Download - GSM Introduction

04/07/23Tempus Telcosys 1

ADA CELLWORKS PVT LTD

INTRODUCTION

The global system for mobile communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN). These recommendations ensure the compatibility of equipment from different GSM manufacturers, and interconnectivity between different administrations, including operations across international boundaries.


THE GSM NETWORK The GSM network is comprised of the following

components: Network Elements The GSM network incorporates a number of network

elements to support mobile equipment. They are listed and described in the GSM network elements section of this chapter.

GSM subsystems In addition, the network includes subsystems that are not

formally recognized as network elements but are necessary for network operation. These are described in the GSM subsystems (non-network elements) section of this chapter.

Standardized Interfaces GSM specifies standards for interfaces between network

elements, which ensure the connectivity of GSM equipment from different manufacturers. These are listed in the Standardized interfaces section of this chapter.


THE GSM NETWORK - CONTINUED

Network Protocols For most of the network communications on these

interfaces, internationally recognized communications protocols have been used

These are identified in the Network protocols section of this chapter.

GSM Frequencies The frequency allocations for GSM 900, Extended GSM

and Digital Communications Systems are identified in the GSM frequencies section of this chapter.


DIGITAL NETWORKS

GSM networks are digital and can cater for high system capacities. They are consistent with the world wide digitization of the telephone network, and are an extension of the Integrated Services Digital Network (ISDN), using a digital radio interface between the cellular network and the mobile subscriber equipment.


INCREASED CAPACITY

The GSM system provides a greater subscriber capacity than analogue systems. GSM allows 25 kHz. Per user, that is, eight conversations per 200kHz. Channel pair (a pair comprising one transmit channel and one receive channel). Digital channel coding and the modulation used makes the signal resistant to interference from the cells where the same frequencies are re-used (co-channel interference); a Carrier to Interference Ratio (C/I) level of 9 dB is achieved, as opposed to the 18 dB typical with analogue cellular. This allows increased geographic reuse by permitting a reduction in the number of cells in the reuse pattern. Since this number is directly controlled by the amount of interference, the radio transmission design can deliver acceptable performance.


CGI : CELL GLOBAL IDENTITY


MCC MNC LAC CI

LAI

CGI

MCC = Mobile Country CodeMNC = Mobile Network CodeLAC = Location Area CodeCI = Cell Identity

MSISDN


CC NDC SN

98 XXX 12345

CC = Country CodeNDC = National Destination CodeSN = Subscriber Number

MSISDN

The Mobile Subscriber ISDN (MSISDN) number is the telephone number of the MS. This is the number a calling party dials to reach the subscriber. It is used by the land network to route calls towards the MSC.


IMSI

IMSI (International Mobile Subscriber Identity) Network Identity Unique To A Sim.


MCC MNC MSIN

404 XX 12345..10

SIM = Subscriber Identity ModuleMCC = Mobile Country CodeMNC = Mobile Network CodeMSIN = Mobile Subscriber Identity Number

IMEI

IMEI : Serial number unique to each mobile


TAC FAC SNR SP

6 2 6 1

IMEI = International Mobile Equipment IdentityTAC = Type Approval CodeFAC = Final Assembly CodeSNR = Serial NumberSP = Spare

SUBSCRIBER IDENTIFICATION International Mobile Subscriber Identity (IMSI) Just the IMEI identifies the mobile equipment, other

numbers are used to identify the mobile subscriber. Different subscriber identities are used in different phases of call setup. The International Mobile Subscriber Identity (IMSI) is the primary identity of the subscriber within the mobile network and is permanently assigned to that subscriber.

Temporary Mobile Subscriber Identity (TMSI) The GSM system can also assign a Temporary Mobile

Subscriber Identity (TMSI). After the subscriber’s IMSI has been initialized on the system, the TMSI can be used for sending backward and forward across the network to identify the subscriber. The system automatically changes the TMSI at regular intervals, thus protecting the subscriber from being identified by someone attempting to monitor the radio channels. The TMSI is a local number and is always transmitted with the Local numbers and is always transmitted with the Location Area Identification (LAI) to avoid ambiguities.


SUBSCRIBER IDENTIFICATION MODULE (SIM) By making a distinction between the subscriber

identity and the mobile equipment identity, a GSM PLMN can route calls and perform billing based on the identity of the subscriber rather than the mobile equipment being used. This can be done using a removable Subscriber Information Module (SIM). A ”smart card” is one possible implementation of a SIM module.

IMSI. This is transmitted at initialization of the mobile equipment.

TMSI This is updated periodically by the PLMN MSISDN This is made up of a country code, a national

code and a subscriber number. Location Area Identity (LAI) This identified the

current location of the subscriber. Subscriber Authentication Key (KI) This is used to

authenticate the SIM.04/07/23Tempus Telcosys 13

EQUIPMENT IDENTITY NUMBER

International Mobile station Equipment Identity (IMEI)

Each MS is identified by an International Mobile station Equipment Identity (IMEI) number which is permanently stored in the mobile equipment. On request, the MS sends this number over the signalling channel to the MSC. The IMEI can be used to identify MS,s that are reported stolen or operating incorrectly.

Equipment Identity Register ( EIR ) A listing of the allowed IMEI is maintained by the

PLMN’s in the Equipment Identity Register (EIR) to validate the mobile equipment.


Frequency Bands

Uplink 890 – 915 MHz 25 MHz


Downlink 935 – 960 MHz 25 MHz

100 KHz 200 KHz 100 KHz

1 43 1242 …………….

A 200 KHz carrier spacing has been chosen. Excluding 2x100 KHz edges of

the band, this gives 124 possible carriers for the uplink and downlink. The

use of carrier 1 and 124 are optional for operators.

GSM Network Architecture


BTS

BTS

BTS

BTS

BTS

BSC

BSC

TRAU

MSC

HLRAUC

VLR

EIR

PSTN

SMSC

MS – Mobile Station

Mobile station provides user access to GSM network for voice and data

All GSM mobiles comply to GSM standards Subscriber data is read from a SIM card

that plugs into ME


SIM ME

MS

MS (cont..) Each MS has a unique number called

as IMEI number, which is stored in EIR for authentication purposes

Mobile camps on to the GSM network through the BTS serving the cell

Mobile also scans neighboring cells and reports signal strengths

Mobile transmits and receives voice at 13 kb/s over the air interface


Mobile Station Output Power

CLASS 1 20 watts Vehicle and Portable CLASS 2 8 watts Portable and Vehicle CLASS 3 5 watts Hand-Held CLASS 4 2 watts Hand-Held (GSM) CLASS 5 0.8 watts Hand-Held (DCS

1800) Output power determines:

Accessibility in areas of coverage Talk Time and Standby time


Mobile Station Identities

CC – Country Code NDC – National Destination Code SN – Serial Number


MSISDN : Mobile Station ISDN Number

It is the human identity used to call a Mobile Station

CC SNNDC MSISDN98 250 00134

IMSI (International Mobile Subscriber Identity)

MCC – Mobile Country Code MNC – Mobile Network Code MSIN – Mobile Subscriber Identity

Number 04/07/23Tempus Telcosys 21

MCC MSINMNC IMSI

3 2 or 3

Not more than 15NMSI

IMEI (International Mobile Equipment Identity)

