RF Interface P

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GSM - RADIO INTERFACE


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IN THIS PRESENTATION Radio Interface

Frequency Bands & Specifications

Multiple Access Method FDMA & TDMA

FDMA /TDMA Frame Representation

Logical Channels : Traffic & Control

Operational Concepts

Other Salient Features of RF I/F- DTX, Time Alignment

Diversity, Fr. Hopping,

Power Control.


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Communication - Mobile

TelephoneExchange

SubscriberLine(2W)

Inter-Exchange

Junction

Mobile SwitchingCentre (MSC)

BSC BTS

MS


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GSMRADIO INTERFACE

Most Important Interface

To increase spectral efficiency

-- Large number of simultaneous calls in agiven bandwidth

-- Frequency Reuse

-- Interference

-- Use of Interference Reduction Techniques

Full Compatibility between mobile stations ofvarious Manufacturers & Networks of differentvendors to help roaming


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GSMUplink & Downlink

Frequency Bands

GSM 900 Mhz

DCS 1800 MHz

BTS


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GSM Specifications

GSM 900

Mobile to BS (UP-LINK) - 890 to 915 MHz

BS to Mobile (DOWN -LINK) - 935 to 960 MHz

Bandwidth - 25 MHz

GSM 1800 ( DCS ) :

Mobile to Cell (UP-LINK) - 1710 to 1785 MHz

Cell to Mobile (DOWN -LINK) - 1805 to 1880 MHz

Bandwidth - 75 MHz

RF Spectrum :


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GSM Specifications

Carrier Separation - 200 kHz

No. of RF Carriers - 124Access Method - TDMA/FDMA

Modulation Method - GMSK

Transmission Rate - 270.833 Kbps

Speech Coding - Full rate 13 KbpsHalf rate 6.5 Kbps

Duplex Distance - 45 MHz


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GSM - 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

Each RF carrier 200khz apart

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

...


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

Thus for practical purposes only 122 RF Carriers are available.

F up-link (n) = 890.2 +0.2* ( n-1 ) MHz

F down-link (n) = 935.2 +0.2* ( n-1 ) MHz

Frequency for any ARFCN ( n) can be calculated from :

Here 124.

GSM - MULTIPLE ACCESS


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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 kHz45MHzChannel layout and frequency bands of operation

890 935 960915

200 kHz


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GSM

TDMA

87

65

43

21

87

65

43

21

45 MHz

Frequency

F2F1(Cell transmit)

F2F1(Cell Rx)

Amplitude

Typical TDMA/ FDMA frame structure

GSM


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GSMDigital Voice Transmission

- In GSM speech coding a block of 20 ms is encoded in one set of260 bits.

- This calculates as 50X 260 = 13 kbps. Thus GSM speech coderproduces a bit rate of 13 kbps per subscriber.

- This provides speech quality which is acceptable for mobile

telephony and comparable with wire-line PSTN phones.

Speech Coding


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01100011000111110011100

Speech Code

20 ms

Speech Signal

Speech Coding

Parameters like tone,length of tone,pitch are transmittedSampling=50 times/sec instead of 8000 of 260 bits each

overall bitrate= 50x260x8 subs=104kbps


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Channel Coding

Block

coder50 Very important bits

132 Important bits

78 Not so important bits

1:2Convolutional

Coder

456

4 Tail bits

53 bits 378 bits

3 parity bits260 bits

Detection & correction of errors


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- It uses 260 bits from speech coding as input and outputs 456encoded bits.

- In one burst one block of 57 bits from one sample and anotherblock from another sample are sent together.

- These 456 bits for every 20 ms of speech are interleaved formingeight blocks of 57 bits each.

Channel Coding

Interleaving


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To counteract the problems encountered in radio path:

Burst Formatting

- Additional bits as training sequence added to basic speech/data.

- Total of 136 bits added, bringing overall total to 592 bits.

- Each TS of TDMA frame is 0.577 ms long and during this time156.25 bits are transmitted.

- One burst contains only 148 bits. Rest of the space, 8.25 bitstime, is empty and is called Guard Period ( GP ).

- GP enables MS/BTS to ramp up and ramp down.


