Mobile Communication Systems -GSM- G S M roupe peciale … Part 3(1) 2001 - Mobile... · Principles of Communication Theory & Systems Compact Lecture Notes Mobile Communication Systems

E303 & ISE3.2E

IMPERIAL COLLEGE of SCIENCE, TECHNOLOGY and MEDICINE,DEPARTMENT of ELECTRICAL and ELECTRONIC ENGINEERING.

COMPACT LECTURE NOTES on COMMUNICATION THEORY.Prof Athanassios Manikas, version November 2001

Mobile CommunicationSystems

-GSM- G S Mroupe peciale obile

Outline:

ì Introductory concepts and GSM Propertiesì GSM Source Encoderì Channel Encoder & Interleaverì GSM Mobile-to-Base Linkì GSM Hierarchyì Gaussian MSK Modulation

Principles of Communication Theory & Systems Compact Lecture Notes

Mobile Communication Systems 2 A. Manikas

1. Multiple Access

FDMAì it is an techniqueANALOQUEì need of in time.no synchronizationì the transmission bandwidth is partitioned to frequency slotsì different users have different RF carrier frequencies, i.e. each user is

assigned a particular frequency slot.ì users/signals are at the receiver by separated out FILTERINGì if all frequency slots are occupied then the system has reached its

CAPACITY

TDMAì it is a techniqueDIGITALì requires between userssynchronizationì each user/signal is assigned a particular (within a time-frame)time slotì if all time slots are occupied then the system has reached its CAPACITY





2 Mobile Cellular SystemsÞ

ì A mobile cellular system consists of

ˆ Base Stations (BS), or Base Transceiver Stations (BTS)

ˆ cells (a cell is the area serviced by a base station) and

ˆ mobiles (subscribers), or Mobile Stations (MS)

When a call originates, the base station nagotiates with the mobile onvarious aspects (such as the channel used etc.), before establishingcommunications.

After this, as the mobile moves from cell-to-cell, the service is handed( ) from one base station to another.hand-off handover or



ì Only one base station will service a mobile at any one time.

ì Note:ˆ base station to mobile is known as FORWARD LINKˆ mobile to base station is known as REVERSE LINK



ì Channel Reuse and Reuse Distanceˆ Most of the current cellular systems, such as GSM, use frequency

division multiplex - time division multiplex (FDM-TDM) technique toimprove the system capacity.

In these systems, .each user is assigned one time-frequency slot

When the system gets larger, these slots cannot be unique for eachand every user, as this will limit the system capacity.

Therefore . The same communicationthese slots have to be reusedchannels (time-frequency slots) will then be used in cells separated byH (cells), which is the of the system.reuse distance

ì Cluster of cells:a set of cells where each user is assigned one channeluniquely(i.e. no channel re-use)



ì Cluster of 7 CellsÐ H Ñreuse distance =3

The by increasing the number of channelssystem capacity could be increasedavailable in a single cell, i.e. .reducing the reuse distance H

But this reduction is limited by the , (i.e. theco-channel interferenceinterference from other cells sharing the same channels). The reuse distanceH, in these systems, is determined by the worst case interference situation.



3 GSM: An Example of a TDMA/FDMA SystemsÞ

ì p roupe peciale obile,GSM G S M

or Pan-European Cellular Comm. System,

or lobal ystem for obile Comms.G S M

ì GSM system: FDMA/TDMA transmits both and in digital form.p data voice

ì allocated to GSM: Bands 890-915MHz (downlink)

MHz (uplink)935-960



ì GSM System Architecture

.

.

.

.

.

.

.

.

BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

VLR

OMC

HLR AUC

PSTN

ISDN

Data Net.