TAC – Type Approval Code FAC – Factory Assembly Code SNR – Serial Number SP – Spare digit (usually used to specify

software version)04/07/23Tempus Telcosys 22

TAC SPFAC IMEISNR6 162 15

SIM ( Subscriber Identity Module)

Removable module inserted when the subscriber wants to use the ME

Two sizes: credit card size and stamp size

SIM features and contents are personalized by the Service Activator

ROM – 6kb to 16 kb RAM – 128 bytes to 256 bytes EEPROM – 3kb to 8 kb 04/07/23Tempus Telcosys 23

Space to insert SIM photo

Contents of SIM

Serial Number IMSI, Subscriber Key Ki, Ciphering Key Kc Algorithms for authentication and

ciphering Network Code PIN, PUK Charging Information Abbreviated Dialling Supplementary Features (e.g. Call

barring) 04/07/23Tempus Telcosys 24

SIM Security Two level protection When mobile is turned on, it will ask

for user to enter PIN (Personal Id Number)

3 tries for PIN, after that PIN locked To unblock PIN, there is PUK (Pin

Unblock Key) 10 attempts of PUK allowed After that SIM is blocked


BTS (Base Transceiver Station) BTS has a set of Transceivers (TRXs) to

communicate with mobiles in its area One BTS covers one or more than one cell The capacity of a cell depends on number of

transceivers in the cell BTS is connected to the BSC through Abis

Interface which is 2Mbps BTS transmits and receives voice at 13kbps

over air interface to the mobiles. BTS commands mobiles to set Tx. Power,

timing advance and Handovers04/07/23Tempus Telcosys 26

BTS


BSC – Base Station Controller Several BTSs are connected to the BSC BSC Manages channel allocation,

handovers and release of channels at connected BTSs

BSC connects to the BTS via the Abis interface and to the MSC on A interface

BSC has the entire database of cell parameters associated with the BTSs.

No mobile data is stored in the BSC Less connections for MSC as intelligence

is made common to all BTSs by the BSC04/07/23Tempus Telcosys 28

BSC


TRAU – Transcoder Rate Adaptation Unit


BTS

BSC PSTN

13 kbps 16 kbps 16 kbps 64 kbps

MSC and TRAU

TRAU (cont..) The MSC is based on ISDN switching.

The Fixed Network is also ISDN based.

ISDN has speech rate of 64 kbps. Mobile communicates at 13 kbps.

TRAU converts the data rates between 13kbps GSM rate to 64kbps Standard ISDN rate

TRAU can be collocated with the BTS, BSC or MSC or it can be a separate unit. 04/07/23Tempus Telcosys 31

Location of Transcoder

Collocated with MSC, BSC, BTS Separate Unit


MSC Transcoder BSC

MSC – Mobile Switching Centre


BSC

BSC

BSCBTSs PSTN

HLR

VLR

MSC (cont..) Exchange where calls are established,

maintained and released Database for all subscribers and their

associated features. Communicates with the BSCs on the A

interface and with PSTN on fixed line. MSC is weighted on the number of

subscribers it can support. E.g. an MSC of 1 lac subscribers means one MSC is enough till subscriber base increases upto 1 lac, beyond which another MSC is required.


Multiple MSCs When there is more capacity, there are

more than one MSCs. All MSCs have to communicate with one

another and to the outside world. Very complicated to connect each MSC to

each other and each MSC to PSTN So there is a concept of GMSC (Gateway

MSC)


BSC

BSC

MSC

MSC

GMSC PSTN

HLR – Home Location Register

MSC has all subscriber database stored in HLR

HLR has all permanent subscriber database

HLR has a database which describes the subscriber’s profile i.e. basic features and supplementary services

MSC communicates with the HLR to get data for subscribers on call04/07/23Tempus Telcosys 36

VLR – Visiting Location Register A subscription when activated is

registered in VLR VLR has all the subscriber numbers

which are active. VLR has a temporary database of all

active subscribers (on/off, location information)

04/07/23Tempus Telcosys 37MSC VLR

HLR

VLR (cont..)

MSC communicates with HLR for subscribers coming from different MSCs. If the subscriber is found valid, then it registers the subscriber in the VLR


MSC MSCVLR

HLR

VLR

AUC – Authentication Centre Authentication is a process by which a

SIM is verified Secret data and the verification process

algorithm are stored in AUC AUC is the element which carries out

the verification of the SIM AUC is associated with the HLR


MS MSC HLR AUC

EIR (Equipment Identity Register)

EIR is the Mobile Equipment Database which has a series of IMEIs

MSC asks the Mobile to send its IMEI MSC then checks the validity of IMEI

with the EIR All IMEIs are stored in EIR with

relevant classifications


EIRMSC

Classification of IMEIs


White list: This contains the IMEI of type approved mobiles

Black List: List of IMEIs which should be barred because either they are stolen or are not functioning properly

Grey list: List of IMEIs which are to be evaluated before they are put in black list

Billing Centre (BC)

BC Generates the billing statement for each subscriber

BC may be directly connected to the MSC or through a mediation device

MSC sends CDRs (Call Detail Records) to the BC

According to the template of pulse rates and units set, BC creates a bill according to the destination called and the call duration


Billing Centre (BC) (cont..)


CDRs

Templates for unit costs

OMC – Operations and Maintenance Centre

Also called the NOC (Network Operations centre)

It is the central monitoring and remote maintenance centre for all network elements

OMC has links to BSCs and MSCs


OMC


OMC System

BSC

BSC

BSC

BTSs

BTSs

BTSs

OMC Terminals


GSM Channels

GSM Channels

Physical Channel One time slot on one carrier is called

physical channel. Logical Channel

Information carried by physical channels is called logical Channels.

Logical channels are mapped on physical channels.


Logical Channels

Traffic channels: Used for speech and data Full Rate(TCH/F) Half Rate(TCH/H)

Control channels: Used for signaling .i.e. setting up a radio connection, call or controlling an MS during conversation BCH(Broadcast channels) CCCH(common control channels) DCCH(dedicated control channels)


Traffic Channels(TCH)


TCH/F(full Rate)

TCH/H(half Rate)

Traffic Channels(TCH)

Control Channels(CCH)


CCH(Control Channel)

BCH CCCH DCCH

CCH RACH CBCH SDCCH ACCHSynch.Chanels

SACCHFACCHPCH/AGCHFCCHSCH

BCH(Broadcast Channels)

BCCH(Broadcast Control Channels) Downlink Only. Broadcast information of the serving

cell (System Information). Transmitted on timeslot zero of BCCH

carrier. Reads only by idle mobile at least once

every 30 secs.


BCH(Broadcast Channels) cont’d SCH(Synchronisation Channels)

Downlink Only Carries information for frame

synchronisation. Contains frame number and BSIC(Base

Station Identity Code).


BCH(Broadcast Channels) cont’d FCCH(Frequency Correction

Channels) Downlink Only. Enable MS to synchronies to the

frequency.


CCCH(Common Control Channel) RACH(Random Access Channel)

Uplink only. Used by the MS when making its first

access to the Network. The reason for access could be initiation

of a call or a page response.


CCCH(Common Control Channel) cont’d AGCH(Assess Grant Channel)

Downlink only. Used for acknowledgement of the access

attempt sent on RACH. Used by the network to assign a

signaling cannel upon successful decoding of access bursts.


CCCH(Common Control Channel) cont’d PCH(Paging Channel)

Downlink only. The network will page the MS ,if there is

a incoming call or a short Message. It contains the MS identity number, the

IMSI or TMSI.


