Introduction of Gsm

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ADVANCED LEVEL TELECOM TRAINING CENTRE

INTRODUCTION TO GSM

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HISTORY 1979-80 Came the first Cellular

Mobile system such as AMPS ( Advance Mobile Phone Service) NMT ( Nordic Mobile Telephony ) TACS ( Total Access Communication

System)These were analog in Nature and known as First Generation System

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HISTORY During 1990 many digital mobile

standards were introduced in different regions/ countries DAMPS ( Digital Advance Mobile Phone

Service) GSM ( Global System for Mobile

Communications) CDMA ( Code Division Multiple Access )These are known as 2nd Generation System

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HISTORY For introduction of data transfer

capability on the Mobile ,additional Hardware was introduced in GSM systems, known as GPRS ( General Packet Radio Service ) . These are being referred as 2.5 G System

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FUTURE Now a Global Standard IMT-2000

(International Mobile Telecom.-2000) is proposed which shall encapsulate all the existing networks and different access technologies to provide any time and any where type of communication known as 3rd Generation standard.

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GSM As early as 1982, the CEPT

( Confederation of European P&T ) formed a Group called Global system for Mobile Communications (GSM)

The objective was to develop a building mobile radio standard for Europe

GSM is an European digital mobile telephony Standard specified by the ETSI and provides a common standard.

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GSM The GSM Standard is the first

international that gives subscriber full access to the network of various operating companies in all participating countries.

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GSM Operational Requirements International Roaming High Spectral Efficiency Improved Transmission Quality as

compared to Analog Systems High Link Integrity Provision of Tele Services & Bearer

Services

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GSM Operational Requirements Economy in both sparsely and

heavily populated areas Better use of available frequencies

in order to improve the user capacity of Mobile Communications.

Large selection of interfaces to other services such as PSTN/ISDN, PSPDN etc.

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SPECIFICATIONS Trunked Radio system. Access Method - TDMA/FDMA Frequency Bands

Mobile to Cell - 890-915 MhzCell to Mobile - 935-960 Mhz

Therefore the frequencies for the uplink and the downlink are different ( Frequency Division Duplex) and the bandwidth of the GSM System is 25 Mhz..

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SPECIFICATIONS

Cell To Mobile – Down Link

Mobile to Cell – Up Link

935-960 Mhz

1805-1880 Mhz

890-915 Mhz

1710-1785 Mhz

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SPECIFICATIONS Channel Bandwidth – 200 Khz Number of Channels – 124 channels

pairs ( 25 Mhz/200 Khz )are available spaced at 200 Khz intervals

Due to interference to other systems, the very first carrier is not used.

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ADVANCED LEVEL TELECOM TRAINING CENTREGSM - MULTIPLE ACCESS GSM uses both FDMA & TDMA

Freq

Mhz. 890.2

1

890.4

2

890.6

3

890.8

4

891.0

5 6

914.8

124

• FDMA Access along Frequency axis

• Total 124 RF Channels available. One or more carrier assigned to each base station

• Absolute Radio Freq Carrier Number (ARFCN) 1 & 124 not used untill it is co-ordinated with Non -GSM operators in adjacent freq. bands.

• In most cases 124 RF Channels are used

……...

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Specifications Therefore the actual frequency of

an ARFCN is Ful(n)= 890.0 Mhz +(0.2 Mhz)*n And Fdl(n)= Ful(n) + 45 Mhz Where n= 1<=n<=124

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FDMA/TDMA Each channel pair serves eight ( Full Rate ) or

Sixteen (half rate) channels successively in a Timed Cycle.

In FDMA system each channel requires a transmitter and receiver

8 Voice channels, staggered in Time are transmitted via one carrier frequency

Each of the 124 FDMA linked carrier frequency is used to transmit 8 TDMA voice channels

Discontinuous nature of TDMA makes the entire 200 Khz bandwidth available to all 8 Voice channels.

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GSMFDMA

25 MHz 25 MHz

Mobile to Base

0 1 2

890.2 890.4 890.6(MHz)

Base to Mobile

0 1 2

935.2 935.4 935.6

200 kHz45MHz

Channel layout and frequency bands of operation

890 935 960915

200 kHz

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GSMTDMA

8765

43

21

8765

43

21

45 MHz

FrequencyF2’F1’

(Cell transmit)F2F1

(Cell Rx)

Amplitude

Typical TDMA/ FDMA frame structure

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ADVANCED LEVEL TELECOM TRAINING CENTREFDMA/TDMA Scheme

BP1BP2BP3BP4BP5BP6BP7BP8BP1BP2

TIME

890.0

890.2890.4

890.6890.8

891.0891.2 915.8

FREQ

MHz

BURST

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8 7 6 5 4 3 2 1

1 2 3 4 5 6 7 8

1

1

1

1

1

2

2

2

2

2

3

3

3

3

3

4

4

4

4

4

6

6

6

6

6

7

7

7

7

7

8

8

8

8

8

5

5

5

5

5

FRAME OF 8 TIME SLOTS

FRAME REPETITION

PHYSICAL CHANNELS

PHYSICAL CHANNELS

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

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BURSTING Therefore the mobile transmits

only in one time slot and stay idle for the remaining seven time slots with its transmitter off.

