) or umbrella (hoPeriodUmbrella) period. In order to be performed the periodic checks require some data for the neighboring cells: the comparison process uses the calculated PBGT(n) or AV_RXLEV_NCELL(n) for the neighboring cells instead of fixed thresholds.
Like with the PC it is possible by changing the HO related cell parameters to affect the HO algorithm at all stages: preprocessing, threshold comparison, decision making.
2.3 HO and PC Interaction
Power control and handover processes run independently in parallel, but the PC is aware of the status of HO and vice versa. The BSC therefore does not try to adjust the power levels and perform a handover simultaneously. In the HO algorithm the current power levels of the BTS and the MS are taken into account in order to recognize the state when the PC can no longer maintain the call quality.
With a proper choice of the PC and HO parameters the BSC will keep the call quality by means of PC and will propose HO only when the MS actually reaches the border of the serving cell. If both HO and PC threshold conditions are fulfilled the HO has greater priority than the PC. In case of HO failure power increase can be used.
The priority order is as follows:
* HO quality uplink/downlink
* HO level uplink/downlink
* Distance
* Better cell (periodic checks for PBGT or Umbrella)
* PC Lower quality/level thresholds uplink/downlink
* PC Upper quality/level thresholds uplink/downlink
2.4 BSS Parameters
In the following figures some of the radio parameters are shown.It’s not the scope of this course to show all the parameters, but someexaamples of general parameters on how the structure of the network is defined (location area codes and BS identification), TDMA frame structure handover and power control.
NETWORK VERIFICATION AND OPTIMIZATION
This is the last step of the network planning procedure. It can start during the network trial period and continues after opening the commercial service and during the network expansion.
The aim of this process is to evaluate and maximize the quality of service in the network with the corresponding set of quality criteria.
1 Network Verification
The purpose of the Network Verification (NV) is to evaluate an independent and objective Quality Of Service (QOS) inside a given service area. This is done with Network Measurement System (NMS/X). Some OMC traffic measurements are done in parallel to provide a statistical data and to complete the network picture.
The network verification procedure consists of the following steps:
1. Planning of the measurement resources (including tools), reference network,
schedule, and test route(s)
2. Setting of the network performance objectives and quality criteria
3. Measurement execution and analysis of the statistical results
4. Reporting to the customer the results of analysis
5. Agreement on possible corrective actions if the set quality criteria is not met
The field verification takes place after successful completion of Site Acceptance. It should be repeated before and after any major network hardware/software changes to verify their affect on the network quality.
The service area, or the part of the network to be verified, is defined as a group of cells giving continuous coverage. It is always connected with a selection of Test Routes, since all verification and optimization activities are based on recurrent measurements over the same routes.
1.1 Network Quality Criteria
The quality objectives are specified according to the capacity requirements and customer's QOS strategy which are agreed with the customer.
Some examples are listed here.
Number of successful mobile originating (MO) call attempts
with normal cell releases: 90-95 %
Minimum overall downlink quality: value 0-5
in 90-95 %
Minimum overall downlink level: >37dBuV/m
on street level for 2 W mobile in 90-95 %
Minimum successful rate for hanovers (HO): 90-95 %
Maximum system response time: 0-7 s
(Time for TCH assigment) in 90-95 %
One basic requirement for Network Quality Survey is to monitor continuously QOS and NP behaviour, compare the network against other similar networks and present the results in such a way, that they are easy to understand for non technical parts of the operator’s organization as well. To be capable of doing that requires additional metrics that are sensitive enough and very easy to understand.
For this purpose there are two Nokia specific metrics. NQ&E (Network Quality and Efficiency) factor driven from NMS/X tries to capture QOS and NQ&E driven from OMC tries to capture NP in one figure. Both metrics are working in 0 to 100 scale.
1.2 Measurement Procedure
The field test measurements are made with Nokia's NMS/X Network Measurement System or a compatible system which contains a GSM/DCS-1800 mobile as measuring equipment and a connection from this mobile to a microcomputer. Optionally there can be integrated navigation equipment, e.g. GPS, for the collection of location information for network optimization purposes.
The test vehicle is driven along the pre-defined test routes while the NMS/X will generate mobile originating (MO) calls to another mobile or a terminal in the PSTN. A call that is successfully established is held for two minutes and then cleared. During the call the down link speech quality, serving cell down link level, and mobile output power level are measured. Statistical information of the previous measurements, system response times, and hand-overs is collected. The number of call attempts and the ratio between normal call releases and failures from all call attempts are recorded. The recommended minimum number of total call attempts is 500 to get statistically reliable results.
To be able to verify the effect of hardware/software changes of network elements on the network quality, the quality measures are performed by using the same reference network (and the same test routes).