DCCH(Dedicated Control Channel) SDCCH (Stand-alone Dedicated

Control Channel) Uplink and Downlink. Used for call setup, authentication,

ciphering location update and SMS.


DCCH(Dedicated Control Channel) cont’d SACCH(Slow Associated Control

Channel) Downlink and Uplink. Used to transfer signal while MS have

ongoing conversation on traffic or while SDCCH is being used.

On the forward link, the SACCH is used to send slow but regularly changing control information to each mobile on that ARFCN, such as power control instructions and specific timing advance instructions


SACCH(Slow Associated Control Channel) cont’d The reverse SACCH carries information

about the received signal strength and quality of the TCH, as well as BCH measurement results from neighboring cells.


DCCH(Dedicated Control Channel) cont’d FACCH(Fast Associated Control

Channel) Downlink and uplink. Associate with TCH only. It is used to send fast message like hand

over message. Work by stealing traffic bursts.


Mapping on Physical Channels The Logical channels are mapped on

the physical channels. The TDMA frames are grouped

together into multi-frame. 26 TDMA multi-frame for Traffic. 51 TDMA multi-frame for control signal.


Channel Combination

Combined All the controlling signals are in the time

slot 0 of the Multi-frame. Non Combined

Dedicated controlling signals are in time slot 1 of the Multi-frame.


Combined

Cell with single carrier. Timeslot 0 :BCCH+CCCH+SDCCH. Timeslot 1-7 :TCH/FACCH+SACCH.


Non Combined

Cell with Two carrier Timeslot 0 (of carrier 1) BCCH+CCCH. Timeslot 1 (of carrier1) SDCCH+SACCH. Timeslot 2-7 & 0-7(of both carriers)

TCH/FACCH+SACCH.


BROADCAST MESSAGES

System information 5 and 6 sent on the SACCH immediately after Handover or whenever nothing else is being sent.

Downlink SACCH is used for system information messages while uplink SACCH is used for measurement reports.

System Information types 7 and 8 (optional) are an extension to type 4 and broadcast on the BCCH.


SYSTEM INFORMATION


SYSTEM INFORMATION 1

When frequency hopping is used in cell MS needs to know which frequency band to use and what frequency within the band it should use in hopping algorithm.

Cell channel descriptionCell Allocation Number(CANO)-Informs the band number of the frequency channels used.

00-Band 0(current GSM band)Cell Allocation ARFCN(CA ARFCN):- ARFCN’s used for hopping.It is coded in a bitmap of 124 bits.


SYTEM INFORMATION 1


124 123 122 121

024 023 022 021 020 019 018 017

016 015 014 013 012 011 010 009

008 007 006 005 004 003 002 001


RACH Control Parameters Access Control Class(ACC) :-Bitmap with 16 bits. All MS spread out on class 0 –9 . Priority groups use

class 11-15. A bit set to 1 barred access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not.0 – All MS can make emergency call. 1 - MS with class 11-15 only can make emergency calls.

Cell barred for access(CB):- 0- Yes 1- No



RACH Control Parameters Re-establishment allowed(RE):-0- Yes1- No Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7].Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system. Emergency call allowed:- Yes/No.



System Information Type 2 message consists of the Double BA list which defines the BCCH frequencies used in the neighboring cells.

The Double BA list provides the MS with different frequencies on which to measure, depending on whether the MS is in idle or active mode.

In active mode, the MS should measure on a reduced number of frequencies in order to improve the accuracy of measurements.



In Idle mode,the MS should measure on larger number of frequencies, so that the time required for the MS to access the network after power on is reduced.

The MS is also informed which PLMN’s it may use.

As well as System Information Type 2,it is also possible to have System Information Type 2 Bis and System information Type 2 Ater, depending on the size of the BA List.

System Information Type 2 Bis/Ter are optional.



Neighbor Cell Description:- BA Indicator(BA IND):- Allows to differentiate measurement results related to different list of BCCH frequencies sent to MS.BCCH Allocation number(BANO):- Band 0 is used.

PLMN Permitted(NCCPERM):-This the PLMN color codes permitted and tells the MS which network color codes(NCC) on the BCCH carriers it is allowed to monitor when it is in this cell..



RACH Control Parameters Access Control Class(ACC) :-Bitmap with 16 bits. All

MS spread out on class 0 –9 . Priority groups use class 11-15. A bit set to 1 barred

access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not.0 – All MS can make emergency call. 1 - MS with class 11-15 only can make emergency calls.

Cell barred for access(CB):- 0- Yes 1- No



Re-establishment allowed(RE):-0- Yes1- No Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7].Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system.Emergency call allowed:- Yes/No.



BCCH ARFCN Number(BAIND):- ARFCN’s used for in a Bitmap of 124 bits


124 123 122 121

024 023 022 021 020 019 018 017

016 015 014 013 012 011 010 009

008 007 006 005 004 003 002 001


The System Information Type 3 contains information on the identity of the current LA and cell identity, because a change means that the MS must update the network.

System Information 3 also as Control Channel Description parameters used to calculate the Paging group.

When the MS is in idle mode it decides which cells to lock to. Information needed by the MS for cell selection is also broadcast in the Type 3 information.



8 7 6 5 4 3 2 1

1 1 1 1

LAC

LOCATION AREA IDENTITTY(LAI)

MCC DIG 1MCC DIG 2MCC DIG 1MNC DIG 1MNC DIG 2

CICI

CELL IDENTITY

LAC



Control Channel DescriptionAttach / Detach(ATT):-

0 = Allowed 1 = Not Allowed

bs_agblk:-Number of block reserved for AGCH [0-7] Ba_pmfrms:-Number of 51 frame multi-frames between transmission of paging messages to MS of the same group

T3212:- Periodic location update timer . [1-255 deci hours].



cch_conf Physical channels combined No. of CCH0 1 timeslot(0) No 91 1 timeslot(0) Yes 32 2 timeslot(0,2) No 184 3 timeslot(0,2,4) No 276 4 timeslot(0,2,4,6) No 36



Cell optionsDTX:-Whether Discontinuous

Transmission used or not.PWRC:-Power control on the

downlink. 0 = Not used.1 =

Used.Radio link timeout(RLINKT):-Radio link time-out is the time before an MS disconnects due to failure in decoding SACCH message. Sets the timer T100 in the MS.



Cell Selection ParametersRxlev_access_min:-

Minimum received signal level at the MS for which it is permitted to access the system.

0-63 = -100 dBm to –47 dBm.Mx_txpwr_cch:- Maximum power

the MS will use when accessing the system.Cell_reselect_hysteresis:- Used for

cell reselection. RACH Control Parameters.



Location Area Identification. Cell Selection Parameters

Rxlev_access_min:- Minimum received signal level at the MS for which it is permitted to access the system.

0-63 = -100 dBm to –47 dBm.Mx_txpwr_cch:- Maximum power

the MS will use when accessing the system.Cell_reselect_hysteresis:- Used for

cell reselection.



RACH Control Parametersmax_retransmissions(MAXRET)

tx_integer(TX)Cell barred for

access(CB). Re-establishment allowed(RE)

Emergency Call AllowedAccess Control Class (ACC)



CBCH Description(Optional) :CHN:- This is the channel number for CBCH. It is controlled internally in BSC.TSC:- Training Sequence Code. Base Station Color Code(BCC) part of BSIC is used.

