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By:

Eng\ WALEED ELSAFOURY

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GSM 900

0.8 0.9 1.0 1.1 1.6 1.7 1.8 1.9 2.0 GHz

GSM 1800 GSM 1900

200 kHz

f

Carrier nCarrier (n+1)

GSM bands

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Uplink and downlink

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GSM Frequency Bands

System P-GSM 900 E-GSM 900GSM(DCS)

1800GSM(PCS) 1900

Uplink (MS BS)Downlink(BS MS)

890 915 MHz935 960 MHz

880 915 MHz925 - 960 MHz

1710 1785 MHz1805 - 1880 MHz

1850 1910 MHz1930 - 1990 MHz

Wavelength 33 cm 33 cm 17 cm 16 cm

Bandwidth 25 MHz 35 MHz 75 MHz 60 MHz

Duplex distance 45 MHz 45 MHz 95 MHz 80 MHz

Carrier separation 200 kHz 200 kHz 200 kHz 200 kHz

No. of carriers 124 174 374 299

Channel rate 270.8 kbps 270.8 kbps 270.8 kbps 270.8 kbps

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- TDMA in GSM

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TDMA frame

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TDMA frame structure

3113 57 5726

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TDMA frame structure

-Bit rate of the radio carrier is 270.833 Kbps

-Bit duration =1/270.833=3.69 sec

-One time slot =148 bits+8.25 guard bits=156.25 bits-Time slot duration =156.25x3.69 sec= 0.577 msec

-Frame duration=0.577x8= 4.615 msec

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Guard band is used for :- burst alignment errors

- time dispersion on the propagation path

- Time required for smooth switch on-off in transmitter

1 2 3

The numbering scheme of the time slots is staggeredby 3 time slots to remove necessity for MS to sendand receive in the same time

4 5 6

1 2 3 4 5 60

0

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Types of channels1. Physical channels

2. Logic channels

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Types of logic channels1. Traffic channels

2. Control channels

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Types of Traffic channels1. Full rate

- 13 Kbps data rate

2. Enhanced full rate

- 12.2 Kbps data rate

3. Half rate

- increase network capacity

13 Kbps 9.8Kbps

12.2 Kbps 10.6Kbps

5.6 Kbps 5.8 Kbps

11.4Kbps

22.8Kbps

Data

Redundancy

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Types of control channels

Broadcast control channel

Dedicated control channel

Common control channel

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Common Control Channels (CCCHs)Transfer control information between all MSs and BTS.Necessary for call origination and paging.

Logical ch. Function BTS MS

Paging CHannel(PCH)

Downlink

Alert the MS of anincoming call

Transmits paging

message to indicatean incoming callThe paging messagecontains MS identity.

Listens to PCHregularly.When identifies itsidentity, it responds.

Random AccessCHannel (RACH)

Uplink

Used by MS torequest access to

the network.

Receives requestfrom MS for a

signaling channel.

Answers pagingmessage on RACH byrequesting signaling

channel.

Access GrantCHannel(AGCH)Downlink

Used by BS toinform MS aboutthe channel to use.

Assigns a SDCCHsignaling channel toMS.

Receives a signalingchannel assignmentSDCCH.

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Dedicated Control Channels (DCCHs)

Carry messages between MS and network.SDCCH is used for call setup, update, authentication

Logical ch. Function BTS MS

Stand-alone

Dedicated ControlChannel (SDCCH)

UL+DL

Exchange signalinginformation in uplinkand downlink.

Switches to SDCCH.

Call set-up isperformed in idlemode.BSC assigns a TCH.

Switches to SDCCH.

Call set up is performed.MS receives TCHassignment [carrier +time slot]

Slow Associated

Control Channel(SACCH)UL+DL

Conveys power controland timing informationin downlink.

Conveys link qualityreports in uplink.

Instructs MS about:Transmit powerTime advance.

Sends measurements ofits BTS and neighboringBTSs during a call.

Fast AssociatedControl Channel(FACCH)

UL+DL

Steals TCH to carryhandover and channelreassignment.

Transmits handoverinformation.

Transmits handoverrequest..

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The Broadcast Channels (BCHs) Is transmitted by BS all the time.Monitored by MSs periodically (every 30 sec).All BCHs are downlink.

Logical channel Function BTS MS

FrequencyCorrectionChannel(FCCH)

Supply MS withreference freq.

Transmits a carrierfrequency.

Identifies BCCHcarrier

synchronizes withthe frequency.

Synch.Channel (SCH)

For TDMA framesynchronization .

Transmits framenumber and BSIC.

Synchronizes withframe structure.

BroadcastControlChannel(BCCH)

Carries parametersneeded to identifyand access network.

It is transmitted atconstant power at alltimes and checked byall MSs.

Broadcasts cellinformation suchas

LAI,maximum output

power needed

Receives LAIMay updateMS sets its outputpower level.

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Adaptive Frame Alignment The BS-MS delay depends on distance. If BSMS separation= 10km, Propagation delay=33 sec. Long guard interval is inefficient. GSM uses adaptive time alignment system whereby BS

transmits timing advance at the MS. This instructs the mis-aligned MS to transmit earlier

TDMA frame

Propagation delay

TS0 TS2TS1

B: Far away from BS

BA

A: Close to BS

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Rayleigh fading is frequency-selective.

BS&MS hop from one frequency to other.

Frequency hopping is optional.

Rate of hopping =1/4.615msec=217 hop/sec.

Slow Frequency Hopping

E E

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0 1 5 6432 7

0 1 5 6432 7

0 1 5 6432 7

TDMAframe

3 4210 765

3 4210 765

3 4210 765

Frame 1Carrier 1

slot 3

Frame 2Carrier 3

slot 3

2 310

2 310

2 310

Frame 3Carrier 2

slot 3