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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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
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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)
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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
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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
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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)
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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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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,...
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GSM MODULATOR
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