CBCHNO:- Absolute RF channel number of CBCH.MAC:- Mobile Allocation in the cell, describes the frequencies to be used in the hopping sequence if frequency hopping is used.



Hopping Channel(H):-Informs if CBCH Channel is hopping or single.ARFCN:- If H=0; MAIO:- If H=1, informs the MS where to start hopping. Values [0-63]. HSN:- If H=1, informs the MS in what order the hopping should take place. Values[0 –63]. HSN=0 Cyclic Hopping. MA:-Indicates which RF Channels are used for hopping. ARFCN numbers coded in bitmap.



Sent on the SACCH on the downlink to the MS in dedicated mode.

On SAACH, the MS also receives information about the BCCH carrier in each neighboring cell. This may differ from those sent in System information type 2.

It is also possible to have system Information Type 5 Bis and System Information Type 5Ter, depending on the size of the BA list.



Neighbor Cell Description:-

BA-IND:-Used by the Network to discriminate measurements results related to different lists of BCCH carriers sent by the MS(Type 2 or 5).

Values 0 or 1(different from type 2).

BCCH Allocation number:-00-Band 0(current GSM band).



BCCH ARFCN:-Neighboring cells ARFCN’s. Sent as a bitmap.

0-Not used1-Used.

124 123 122 121

024 023 022 021 020 019 018 017

016 015 014 013 012 011 010 009

008 007 006 005 004 003 002 001



Ms in dedicated mode needs to know if the LA has changed.If so, it must perform location updating when the call is released.

MS may change between cells with different Radio link timeout and DTX.

Cell Identity. Location Area Identification. PLMN permitted.



Cell options:DTX

PWRCRadio Link timeout.


SYSTEM INFORMATION 7/8

System Information Types 7 and 8 contain Cell Reselect parameters. Their function is to supplement System Information Type 4.


GSM Interfaces

(Um) Air interface - MS to BTS A bis interface - BTS to BSC A Interface - BSC to MSC B Interface - MSC to VLR C interface - MSC to HLR


MSC

BSC

VLRHLR

AUC

EIR

GMSC

MS

A Interface

A bis Interface

Air Interface

B Interface C Interface

F Interface

D Interface H Interface

To otherNetworks


GSM Interfaces

The interfaces between MSC and MS is called A, Abis and Um interfaces.

On these interfaces only three layers are defined.They are not corresponding to the OSI (Open System Interconnection) model.


A Interface

A interface between the BSC and the MSC

The A interface provides two distinct types of information, signalling and traffic, between the MSC and the BSC.

The speech is transcoded in the TRC and the SS7 (Signalling system) signalling is transparently connected through the TRC or on a separate link to the BSC.


Abis Interface

The A-bis interface responsible for transmitting traffic and signalling information between the BSC and the BTS.

The transmission protocol used for sending signalling information on the A-bis interface is Link Access Protocol on the D Channel (LAPD)


(Um) Air Interface

This is the interface between the mobile station and the Base station.

The Air interface uses the Time Division Multiple Access (TDMA) technique to transmit and receive traffic and signalling information between the BTS and MS.

The TDMA technique is used to divide each carrier into eight time slots.These time slots are then assigned to specific users,allowing up to eight conversations to be handled Simultaneously by the same carrier.


7 5 6 3 4 1 2 0

1 2 43 5 7 6

Down Link

Up Link 0

Time Slot


• This interface is the radio interface between the mobile station and the network and uses layer Three messages. • On Layer three messages we have the division of message types into CM (communication Management), MM (Mobility Management), and RR (Radio Resource Management).

Connection Management (CM)

There are three entities within CM: Call Control(CC) – Which handles the

procedures concerning call control. e.g. setup,Change of bearer service.

Supplementary Service (SS) – Which handles such as call bearing, call waiting , call forwarding etc.

Short Message Service (SMS) – Enables the MS to handle short message transfer to and from the network.


Mobility Management (MM)

Mobility management handles functions for authentication, location updating, identification and others concerning the mobility of the mobile station.


Radio Resource Management (RR)

It contains the functions concerning the radio link. Here we find the capability to establish,maintain and release the radio connection between the network and the mobile station, which includes the handover procedure.


B Interface

The B interface between the MSC and the VLR uses the MAP/TCAP protocol.

Most MSCs are associated with a VLR, making the B interface "internal".

Whenever the MSC needs access to data regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.


C Interface

The C interface is between the HLR and a MSC.

Each call originating outside of GSM (i.e., a MS terminating call from the PSTN) has to go through a Gateway to obtain the routing information required to complete the call, and the MAP/TCAP protocol over the C interface is used for this purpose.

Also, the MSC may optionally forward billing information to the HLR after call clearing.


D Interface

The D interface is between the VLR and HLR. It uses the MAP/TCAP protocol to exchange

the data related to the location of the MS and to the management of the subscriber.


E Interface

The E interface interconnects two MSCs. The E interface exchanges data related to

handover between the anchor and relay MSCs using the -MAP/TCAP+ISUP/TUP protocol.


F Interface

The F interface connects the MSC to the EIR. It uses the MAP/TCAP protocol to verify the

status of the IMEI that the MSC has retrieved from the MS.


G Interface

The G interface interconnects two VLRs of different MSCs.

It uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure.


Topics for discussion

Speech Encoding Data Encoding Interleaving for Voice,Control and

Data signals


Speech Encoding

We shall start with a raw voice signal fed into the microphone, travel through the various stages involving vocoding, channel coding etc till it reaches the final burst format on the Air Interface.


Speech Encoding ckt


Voice Encoding

Channel coding interleaving

RF Modulation

Raw Voicesignal

Speech Encoding ckt

The voice is sampled at the rate of 50 samples per second.

This results in 20 msec blocks of speech

Each of this 20 msec block is passed on to the 13Kbps vocoder.

There are 260 information bits from the output of the vocoder for every 20 msec input i.e.; 13Kbps *20msec = 260 bits.


Voice Encoding ckt


Vocoder I/p20 msec speech

blocks

13Kbps Vocoder Vocoder O/p260 bits

Channel coding

Channel Coding is done to protect the logical channels from transmission errors introduced by the radio path.

The coding schemes depend on the type of the logical channels, hence the coding can differ from speech, control and data .


Channel Coding for speech


Class class 1b class 21a

50 3 132 4 tailBits parity bits

Convolutional coder½ coder, k=5

456 bits=378 bits from Convolution coder + 78 class 2 bits

260 bits

Channel coding for Speech The 260 bits of speech info from the

vocoder is broken down into three parts. Class 1a- 50 bits , these represent the

filter coefficients of the speech and are the most important for proper detection of the speech at the receiver and hence are given maximum protection. 3 additional parity bits are derived from the class 1a bits for cyclic redundancy check (CRC).


Channel coding for Speech cont’d Class 1b - 132 bits are not parity

checked but are fed into the convolutional coder along with 4 tail bits which are used to set the registers in the receiver to a known state for decoding purpose.

Class 1b- 78 bits, these are not so important and are not protected but are combined with the output of the convolution coder.


Convolutional coder CC

The Convolutional coder is a series of shift registers implemented using logic gates, where for every one input bit we get 2 output bits. Hence it is called ½ coder.

