CS Part 3(1) 2001 - Mobile Communication Systems (GSM)

8/14/2019 CS Part 3(1) 2001 - Mobile Communication Systems (GSM)

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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 Communication

Systems

-GSM-

G S Mroupe peciale obile

Outline:

Introductory concepts and GSM Properties

GSM Source Encoder

Channel Encoder & Interleaver

GSM Mobile-to-Base Link

GSM Hierarchy

Gaussian MSK Modulation


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Principles of Communication Theory & Systems Compact Lecture Notes

Mobile Communication Systems 2 A. Manik

1. Multiple Access

FDMA

it is an techniqueANALOQUE need of in time.no synchronization the transmission bandwidth is partitioned to frequency slots

different users have different RF carrier frequencies, i.e. each user isassigned a particular frequency slot. users/signals are at the receiver byseparated out FILTERING if all frequency slots are occupied then the system has reached its

CAPACITY

TDMA

it is a techniqueDIGITAL

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




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2 Mobile Cellular Systems

A mobile cellular system consists of

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

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

mobiles (subscribers), or Mobile Stations (MS)

When a call originates, the base station nagotiates with the mobile on

various aspects (such as the channel used etc.), before establishing

communications.

After this, as the mobile moves from cell-to-cell, the service is handed

( ) from one base station to another.hand-off handoveror



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

Note:

base station to mobile is known as FORWARD LINK mobile to base station is known as REVERSE LINK


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

division multiplex - time division multiplex (FDM-TDM) technique to

improve 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 each

and every user, as this will limit the system capacity.

Therefore . The same communicationthese slots have to be reused

channels (time-frequency slots) will then be used in cells separated by

H(cells), which is the of the system.reuse distance

Cluster of cells:a set of cells where each user is assigned one channeluniquely

(i.e. no channel re-use)



Cluster of 7 Cells H reuse distance =3

The by increasing the number of channelssystem capacity could be increased

available in a single cell, i.e. .reducing the reuse distance H

But this reduction is limited by the , (i.e. theco-channel interference

interference from other cells sharing the same channels). The reuse distance

H, in these systems, is determined by the worst case interference situation.


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3 GSM: An Example of a TDMA/FDMA Systems

p roupe peciale obile,GSM G S M

or Pan-European Cellular Comm. System,

or lobal ystem for obile Comms.G S M

GSM system: FDMA/TDMAtransmits both and in digital form.p data voice

allocated to GSM:Bands

890-915MHz (downlink)

MHz (uplink)935-960



GSM System Architecture

.

.

.

.

.

.

.

.

BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

VLR

OMC

HLR AUC

PSTN

ISDN

Data Net.AbisInterface

A

Inter

face

SS7

Base StationSubsystem

Network SwitchingSubsystem

PublicNetworks

MS

MS

Air

Interface


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

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

Each 200kHz FDMA channel uses an aggregated bit-rate of 270.833

kbits/sec GMSK BUE=0.3carried over the radio channel using with a

The is divided into270.833kbits/sec bit stream in each FDMA channel 8fixed-assignment TDMA channels or (known as logical channels).time slots

Each logical channel (time-slot) is which corresponds to the577 sec.transmission time of (actually only are transmitted in156.25bits 148 bits

each slot time duration time =156.25bits)+ 8.25 bits guard

Note a TDMA/GSM frame = 4.615msec



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

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

When these signals arrive at the receiver, constructive or destructive

interference takes place (principle of superposition).

The destructive interference is known as multipath or Rayleighfading.

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 to

reduce the effects of "fading" and "channel noise".


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Mobile-to-BS = UPLINK

BS-to-Mobile = DOWNLINK

In GSM the source encoder operates on which20msec Frame of datacorresponds to blocks of (quantization levels) with160 message-symbols

each message symbol (quantization level) being a (i.e. total13 bit integer

equivalent ).13bits 160 bits every 20 msec #!)!

At the output of a GSM source encoder we get (i.e.260 bits every 20msec

2080 bits at the input are transformed to 260 bits at the output)



Messagesignal

bandwidth=4kHzFg

SamplingFrequency

=8kHzFs

Uniformquantizer

=2Q13

160 levels20 msec

2080 bits20 msec

=

i.e. =104kbits/srb

260 bits20 msec

i.e. =13kbits/srb

456 bits20 msec

i.e. =22.8kbits/srb

Gaussian MSKM=4

operating on148 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


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Principles of Communication Theory & Systems

Mobile Communication Systems 16


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GSM CHANNEL ENCODER

Take a block of 260 bits260 bits bit rate 13k < , 260 bits bits20 msec sec

20msec

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

Ia Ib

50 bits 132 bits78 bits

=260 bits

(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

CC(2,1,5) convolutional encoder for the above 189 bits (using the polynomial: D D D D D+1)% $ % $ " 78 bits378 bits bit rate= = =22.8k


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GSM HIERARCHY

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7148bits

+8.25bits(guard bits)

0 1 2 3 42047

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 21 A. M


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CS Part 3(1) 2001 - Mobile Communication Systems (GSM)

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