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8.253571261573

1 2 3 4 5 6 7 8

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

D D D D DDD D D D D D DDD D

456 bits

Sample 1

456 bits

Sample 2

Normal Burst

Stream of Time Slots

Interleaving & Burst Formatting

1st Sample of 20 ms speech 2nd Sample of 20 ms speech

GS


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GSMSpeech to Radio waves

Speech Coding

Channel Coding

Interleaving

Burst formatting

Ciphering

Burst formatting

Deciphering

Analog

Modulation

200kHz BW

Speech Decoding

Analog

200kHz BW

Demodulation

De-interleaving

Channel Decoding


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FDMA/TDMA Scheme

BP1

BP2

BP3

BP4

BP5

BP6

BP7

BP8

BP1

BP2

TIME

890.0890.2

890.4890.6

890.8891.0

891.2 915.8

FREQ

MHz

BURSTF

R

A

M

E


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CM

CC SMS SS

Normal Burst

3

T

57

Encrypted

1

S

26

Training

1

S

57

Encrypted

3

T

8.25

GP

CM

CC SMS SS3

T

142

Fixed Bits

3

T

8.25

GP

FCCH Burst


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Other Salient Features Of GSM RF INTERFACE:

GSM RF INTERFACE

- Control of Transmitted Power.

- Timing Advance.

-Discontinuous Transmission.

- Diversity.

- Frequency Hopping.


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POWER CONTROL

To minimize co-channel interference and toconserve power, both the Mobile and BTSoperate at the lowest power level that will

maintain an acceptable signal quality.

Mobile decides that power level is acceptableusing bit errors ratio.


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DISCONTINUOUS TRANSMISSION(DTX)

The idea is based on the fact that a personspeaks less than 40% of time in normalconversation, so turning the transmitter off cansave power.

In order to distinguish voice and backgroundnoise, very accurate Voice Activity Detectorshould be used.

While transmitter is off, the receiving end will heara total silence, i.e. due to digital transmission.

To avoid this, comfort noise is generated trying tomatch the characteristics of background noise.


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DISCONTINUOUS RECEPTION (DRX)

While being in Idle Mobile Station has to listenonly to Paging Channel, that uses almost no

power.


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DISCONTINUOUS TRANSMISSION(DTX)

Speech activity only 40% of time.

Needs Voice activity detection.

Determination of voice threshold vis--vis noise.

Annoying clicks/inefficient DTX.

Generation of Comfort Noise at receiver to avoidthe feeling of the set being dead.


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Timing Alignment :

GSM RF Interface

- Large distance between BTS and MS causes the problem.

- Each MS on call is allocated a timeslot on TDMA frame.

- The problem occurs when the information transmitted by MS does not

reach BTS on allocated timeslot.

0 1 2 3 4 5 6 7

BTS

TDMA Frame

A on TS3

B

on TS2TS3

TS2


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Timing Advance : ( To counteract problem of Time Alignment )

- MS instructed to do its transmission certain bit-times earlier or laterto reach its timeslot at BTS in right time.

- In GSM systems maximum 63 bit-times can be used.

- This limits the GSM cell size to 35 Km radius.

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

Time

StartSending

GSM RF Interface


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Antenna Diversity :

GSM RF Interface

- Mounting two receiver antenna physically separated a distance.

- At 900 MHz with antenna spacing of 5-6 m we get 3 db gain.

Space Diversity

- Probability of both of them being affected by a deep fading dip

at same time is low.

No Diversit Antenna Diversit

Tx Rx Rx (A) Rx ( B)Tx

Polarization Diversity -

- Dual polarized antenna vertical and horizontal arrays.


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FREQUENCY HOPPING

Change of frequency after every frame in apre-determined manner

SFH improves performance in multi-path fading

Decreases required C/I

Mandatory for MS when requested by BS

FCCH ,SCH ,BCCH are not hopped Algorithm : Cyclic or pseudorandom

Provides interference diversity


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SUMMARY


Radio Interface Frequency Bands & Specifications

Multiple Access Method FDMA & TDMA

FDMA /TDMA Frame Representation

Logical Channels Traffic & Control

Operational Concepts

Other Salient Features of RF I/F- DTX, Time AlignmentDiversity, Fr. Hopping,

Power Control.

RF Interface P

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