Here k=5 is the constraint length, it means there are 5 shift register and each bit has memory depth of 4 , meaning it can influence the output of up to four next successive bits. This is useful during reception as bits can be derived even if a few consecutive bits are lost due to errors or corruption.


½ Convolutional coder


R1 R2 R3 R5R4

+

+

C0output

C1output

0110..Input bits

+ EX-OR R=register

Convolutional coder cont’d The output of the CC* is now 378 bits. (50+3+132+4)*2=378The total number of bits now is

378+78=456 bits.*Note : The bit rate from the vocoder was

13Kbps for the 20 msec speech block, but after CC the bit rate increases to 22.8Kbps.

456 bits *20msecs=22.8Kbps * CC = Convolutional Coder.


Control Channel Coding


184 bits Control data

184 40 4 tailFire coded parity bits

½ Convolutional Coder456 bits output

Control Channel Coding

The control information is received in blocks of 184 bits.

These bits are first protected with a cyclic code called as Fire code, which is useful in correction and detection of burst errors.

40 Parity bits are added, along with 4 tail bits.

These 228 bits are given to the CC whose output is again 456 bits at a bitrate of 22.8Kbps.

The control channels include the RACH, PCH, AGCH etc.


Data Channel Coding


240 bits 4 tail Data bits

½ Convolutional CoderOutput= 488 bits

After PuncturingOutput=456 bits

Data Channel Coding

The data bits are received in blocks of 240 bits. These are directly convolution coded after adding 4 tail bits.

The output of the CC is now 488 bits, which actually increases the bitrate to 24.4 Kbps.

To keep the bitrate constant on the air interface we need to puncture the output of the CC. Hence, we have a final bitrate of 22.8 Kbps again .


Channel Coding cont’d

The above explanation was given keeping in view a full rate Traffic, Control, or Data channel.

For Half rate or Lesser rates the same principle of channel coding holds good, with slight differences in the encoding process.


Interleaving

Having encoded the logical channel information, the next step is to build its bit stream into bursts that can be transmitted within the TDMA frame structure. This is the stage where the interleaving process is carried out.

Interleaving spreads the content of one information block across several TDMA timeslots or bursts.


Interleaving cont’d

The following interleaving depths are used :

Speech – 8 blocks Control – 4 blocks Data – 22 blocks The interleaving process for a speech

block is shown wherein which a 456 bit speech block is divided into 8 blocks of 57 bits each and each of these odd and even 57 bit blocks are interleaved diagonally on to alternate bursts on the TDMA frame.


Speech Interleaving


8* 57 bits each = 456 bitsOf Speech block N

57Even

Of N-1

57EvenOf N

Speech blockN-1

57odd

Of N-1

57odd

Of N

The speech is spread over 8 such normal burstsEach normal burst consists of two blocks of 57 bit speech

from different 20msec blocks (say N, N-1) along with26 bit training sequence T and 2 flag F plus 6 start stop bits .

T+FT+FT+F

456 bit speech data

Control Data Interleaving


114 114 114 114

456 bits control data

The control data is spread over 4 blocks using rectangular interleaving instead of diagonal interleaving as in speech the receiver will have to wait for at least

2 multiframes before being able to decode the controlmessage

TDMABurst blocks

Data Interleaving


114 114 114 114

Burst 1 Burst 22Burst 2 Burst 3 Burst 4 Burst 19

First 6bits

First 6bits

Last 6bits

Last 6bits

456 bit data block

Data Interleaving cont’d

Here the data block of 456 bits is divided into 4 blocks of 114 bits each.

The first 6 bits from each of the 114 bit blocks is inserted in to each frame, the second 6 bits from each of the 114 bits into the next frame and so on spreading each 114 block over 19 TDMA bursts while the entire 456 bits is spread over 22 TDMA bursts.

Thus the data interleaving is said to have a depth of 22 bursts.


Data Interleaving cont’d

The reason why data is spread over such along period of time is that if data burst is corrupted or lost, only a small part of it is lost which can be reproduced at the receiver.

This wide interleaving depth does produce a time delay during transmission but that is acceptable since it does not affect the data signal quality at the receiver, unlike speech where delay could result in bad quality of signal to the subscriber.

*Note – The interleaving used in data is diagonal interleaving.


Before Deinterleaving3 successive bursts corrupted

After DeinterleavingThe corrupted bursts are spread over a length equal to the

interleaving depth so that the effect of the errors isminimized.


Interleaving Advantage

Air Interface Bitrate

The information which is now coded and interleaved at 22.8 Kbps now has to be transmitted over the Air interface to the BTS.

The information burst is not sent directly , but is sent in ciphered form within a burst envelope. This ciphering is done using ciphering keys and algorithms known both by the mobile and the BSS.


Air Interface Bitrate cont’d The Kc is the ciphering key and A5

algorithm are applied to the information(speech or data) which increases the bitrate to a final rate of 33.8 Kbps from/to each mobile.

If we assume all 8 timeslots of the cell to be occupied then the bitrate of the Air interface comes to 33.8 * 8= 270.4 Kbps/channel.


Air Interface Bitrate cont’d


A5 Algorithm

Kc Information Block 22.8 Kbps

Sent on Air interfaceCiphered information burst

33.8 Kbps

Air Interface Bitrate cont’d


1 2 3 4 5 6 7 8

Mobile Tx’s at

33.8 Kbps

Cell rx’s 8*33.8 KBps = 270.4 KbpsPer TDMA frame

Cell coverage area

TDMA Fn TDMA Fn+1

Decoding and Deinterleaving at the Receiver At the receiver the reverse process of

Deinterleaving and decoding have to take place respectively, so as to recover the information from the signal.

After Deinterleaving the signal will be decoded which is the reverse process of the Convolutional coding, using Viterbi decoders.

The decoder can recover lost or corrupted data up to 4 successive bits, because the memory depth of the CC is 4(for k=5).


Channelization

Frequency band has several application segments

Certain blocks of the Band are reserved for certain applications by regulating authorities

Technologies have decided their frequency bands

E.g. AMPS/DAMPS: 824-894 MHz


Channelization methods

Channelization can be done primarily by three methods: FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access) CDMA (Code Division Multiple Access)


FDMA

E.g. AMPS band is divided into 30 KHz channels (1666 Freq. channels)

Television Channels (Star, Zee, Sony,..)04/07/23Tempus Telcosys 14

3

Frequency

TimePower

TDMA

E.g. AMPS has 3 timeslots on each 30 KHz channel


Frequency

TimePower

CDMA

Frequency channel is divided into code channels

E.g. in IS-95 CDMA, 1.228 MHz channel is divided into 64 Code Channels

Each user has a particular code Codes are orthogonal to each other,

do not interfere with each other


Duplex Access Methods

Frequency Division Duplex (FDD) Transmit on one frequency and receive

on another frequency04/07/23Tempus Telcosys 14

6

F1 F2 Frequency

Amplitude

Time

Tx Rx

Time Division Duplex

Time division duplex Tx and Rx is on the same frequency but

on different times


F1 Frequency

Amplitude

Time

Tx

Rx

GSM Air Interface Separate Bands for Uplink and Downlink

Downlink: 935-960Mhz (EGSM: 925-960MHz) Uplink: 890-915 MHz (EGSM: 880-915 MHz)


• TDMA and TDMA Multiplex– 124 Frequency Channels (ARFCN) for

GSM900– 1 to 124 fro current band– 975 to 1023 for E-GSM – 200kHz Channels– 8 Mobiles share ARFCN by TDMA