Abis

Inte

rfac

e

A In

terfa

ce SS

7

Base StationSubsystem

Network SwitchingSubsystem

PublicNetworks

MS

MS

Air I

nte

rfac

e



ì in a GSM system has a (twoEach cell fixed number of frequency channelsway) ranging from only .1 to 15

ì Both downlink and uplink are divided into frequency channels (carriers).124Bandwidth 200 per frequency channel = kHz

ì Each 200kHz FDMA channel uses an aggregated bit-rate of 270.833kbits/sec GMSK BUE=0.3 carried over the radio channel using with a

ì The is divided into 270.833kbits/sec bit stream in each FDMA channel 8fixed-assignment TDMA channels or (known as logical channels).time slots

ì Each logical channel (time-slot) is which corresponds to the577 sec.transmission time of (actually only are transmitted in156.25bits 148 bitseach slot time duration time =156.25bits)+ 8.25 bits guard

ì Note a TDMA/GSM frame = 4.615msec



ì In mobile environments the received signals are heavily affected bymultipath-generated-fading plus noise.

ˆ In a mobile cellular system (TDM/FDM) the signals from the basestation to a mobile follow many different paths of different lengths.

ˆ When these signals arrive at the receiver, constructive or destructiveinterference takes place (principle of superposition).

ˆ The destructive interference is known as multipath or Rayleighfading.

ˆ This occurs more frequently when the mobile is moving. This fading isdetrimental to the system performance.

THEREFORE "discrete channel encoder" and "interleaver" should be used toreduce the effects of "fading" and "channel noise".



ì Mobile-to-BS = UPLINK

BS-to-Mobile = DOWNLINK

ì In GSM the source encoder operates on which20msec Frame of data corresponds to blocks of (quantization levels) with160 message-symbolseach message symbol (quantization level) being a (i.e. total13 bit integerequivalent ).13bits 160 bits every 20 msec‚ œ #!)!

At the output of a GSM source encoder we get (i.e.260 bits every 20msec2080 bits at the input are transformed to 260 bits at the output)



Message signal

bandwidth=4kHzFg

SamplingFrequency

=8kHzFs

Uniformquantizer

=2Q 13

160 levels20 msec

2080 bits20 msec

=

i.e. =104kbits/s rb

260 bits20 msec

i.e. =13kbits/s rb

456 bits20 msec

i.e. =22.8kbits/s rb

Gaussian MSKM=4

operating on 148 bits

per TDMA frame

148bitsplus 8.25 guard bits0.577ms

TDMA FRAME=4.615ms

0

f

Downlink 25MHz Uplink=25MHz

200kHz

890MHz 915MHz 935MHz 960MHz

SPECTRUM

BUE=0.3









GSM CHANNEL ENCODER

ì Take a block of 260 bits

260 bits bit rate 13kœ < œ œ,260 bits bits20 msec sec

20msec

ì divide the above block into two groups CLASS I = 182 bits CLASS II = 78 bits

Ia Ib50 bits 132 bits

78 bits=260 bits

ì (53,50) block encoder for Ia, andadd 0000 to reset the CC encoder which is used in the next step

50 bits 132 bits3 bits(parity) 0000 =189 bits

ì CC(2,1,5) convolutional encoder for the above 189 bits (using the polynomial: D D D D D+1)% $ % $ "à

78 bits378 bits bit rate= = =22.8k<,456 bits bits20 msec sec

TCS/FS, 20msec, 456 bits



GSM INTERLEAVER (clasifies if burst is TCH/FS burst or not)I/P: O/P: flag bit one burst Æ

1 2 3 4 5 6 7 81st

2nd

57th

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 83

bits

57 bits

1 bit

57 bits

1 bit

57 bits

1 bit

57 bits

....etc.

57 bits

1 bit

26 bits

1 bit

57 bits

3 bits

for equalisation (training sequence)

GSM MOBILE-to-BASE LINK (UPLINK)

BUE=0.3

bit rate= = =22.8k <,456 bits bits20 msec sec

1bit= 3.69 sec.

0.577msec

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7148bits

+8.25bits(guard bits)



GSM HIERARCHY

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7148bits

+8.25bits(guard bits)

0 1 2 3 4

2047

0 1 50

0 1 25

0.577msec

1 multiframe =26 TDMA frames= 120msec

1 superframe =1326 TDMA frames= 6.12sec

1 hyperframe =2,715,648 TDMA frames= 3h,28min,...



GSM MODULATOR













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Mobile Communication Systems -GSM- G S M roupe peciale … Part 3(1) 2001 - Mobile... · Principles of Communication Theory & Systems Compact Lecture Notes Mobile Communication Systems