This periodic switching (ON&OFF) is called BURSTING ( 216.6 Hz=1/4.615ms)

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MODULATION METHOD Modulation Method

GMSK ( Gaussian Minimum Shift Keying )

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Duplex Operations Frequency Division Duplex

Frequency for uplink and downlink are different.

Time Division DuplexThe mobile station transmits 3 timeslots latter than the BTS but the Time slot number stays same as if both are using the same time slot at the same time.

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Duplex Operation

0 1 2 3 4 5 6 7 0 1 2 3

5 6 7 0 1 2 3 4 5 6 7 0

BTS Transmits

MS Transmits

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Timing Advance & Power Control In a cell MS can be at different

distances, hence different delay and diff. Attenuations

Due to above the collisions of signals from mobiles assigned to adjacent time slots

Mobiles farthest from the base station are given a head start.

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Timing Advance & Power Control To compensate for the attenuation

over different distances within the cell, the BTS commands the MS to use different power levels in such a way that the power arriving at the base station receiver is approx same for each time slot.

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Timing Advance & Power Control

A Long signal delayHigh signal Attenuation

BShort Signal DelaySmall signal Attenuation

BTS

AB

TSn TSn+1

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Logical channels Traffic Channel :Used for

Transmission of Speech Full Rate Half Rate

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Logical channels Control Channel : Intended to carry

signalling and synchronisation for network management messages and channel maintenance task. Broad Cast Channel BCH Common Control Channel CCCH Dedicated Control Channel DCCH

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Broadcast Channel Transmitted by the BTS and

are intended to provide sufficient information to the mobile station for it to synchronize with the network.

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Broadcast Channel Broadcast Control Channel

BCCHInforms the mobile station about specific parameters it need to identify the network or to gain access to the network

( LAC, MNC, Freq. Of neighboring cells etc.)

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Broadcast Channel Frequency correction Channel

FCCH Informs the mobile station about the frequency reference of the system. This logical channel is mapped onto the freq. Correction burst. It appears on frequency correction burst and freq. Corr. Burst contains only FCCH.

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Broadcast Channel Synchronization Channel SCH

Informs the mobile station about the key it needs to be able to demodulate the information coming from the base station.It is mapped on to synchronization burst.

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Common Control ChannelThey support the

establishment of a dedicated link between a mobile and base station .

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Common Control Channel Random Access Channel

It is used by the MS to request a dedicated channel from the network. It is mapped into the random access burst and contains the first message sent to the BTS.

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Common Control Channel Paging Channel PCH

BTS calls individual MSs within its cell on this channel.

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Common Control Channel Access Grant Channel AGCH

MS gets information from the BTS on which dedicated channel it should use for its immediate needs from the access grant channel. The information about the timing advance also comes on this channel.

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Dedicated Control Channel They are used for message

transfer between the network and the MS, not the traffic. The messages are needed for registration, authentication and call set up. They are also used for low level signalling messages for channel maintenance.

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Dedicated Control Channel Standalone Dedicated Control

Channel : SDCCH It is intended for the transfer of signaling information between a mobile and a BTS

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Dedicated Control Channel Slow Associated Control Channel :

SACCH It is used in association with either traffic channel or SDCCH. The purpose is channel maintenance to maintain a link between mobile and BTS. In the downlink – latest information about the system ( Timing advance and power control ) and in the uplink– measurement performed on the neighboring cells.

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Dedicated Control Channel Fast Associated Control

Channel : FACCH If during a call there is a need for some heavy duty signalling, then FACCH appears in place of the traffic channel.

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ADVANCED LEVEL TELECOM TRAINING CENTREOPERATIONAL CONCEPTS• Subscribers are not allocated dedicated channels

• TCH Allocated to users only when needed

• Hence IDLE MODE & DEDICATED MODE

• DEDICATED MODE -- When a full Bi -directional P to P CHL has been allocated during an established call • IDLE MODE

-- When MS is powered on (active) without being in dedicated mode

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ADVANCED LEVEL TELECOM TRAINING CENTREOPERATIONAL CONCEPTS

IDLE MODE -- When MS is powered on (active) without being in dedicated mode

• MS stays continuously in touch with BS

• Listens to transmissions from BS to intercept Paging Messages ( for incoming calls)

•

• Monitors Radio Environment in order to evaluate Chl Quality & choose the most suitable BS • Listens to BS to avail short message broadcast service

Yuri Sherman

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Introduction of Gsm