GSM Air interface (1800)

1800: Downlink: 1805-1880 MHz 1800: Uplink: 1710-1785 MHx

374 ARFCNs Separation of 95 MHz ARFCNs are numbered from 512 to

885 inclusive


The GSM Burst


3 357 261 571 8.25

Tail Bits

Data

Control Bit

Midamble

Control Bit

Data

Tail Bits

Guard Period

Speech Coder

RPE/LTP coder (Regular Pulse excitation/Long term Prediction)

Converts 64 kbps speech to 13 kbps

At the end we get 13kbps speech i.e. 260 bits in 20 ms


20 ms blocks

Speech Coder

Bits Ordered

50 very important bits

132 important bits

78 other bits

Error Correction


Type 1a 50 3(CRC)Type 1b 132 Type II 78

Reordering

25 66366 25 4 Type II 78Type 1a

Type 1b Type 1b

Type 1a

Tail

Half rate convolutional code

378 Type II 78

456 bits from 20 ms of speech

Diagonal Interleaving

Traffic channel (TCH) bursts carry two 57 bit blocks (114)

Each 120 ms of speech = 456*6 = 2736 bits2736/114 = 24 bursts i.3. 24 frames

Multiframe has 26 frames in 120ms.There are 2 spare frames .. 1 SACCH, 1 Idle


456 bits from 20ms of speech 456 bits from 20ms of speech

57 57575757575757 57 57575757575757

57 57 57 5757 5757 5757 5757 5757 5757 57

Convolutional Coding and Interleaving Bits to be Tx ed: HELLO Convolutionally encoded: HHEELLLLOO Interleaved: EE HH LL LL OO

Bits Rx ed: EE HH LL LL OO De-Interleaved: HHEELLLLOO Viterbi Decoded: HELLO


Speech Coding Process


20 ms

Speech Coder

260 bits 13 kbps

50 1a 132 1b 78 II

Channel Coder

456 bits 22.8 kbps

Transceiver (BTS)

Transcoder Handler

260 bits

456 bits

16 kbps

TRAU frame

260 + 60 = 320 bits

Abis

13 kbps

TRAU frame

260 bits info + 60 TRAU bits = 320 bits/20ms = TRAU frame

60 bits contain frame Information data which indicates speech, data, O&M, full rate/half rate

60 bits = 35 synchronization + 21 control + 4 timing


Midamble or Training Bits

8 midamble patterns (Colour codes) of 26 bits (BSIC)

RACH and SCH have longer 41 and 64 bit Midambles

Equalizer estimates channel impulse response from midamble

Mathematically construct inverse filter Uses inverse to decode bits 04/07/23Tempus Telcosys 15

7

3 357 261 571 8.25

Tail Bits

Data

Control Bit

Midamble

Control Bit

Data

Tail Bits

Guard Period

Downlink and Uplink

Uplink lags downlink by 3 timeslots Uplink and downlink use same timeslot

number Uplink and downlink use same channel

number (ARFCN) Uplink and downlink use different

bands (45 MHz apart for GSM 900)


Measurements made by MS and BTS


RxQual0 < 0.2% 1 0.2 – 0.4 %3 0.4 – 0.8 % 4 0.8 – 0.16 %5 1.6 – 3.2 % 6 3.2 – 6.4 %7 6.4 – 12.8 %

Uplink RXLEV (-48 to -110 dbm)Uplink RXQUAL (0-7)

Uplink RXLEV (-48 to -110 dbm)Uplink RXQUAL (0-7)

Mobile Power Control


Mobile is commanded to change its Transmit Power

Change in Power is proportionate to the Path Loss

Change in Power is done in steps of 2 dbs

Path Loss

Power Command

Timing Advance

TDMA approach requires signals to arrive at BTS at the correct time

A mobile at 30 km will be late by 100micro seconds

Timing advance is in the range of 0-62

One unit is 550m So maximum cell size is 63*0.55 =

~35 kms04/07/23Tempus Telcosys 16

1

Concepts of Channels in GSM

A company vehicle is used for several purposes in a day

Similarly in GSM, the timeslots are used for different purposes at different times


Frames and Multiframes


0 654321 7

3 Data 1Midamble1 Data 3 8.25 bits

156.25 bits 576.92 micro sec

4.615 ms

Time

Slot

Frame

0 50 0 25

Control Channel Multiframe

Traffic Channel Multiframe

GSM Operations

Location Update Mobile

Originated Call Mobile

Terminated Call Handover Security

Procedures Cell Barring

DTX Cell Broadcast Short Message

Service Emergency calls Supplementary

Services Roaming


Mobile Turn On

Mobile Searches for Broadcast Channels (BCH)

Synchronizes Frequency and Timing Decodes BCH sub-channels (BCCH) Checks if Network Allowed by SIM Location Update Authentication


Location Area


Location Area 1

LocationArea 2

BTS

BTSBTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

BSCMSC

Location Area Identity

Location area is the area covered by one or more BTSs where a mobile can move freely without updating the system

One Location area can be covered by one or more BSCs, but ony one MSC.


MCC LACMNC

Importance of Location Area

Reduce Paging load Resource Planning

Smaller Location Areas – Location update increases

Larger Location Areas – Paging load increases


What is Location Update?

MSC should know the location of the Mobile for paging

Mobile is continuously changing location area

Mobile when changes Location Area informs the MSC about its new LA

Process of informing MSC about new Location area is Location Update


Types of Location Updates

1. Normal Location Update

2. IMSI Attach3. Periodic Location

Update


Hi,I am in Location area

xxx

IMSI Attach

Mobile turns off and sends an IMSI Detach to MSC

Mobile turns on again and compares LAI

If same, sends an IMSI attach to MSC


Is the received LAI same as

before

If same,Sends IMSI

attach

Normal Location Update

Mobile Turns on Power Reads the new LAI If different, does a Location Update


Is the received LAI same as

before

If different,does

Location Update

Periodic Location Update

The periodic location Update time is set from OMC/MSC

After the periodic location update timer expires, the mobile has to do a location update


What happens at Location Update?

Mobile changes location area Reads the new Location Area from

BCCH Sends a RACH (request for channel) Gets a SDCCH after AGCH Sends its IMSI and new and old LAI in a

Location Update request to MSC on SDCCH


What happens at location update cont..

….. . . MSC starts Authentication If successful, Updates the new

Location area for the Mobile in the VLR

Sends a confirmation to the Mobile Mobile leaves SDCCH, and comes to

idle mode


Mobile Originated Call


Channel RequestImmediate AssignService Request

Call ProceedingSet Up

CipheringAuthentication

AlertingAssignment

Connection

Mobile Terminated Call


PagingChannel RequestImmediate Assign

Set UpCiphering

AuthenticationPaging Response

AssignmentCall Confirmed

AlertingConnection

Security Features

Authentication Process to verify Authenticity of

SIM Mobile is asked to perform an

operation using identity unique to SIM


• Ciphering– Process of coding speech for

secrecy– The speech bits are EXORed with

bit stream unique to MS

Security Features (TMSI Reallocation)


GSM Infrastructure Mobile

Location UpdateTMSI Allocation

Call SetupTMSI Reallocation

TMSI- Temporary Mobile Subscriber Identity

Security Features (Identity Check)


EIR

Sends IMEI

Identity Check

White listed /Grey Listed/ Black Listed mobiles

Handover


Cell 1 Cell 2

Handover is a GSM feature by which the control/communication of a Mobile is transferred from one cell to another if certain criteria’s are

met. It is a network initiated process.

