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GSM - RADIO INTERFACE. IN THIS PRESENTATION. Radio Interface. Frequency Bands & Specifications. Multiple Access Method FDMA & TDMA. FDMA /TDMA Frame Representation. Logical Channels Traffic & Control. Operational Concepts.

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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.

GSM - RADIO INTERFACE


Communication mobile

Inter-Exchange

Junction

Subscriber

Line

(2W)

BSC

BTS

MS

Telephone

Exchange

Mobile Switching

Centre (MSC)

Communication - Mobile


Gsm radio interface

Most Important Interface

GSMRADIO INTERFACE

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

  • To increase spectral efficiency

    -- Large number of simultaneous calls in a given

    bandwidth

    -- Frequency Reuse

    -- Interference

    -- Use of Interference Reduction Techniques


Gsm uplink downlink

Frequency Bands

GSM 900 Mhz

DCS 1800 MHz

UPLINK

DOWNLINK

B

T

S

GSMUplink & Downlink


Gsm specifications
GSM Specifications

RF Spectrum :

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


Carrier Separation - 200 kHz

Duplex Distance - 45 MHz

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

GSM Specifications


Gsm multiple access

GSM uses both FDMA & TDMA

1

2

3

4

5

6

124

Freq

Mhz.

890.2

890.4

890.6

890.8

914.8

891.0

GSM - MULTIPLE ACCESS

  • FDMA Access along Frequency axis

  • Each RF carrier 200khz apart

  • Total 124 RF Channels available.

  • One or more carrier assigned to each base station

……...


  • Thus for practical purposes only 122 RF Carriers are available.

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

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

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

Here 124.

GSM - MULTIPLE ACCESS


Gsm fdma

890

915

935

960

25 MHz

25 MHz

1

2

1

2

0

0

Mobile to Base

Base to Mobile

(MHz)

890.4

890.6

935.4

935.6

890.2

935.2

200 kHz

200 kHz

45MHz

Channel layout and frequency bands of operation

GSMFDMA


Gsm tdma

8

8

7

7

6

6

5

5

4

4

3

3

2

2

1

1

GSMTDMA

Amplitude

45 MHz

Frequency

F1

(Cell Rx)

F2

F1’

(Cell transmit)

F2’

Typical TDMA/ FDMA frame structure


Gsm digital voice transmission
GSMDigital Voice Transmission

Speech Coding

- 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.


Gsm digital voice transmission1
GSMDigital Voice Transmission

Channel Coding

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

Interleaving

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

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


Gsm digital voice transmission2

- 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 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”.

GSMDigital Voice Transmission

Burst Formatting

To counteract the problems encountered in radio path:


D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

3

57

1

1

57

3

8.25

26

Interleaving & Burst Formating

1st Sample of 20 ms speech

2nd Sample of 20 ms speech

456 bits

Sample 2

456 bits

Sample 1

1 2 3 4 5 6 7 8

Stream of Time Slots

Normal Burst


Gsm speech to radio waves

Analog

Analog

Speech Coding

Speech Decoding

Channel Coding

Channel Decoding

Interleaving

De-interleaving

Burst formatting

Burst formatting

Ciphering

Deciphering

Modulation

Demodulation

200kHz BW

GSMSpeech to Radio waves

200kHz BW


Fdma tdma scheme

TIME

BP2

BP1

BP8

BURST

BP7

BP6

BP5

BP4

BP3

BP2

BP1

FREQ

MHz

890.2

890.6

891.0

915.8

890.0

890.4

890.8

891.2

FDMA/TDMA Scheme

F

R

A

M

E


3

T

142

Fixed Bits

8.25

GP

3

T

CC

SMS

SS

CM

  • FCCH Burst

3

T

57

Encrypted

8.25

GP

3

T

CC

57

Encrypted

SMS

SS

1S

26

Training

1

S

CM

  • Normal Burst


8

8

8

7

7

7

6

6

6

5

5

5

4

4

4

3

3

3

2

2

2

1

1

1

6

6

6

6

4

1

2

3

5

7

8

6

4

1

2

3

5

7

8

8

4

7

1

2

3

5

7

8

6

4

1

2

3

5

7

8

5

4

4

1

2

3

5

7

8

3

2

1

PHYSICAL CHANNELS

8

7

6

5

4

3

2

1

FRAME OF 8 TIME SLOTS

FRAME REPETITION

8

7

6

5

4

3

2

1

PHYSICAL CHANNELS

1


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 radio interface cycles

Hyperframe = 2048 Superframes

3 Hours 28 Minutes 53 Seconds and 760 milliseconds

0

2047

Superframe = 26× 51 multiframes

6.12 Seconds

0

50

0

25

26 Multiframe

120 mS

51 Multiframe

Approx 235 mS

0

1

2

24

25

0

1

48

49

50

TDMA frame

4.615 mS

0

1

2

3

4

5

6

7

GSM Radio Interface - CYCLES


Organisation of speech data

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

BP 0

BP 1

BP 2

BP 3

BP 4

BP 5

BP 6

BP 7

3

57

1

26

1

57

3

8.25

Organisation of Speech & Data

Frames 0-11 : TCH

Frames 12 : SACCH

Frames 13-24 : TCH

Frames 25 : Unused

26 – frame multiframe

Duration: 120 ms

TDMA frame

Duration: 60/13 ms

=4.615 ms

Tail

bits

Tail

bits

Guard

bits

Training sequence

Stealing

bit

Stealing

bit

Data bits

Data bits

Normal burst

Duration 15/26 ms


Logical

Channels

Traffic

Channels

Control

Channels

Common

Control

Channels

Dedicated

Control

Channels

Broadcast

Channels

Logical Channels

Full-Rate

Half-Rate

FCCH

SCH

BCCH

RACH

PCH

AGCH

SDCCH

SACCH

FACCH


GSM

LOGICAL CHANNELS

USER INFORMATION( TRAFFIC)

SIGNALLING INFORMATION (CONTROL)


GSM

  • CONTOL CHHANELS OVER LOGICAL CHANNELS

  • Intended to carry signalling and synchronisation

  • THREE TYPES OF CONTROL CHANNELS

  • 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


  • P- MP For Freq Correction FCCH

  • For Syncronisation SCH

  • BCCH

  • Common control channel CCCH

  • For ACCESS Management PCH

  • RACH

  • AGCH

  • Dedicated control channel DCCH

  • P - P For Registration SDCCH

  • ,authentication SACCH

  • & Handover FACCH

GSM

  • THREE TYPES OF CONTROL CHANNELS


OPERATIONALCONCEPTS

IDLE MODE

ACCESS PROCDURE

DEDICATED MODE

  • IDLE MODE

----FCCH

---- SCH

----BCCH

  • MS O/G Call

----RACH

----AGCH

----SDCCH

----TCH

  • MS I/C Call

----PCH

----RACH

----AGCH

----SDCCH

----TCH


GSM – RF Interface

Other Salient Features Of GSM RF I/F:

- Control of Transmitted Power.

  • Discontinuous Transmission.

- Timing Advance.

- 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.

Discontinuous Transmission (DTX)


GSM – RF Interface

Timing Alignment :

- 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.

TDMA Frame

0

1

2

3

4

5

6

7

B –on TS2

TS3

TS2

A –on TS3

BTS


GSM – RF Interface

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.

Start Sending

- 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


GSM – RF Interface

Antenna Diversity :

Space Diversity -

- Mounting two receiver antenna physically separated a distance.

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

  • at same time is low.

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

Polarization Diversity -

- Dual polarized antenna – vertical and horizontal arrays.

Tx

Tx

Rx

Rx (A)

Rx ( B)

No Diversity

Antenna Diversity


Frequency hopping
FREQUENCY HOPPING

  • Change of frequency after every frame in a pre-determined manner

  • SFH improves performance in multi-path fading

  • Provides interference diversity

  • Decreases required C/I

  • Mandatory for MS when requested by BS

  • FCCH ,SCH ,BCCH are not hopped

  • Algorithm : Cyclic or pseudorandom


GSM - RADIO INTERFACE

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 Alignment

  • Diversity, Fr. Hopping,

  • Power Control.



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