During the measuring phase no hardware/software of network elements affecting the measurement results may be modified.
All the changes to the reference network are done in a controlled manner by Nokia together with the customer.
1.3 Analysis of the Results
The main result of the verification procedure is the statistical Quality Sheet generated by the NPS/X or similar system.
In addition to the statistical Quality Sheet, the network element availability statistics from the OMC also form part of the network verification results. These results are collected either on a monthly or weekly basis, and they concern the network elements as a whole or by unit basis. The statistics of unit restarts and the availability of transceiver units are reported.
The congestion level of each BTS is obtained from the OMC traffic statistics, and reported together with other Network Verification results as the Network Verification Report.
2 Network Optimization
Network Optimisation can be defined as a continuous process of improving overall network quality. Looking at network quality two different views should be considered. The customers (subscribers) view and the more comprehensive operators view. Figure 1. ‘Overall Network Quality’ is illustrating this.
Figure 1. Overall Network Quality.
Usually a subscriber is not interested in site leasing or maintenance costs. As long as his service is not affected things like spectrum efficiency and network traffic are of no interest to him. For the operator these figures are of fundamental importance.
Network Optimisation service and more general the Nokia Quality Cycle service package are designed to support the operator in the most efficient way to improve all different aspects of network quality. Nokia’s tools, experts with detailed system knowledge and the global network of experience provide the operator with the most sophisticated services.
2.1 BSS Default Parameter Assessment
Proper BSS default parameter settings are needed to ensure the best possible performance of the network. The parameter sets are based on experience from optimised networks.
2.2 Small Quality Cycle
The ‘Small Quality Cycle’ is, as the name indicates, the smallest possible quality monitoring and improving process. It is the combination of the Basic Configuration Analysis Module and the Basic Network Optimisation Module which are described in the following two chapters. Figure 2 ‘Small Quality Cycle’ may illustrate the relation between the different service modules.
Figure 2. Small Quality Cycle.
2.2.1 Basic Configuration Analysis Module
Network Configuration Analysis is the smallest possible service module of Network Optimisation. With this service the system configuration as existing in the real network (System Configuration ‘Network’) is compared against the system configuration as provided by network planning (System Configuration ‘NW Planning’). This task ensures consistency between different system configuration databases and therefore is the basis of all following tasks. Basic Configuration Analysis should be repeated on a regular basis. Nokia can support with improving or designing and implementing procedures for regular consistency checks.
2.2.2 Basic Network Optimisation Module
Field tests, OMC measurements and customer complaints are the three main sources to provide a detailed network quality picture (e.g. call and handover success rates, problems reported by customers and field test personnel). The network performance data analysis together with single quality improvement actions raise network quality on a case by case basis. These tasks are combined in the Basic Network Optimisation Module.
2.3 Full Optimisation Service
As Network Optimisation has to be seen as part of a bigger process which is embedded in the operators organisation Nokia is offering services and consultancy in the area of quality definition, monitoring and improvement. This service is called ‘Full Optimisation Service’ and contains the ‘Small Quality Cycle’ as described in the previous chapter (chapter 2.2). Figure 3 ‘Network Quality Cycle’ is showing the relation between the ‘Small Quality Cycle’ and the bigger ‘Network Quality Cycle’.
Figure 3. Network Quality Cycle.
As the ‘Small Quality Cycle’ is required to monitor and improve quality on a more or less ‘problem-by-problem’ basis a more global approach of quality monitoring and improvement is required.
Global quality reporting on regional and network level is required for management, marketing and network planning. Global quality monitoring is similar to quality monitoring as required for Basic Network Optimisation, but it is reporting on a more general level (field tests, OMC and customer complaints). Global quality reporting allows to monitor the impact of major changes in the network (e.g. frequency changes, BSS default parameter changes or massive traffic increase).
Quality definition and quality target setting are needed for agreements between different departments (e.g. planning and marketing). As a conclusion from global quality reporting and experience from the ‘Small Quality Cycle’ work general improvement actions (e.g. testing new system features) and general corrective actions (e.g. major frequency change, capacity extension, introduction of microcells or IUO) might follow.
In all phases of the Network Quality Cycle Nokia is supporting with consultancy, tools and training.
2.4 Optimisation tools
Nokia is providing a variety of tools for network optimisation. NMS/X, Nokia’s Network Measurement System for GSM/DCS and NMT network quality survey, network tracing and for multichannel field strength measurements covers all demands for field measurements. A portable version allows indoor measurements.
NPS/X, Nokia’s Network Planning System, is also a powerful tool for network optimisation. It’s features for down- and uploading data from the OMC, the GSM/DCS simulator, the link to the measurement system NMS/X and many other features provide the optimisation personnel with an advanced tool.
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