Criteria for Handover

Receive Quality (RXQUAL) on uplink and downlink

Receive Signal Strength (RXLEV) on uplink and downlink

Distance (Timing Advance) Interference Level Power Budget


Handover Decision

BSC process the measurements reported by Mobile and the BTS


BTS

BTS

BTS

BTS

BTS

BTS

Mobile has measurements of six neighbors

Handover Decision (cont..)

BSS performs averaging function on these measurements every SACCH frame (480ms)

Handover Decision algorithm is activated after a set number of SACCH frame periods by comparison against thresholds


Types of Handovers

INTRA-CELL HANDOVERS INTER-CELL HANDOVERS INTRA-BSC HANDOVERS INTER-BSC HANDOVERS INTER-MSC HANDOVERS


INTRA-CELL HANDOVER


C0

C1

Handover between timeslots of same frequency

Handover between different frequencies of the same cell (to reduce interference)

MSC is not aware about this

Inter-cell Handover


Handover between cells of the same BTS

BTSCell 1 Cell 2

Inter-cell Handover (cont..)

MSC is told about HO BTS -> BSC -> MSC Why MSC is informed?

In case of change of LA, MSC may need LAC for paging. As MS is busy, a link already exists. So, MSC can send a tone in case of call waiting, and does not need to page again.

This is needed also for billing and call tracing


INTRA-BSC Handover


MSC BSC

BTS

BTS

This HO takes place if the cell to which handover is to be done belongs to the same BSC

Inter BSC Handover


MSC

BSC BTS

BTSBSC

The MSC is completely involved in this Handover

Inter MSC Handover


BSC

BSC

MSC

MSC

BTS

BTS

GMSC/PSTN/

Backbone

In this case the handover takes place through the interconnecting element which can be GMSC or PSTN or private Backbone between the MSCs

Cell Barring


BTS

Cell Barring is a GSM feature by which certain mobiles could be barred access to certain cellsCell barring is activated/deactivated at BTS level

Cell barring is done for mobile categories and priorities

Cell Barring

Every mobile has an access class The access class is stored in the SIM

Classes 0-9 are termed normal calsses Classes 11-15 are emergency classes


• Every cell has a set parameter which defines which access classes are barred for the particular cell. This parameter is broadcasted on the BCCH

What is DTX? DTX (Discontinous Transmission) Each direction of Transmission is only

50% Transmitter is switched ON for useful

information frames


Need for DTX•To increase battery life•To reduce the average interference levelDTX is done by DTX handlers which have the following functions.

VAD (Voice Activity Detector)

Senses for speech in 20ms blocks Removes stationary noise VAD is an energy detector Compares Energy of filtered speech

threshold It determines which 20ms blocks

contain speech and it only forwards those frames


Evaluation of Background Noise Background noise is always present

with speech DTX cuts off this noise with speech Gives an uncomfortable feeling to

the listener VAD takes care of this by inserting

comfort noise at the receiving end when speech discontinues.


Emergency Calls GSM specs define 112 as an

emergency number ‘112’ is accessible with or without

SIM Without SIM it is sent on the best

channel Mobile on sensing ‘112’ sets the

establishment cause to emergency call in the RACH

Routing of this call be done to a desired location defined in the switch


Cell (Re)selection

Cell reselection is done using C1 path loss criterion.

The purpose is to ensure that the MS is camped on to the cell with the best transmission quality.

The MS will camp on to the cell with the highest C1 value if C1 > 0.


The following parameters are used to calculate the C1 criterion

The received signal at the MS side. Rxlev_access_min - broadcast on

the BCCH - The minimum received level at the MS required for access to the network.

Ms_txpwr_max_cch - the maximum power that an MS may use when initially accessing the network.

The maximum power of the MS


C1 = A - Max(B,0)

A = Received level Average - Rxlev_access_min.

B = MS_txpwr_max_cch - maximum output power of the MS


Cell Reselect Hysteresis

Cell reselection on the border of two location areas result in a location update. When an MS moves on the border of two location areas lots of location updates take place. To avoid these location updates, the reselect hysteresis is introduced.

A location update is performed only if: The C1 value of the new location area is

higher than the C1 value in the current location area and

The received signal strengths have at least a difference of the reselect hysteresis.


Cellular concept


Why to use the cellular concept ?Solves the problem of Spectral

congestion and user capacity by means of frequency reuse.

Offers high capacity in a limited spectrum allocation.

Offers system level approach, using low power transmitters instead of a single, high power transmitter (large cell) to cover larger area.


A portion of the total channels available is allocated to each base station.

Neighboring base stations are assigned different groups channels, in order to minimize interference.


Cell shape


1-Omni-directional cell-site (Omni-directional antenna).

2-Rhombus-shaped sectors (Directive antenna).

3-Hexagonal shaped sectors (Directive antenna).


Cell sizeLarge cell : (up to 70km in diameter)It exists where :1-Radio waves are unobstructed.2-Transmission power can cover the area.3-low subscriber density.

Small cell : (up to 2km in diameter)It exists where :1-Radio waves are obstructed.2-Low transmission power to decrease

interference.3-High subscriber density.


Types of cells1-Macro-cells 2-Micro-cells. 3-Pico-cells. 4-Umbrella-

cells.


What is a cluster ?A cluster is a

group of cells.No channels are

reused within a cluster.

It is the unit of design.


Cluster sizeDefinition : It is The number of

cells per cluster N = i^2 + ij + j^2

Where : i = 0, 1, 2….& j = 0,1,2…. etc. N = 1 , 3 , 4 ,7, 9 , 12 ,……


Types of clusters1-N=7 omni frequency plan (2-

directional).2-N=7 trapezoidal frequency plan (1-directional).3-N=9 omni frequency plan.4-Tricellular plans a) N=3 tricellular plan (3/9). b) N=4 tricellular plan (4/12).


Channel assignment strategies Considerations : 1) Max. capacity. 2) Min interference. 3) Perfect handover.

Types of assignment strategies : 1) Fixed : Each cell has permanent predetermined set of

voice channels. New calls served by unused channels of this cell. Borrowing strategy if all channels are occupied. High probabiltity that call is Blocked if channels

are occupied.( disadv.)04/07/23Tempus Telcosys 21

3

2) Dynamic :

Channels are not allocated to different cells permanently.

Each new call BTS requests new channel from MSC.

MSC allocate a channel, by using an algorithm that takes into account:

1- Frequency is not already in use. 2- Min. reuse distance to avoid co-

channel interference.


Adv. of dynamic assignment strategy :

1) Increase channel utilization ( Increase trunking efficiency ).

2) Decrease probability of a blocked call.


Frequency reuseConcept


Reuse cluster


Co-channel Reuse ratio (Q) :

R : cell radius. D : reuse distance.

Q = D/R. = sqrt(3N).

Where :N : cluster size


Handover


Definition : procedure that allows MS to change the cell or time-slot to keep as good link as possible during all the call.


Types of handover IntraCell : bet. 2 channels of same

cell.

InterCell : bet. 2 channels of 2 different cell & same BTS.

InterBTS (intra BSC) : 2 cells of different BTS Same BSC.

