GSM - RADIO INTERFACE

IN THIS PRESENTATION

GSM - RADIO INTERFACE

• Radio Interface

• Frequency Bands & Specifications

• Multiple Access Method FDMA & TDMA

• Modulation Method

• 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.

Communication - Mobile

TelephoneExchange

SubscriberLine (2W)

Inter-ExchangeJunction

Mobile SwitchingCentre (MSC)

BSC BTSMS

GSMRADIO INTERFACE

• Most Important Interface

• To increase spectral efficiency

-- Large number of simultaneous calls in a given

bandwidth

-- Frequency Reuse

-- Interference

-- Use of Interference Reduction Techniques

• Full Compatibility between mobile stations of various Manufacturers & Networks of different vendors to help roaming

GSMUplink & Downlink

• Frequency Bands GSM 900 Mhz DCS 1800 MHz

BTS

UPLINK

DOWNLIN

K

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 :

GSM Specifications

Carrier Separation - 200 kHz

No. of RF Carriers - 124

Access Method - TDMA/FDMA

Modulation Method - GMSK

Transmission Rate - 270.833 Kbps

Speech Coding - Full rate 13 Kbps Half rate 6.5 Kbps

Duplex Distance - 45 MHz

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

……...

• Absolute Radio Freq Carrier Number (ARFCN) 1 and 124 not used until 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 1 ≤ n ≤ 124.

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

GSMTDMA

8765

43

21

8765

43

21

45 MHz

Frequency

F2’F1’(Cell transmit)

F2F1(Cell Rx)

Amplitude

Typical TDMA/ FDMA frame structure

GSMDigital Voice Transmission

- In GSM speech coding a block of 20 ms is encoded in one set of 260 bits.

- This calculates as 50X 260 = 13 kbps. Thus GSM speech coder produces 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

GSMDigital Voice Transmission

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

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

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

Channel Coding

Interleaving

GSMDigital Voice Transmission

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

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

- To counteract the problems encountered in radio path.

Burst Formatting

- Each TS of TDMA frame is 0.577 ms long and during this time 156.25 bits are transmitted.

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

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

GSM Speech 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

FDMA/TDMA Scheme

BP1

BP2

BP3

BP4

BP5

BP6

BP7

BP8

BP1

BP2

TIME

890.0

890.2890.4

890.6890.8

891.0891.2 915.8

FREQ

MHz

BURSTF

R

A

M

E

CM

CC SMS SS

Normal Burst

3

T

57

Encrypted

1S

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

GSM Radio Interface - CYCLES

20470

Hyperframe = 2048 Superframes

3 Hours 28 Minutes 53 Seconds and 760 milliseconds

500

250

Superframe = 26× 51 multiframes6.12 Seconds

51 MultiframeApprox 235 mS

26 Multiframe120 mS

2524210 50494810

76543210

TDMA frame

4.615 mS

Organisation of Speech & Data

24 2523222120191817161514131211109876543210

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

8.253571261573

Frames 0-11 : TCH Frames 12 : SACCH Frames 13-24 : TCH Frames 25 : Unused

26 – frame multiframeDuration: 120 ms

TDMA frameDuration: 60/13 ms

=4.615 ms

TaiTaill

bitbitss

Data Data bitsbits

StealinStealingg

bitbit

Training Training sequencsequenc

ee

StealinStealingg

bitbit

Data Data bitsbits

TailTailbitsbits

GuarGuardd

bitsbitsNormal burstNormal burstDuration 15/26 Duration 15/26 msms

GSM-- TDMA STRUCTURE

• TDMA 8 Time Slots / RF Channel

• Time slot duration 0.577m sec or 15 / 26 m sec

• Frame 8 Burst Periods ( Time Slots)

= 8 15/26 = 4.615 m sec• Multi Frame Traffic 26 4.615 = 120 msec

Control 51 4.615 = 235.365 m sec• Super Frame 51 Traffic Multi frames

26 Control Multi frames • Hyper Frame 2048 Super Frames = 3 28 52.76

hr min sec

GSM

LOGICAL CHANNELS• USER INFORMATION( TRAFFIC)• SIGNALLING INFORMATION (CONTROL)

GSM CONTOL CHHANELS OVER LOGICAL CHANNELS

THREE TYPES OF CONTROL CHANNELS

• Intended to carry signalling and synchronisation

• Broadcast control channel BCCH

• Common control channel CCCH

• Dedicated control channel DCCH

OPERATIONAL 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 MODE

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

GSM

THREE TYPES OF CONTROL CHANNELS

Dedicated control channel DCCH

P - P For Registration SDCCH ,authentication SACCH & Handover FACCH

Broadcast control channel BCCH

P- MP For Freq Correction FCCH For Syncronisation SCH

BCCH Common control channel CCCH

For ACCESS Management PCH RACH

AGCH

OPERATIONAL 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

OPERATIONAL CONCEPTS

ACCESS PROCEDURE

-- Access to system ( switch over from IDLE to DEDICATED Mode)

• MS indicates to BS that it needs a connection

• BS accepts the request & indicates which traffic CHL it may use • For above purpose specific transmission is done over “ Common Channels”

OPERATIONAL CONCEPTS MOBILE O/G Call

• MS sends access over RACH

• System allocates SDCCH through AGCH

• Lastly TCH is assigned through SDCCH when a conversation can start

• Set up information exchanged over SDCCH ( Authentication , Measurement Reports, Power Control)

OPERATIONAL CONCEPTS MOBILE I/C Call

• Paging to MS through PCH since MS is monitoring PAGCH

• MS responds by sending a page response over RACH

• Set up information exchanged over SDCCH ( Authentication, Call set-up messages , Power Control)

• As a result system allocates SDCCH to MS over AGCH

• Lastly TCH is allocated to mobile over SDCCH . Mobile starts conversation.

OPERATIONAL CONCEPTS IDLE MODE

• IDLE MODE

ACCESS PROCDURE DEDICATED MODE

• MS O/G Call

• MS I/C Call

----FCCH---- SCH----BCCH

----RACH----AGCH

----SDCCH----TCH

----PCH----RACH----AGCH

----SDCCH----TCH

Other Salient Features Of GSM RF I/F:

GSM – RF Interface

- Control of Transmitted Power.

- Timing Advance.

- Discontinuous Transmission.

- Diversity.

- Frequency Hopping.

Discontinuous Transmission (DTX)

• Speech activity only 40% of time.

• Needs Voice activity detection.

• Determination of voice threshold vis-à-vis noise.

• Annoying clicks/inefficient DTX.

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

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

Timing Advance : ( To counteract problem of Time Alignment )

- MS instructed to do its transmission certain bit-times earlier or later – to 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

Start Sending

GSM – RF Interface

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 Diversity Antenna Diversity

Tx Rx Rx (A) Rx ( B) Tx

Polarization Diversity -- Dual polarized antenna – vertical and horizontal arrays.

FREQUENCY HOPPING

• Change of frequency after every frame in a pre-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

SUMMARY

GSM - RADIO INTERFACE

• Radio Interface

• Frequency Bands & Specifications

• Multiple Access Method FDMA & TDMA

• Modulation Method

• 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.

MOBILE COMMUNICATION

THANKS !!