InterBSC : bet. 2 cells of different BSC’s & same MSC.


Measurements before handover1- Measurements from MS to BSC : a) Strength of BTS signal. b) Quality of BTS signal. c) Signal strength of 6 neighbor BTS’s.

2-Measurements from BTS to BSC : a) Strength of MS signal. b) Quality of MS signal. c) Distance between serving BTS & MS.


Different causes of handover


Better cell HOEmergency HO

Level Quality PBGT

Traffic causesInterferenceDistance

Different causes ofHandover

Basic handover algorithms

a)“Min. acceptable performance” algorithm:

MS power is increased when quality deceases till handover is the only way.

b) “Power budget “ algorithm: Prefer direct handover when quality

deceases without increasing MS power first .


Handover priority1) UL quality cause (or interference).2) DL quality cause (or interference).3) UL level cause.4) DL level cause.5) Distance cause.6) Better cell cause.


Interference


Sources of interference include: 1) Another mobile in the same cell. 2) A call in progress in the

neighboring cell.

3) Other BTS’s operating in the same frequency band.


Interference effects : In voice channel causes crosstalk

In control channels it leads missed and blocked calls due to errors in the digital signaling.


Main types of interference :

1) Co-channel interference.

2) Adjacent channel interference.


1) Co-channel interference Source : Near cell using same frequency.

It is a function of reuse distance(D/R). General rule :

io = No. of co-channel interfering cells.S = Signal power from a desired BS.Ii = interference power caused by the

ith interfering co-channel cell BS.04/07/23Tempus Telcosys 23

1

Another form : C/I = 10 log {(1/n)(D/R)*m} Where : m = propagation constant

(dep’s on nature of environment)

n = number of co-channel interferers.

Can be minimized by : Choosing minimum reuse distance = (2.5….3)(2R).


2) Adjacent channel interference Source : A cell using a frequency

adjacent to the one in another cell due to imperfect reciever’s filter.


Can be minimized by :1-careful filtering2-careful channel assignments3-Directional antenna.

General rule : ACI= -10 Log[(d1/d2)*m] – Adj ch isolation.

Where : d1: distance between MS & proper

BTs d2: dist. Bet MS & adj BTS causing interference.

Adj ch isolation = Filter isolation = - 26db.


Traffic engineering theory


Why do we need to know traffic?

The amount of traffic during peak hours allows us to dimension our wireless system for a certain GOS.

GOS : probability of having a call blocked during busy hour (block rate).


Traffic intensity (E)Erlang : A unit of traffic intensity

measure.

1 Erlang = 1 circuit in use for 1 hour.

T ( in Erlangs) = [No. of calls per hour*average call holding time(sec.)] / [3600]


Typical traffic profile


Traffic tables

Erlang B Table

Blocked calls are not held

Erlang C Table

Blocked calls are held in the queue indefinitely

Poisson Table

Blocked calls are held in the queue for a time = the mean holding time of a call


Erlang – B table

P(N;T) = [ (T^N)*exp(-T) ] / N!

N GOS 1% GOS 2%

2 0.153 0.2234 0.869 1.093

10 4.46 5.08420 12.0 13.18240 29.0 30.99704/07/23Tempus Telcosys 24

0

Trunking

Sharing channel among several users.

Trunking efficiency (nT) : Measures the number of subscribers that each channel in every cell can accommodate.

nT = (traffic in Erlangs / no. of channels)*100.


Trunking efficiency in presence of one operator :

N = 7 , 312 one direction voice channels

No. of channels / cell = 312 / 7 = 44 ch./cell.

From Erlang-B table @GOS 2%,this’s equivalent to 35 Erlangs

nT = 35 / 44 = 79.55.

Trunking efficiency in presence of two operators :

N = 7 , 312 / 2 = 156 one direction voice channel for each operator.

No. of channels / cell = 156 / 7 = 22 ch./cell.

From Erlang-B table @GOS 2%,this’s equivalent to 15 Erlangs.

nT = 15 / 22 = 68.18.04/07/23Tempus Telcosys 242

System capacity


S : total duplex channels available for use = k*N

Where: N : cluster size. k : No. of channels / cell.

C : total No. of duplex channels in system;

C = M*k*N.

Where : M : No. of times the cluster is

repeated.04/07/23Tempus Telcosys 24

4

Improving system capacity

Cell splitting.Sectoring.


Cell splitting


Sectoring We use directional antennas instead of

being omnidirectional


What does sectoring mean? We can now assign frequency sets

to sectors and decrease the re-use distance to fulfill :

1) More freq reuse. 2) Higher system capacity. 3) Improve S/I ratio ( better signal quality ). How S/I ratio is improved? -e.g. In 120 degree sectoring there’s only 2 interferers instead of 6 incase of

omnidirectional N=7 cluster.


Directional frequency reuse

Here we use 7/21 pattern for frequency allocation.


Comparison between various types of clusters


N = 7 omni frequency plan : n = 6 , m = 4.

D / R = 4.583.

1) Co-channel interference ratio :

C / I = 18.6 dB.

2) Adjacent channel interference :

ACI = -26 dB @ d1= d2. 04/07/23Tempus Telcosys 25

3

N = 7 trapezoidal frequency plan

n = 2 , m = 4.

D / R = 6.245.

1) Co-channel interference ratio :

C / I = 28.8.

2) Adjacent channel interference : disappears because the channels are assigned alternatively to the cells.


Trunking efficiency : 312 one direction voice channels N = 7 312 / 7 = 44.57 ~ 44 ch./cell.

From Erlang-B table @ GOS = 2%T = 35 E. nT = 35 / 44 = 79.55 %.


N = 9 omni frequency plan n = 4 , m = 4.

D / R = sqrt ( 3 * 9 ) = 5.2.

1) Co-channel interference :

C / I = 22.6 dB.

2) Adjacent channel interference :

ACI = -38 dB @ d2 = 2 (d1).


Trunking efficiency : 312 one direction voice channels N = 9 312 / 9 = 34.67 ~ 34 ch./cell. From Erlang-B table @ GOS = 2%T = 25.529 E. nT = 25.529 / 34 = 75.085 %.Conclusion : nT 7 > nT 9 But C/I 7 > C/I 9 ACI 7 > ACI 9


4 / 12 cell pattern n = 1 , m = 4. D / R = sqrt (3* 4) =

3.732.

C / I = 22.87 dB.

Trunking efficiency : No. of channels/cell = 312 / 12 = 26 ch./cell. From Erlang-B table @

GOS = 2 %. T = 18.4 E/cell. nT = 18.4 / 26= 70.77%.


3 / 9 cell pattern n = 1 , m = 4. D / R = sqrt (3* 3) = 3.

C / I = 19.1 dB.

Trunking efficiency : No. of channels/cell =312 / 9 = 34 ch./cell. From Erlang-B table @

GOS = 2 %. T = 25.5 E/cell. nT = 25.5 / 24 = 75 %.


120 degree cell sectoring

n = 2 , m = 4. D / R = sqrt(3 * 7) = 4.583.

Co-channel interference : C / I = 23.436 + 6dB(due to

isolation) = 29.436 dB.

Trunking efficiency : No. of channels/cell = 312 /

21 = 14.857. From Erlang-B @ GOS=2%

T= 8.2003. nT = 8.2003 / 14.857 =56.216%.


References : Motorola CP02 NOKIA SYSTRA


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