GSM Mobile Computing IT644. GSM System Architecture Network Subsystem MSC ?? Radio Subsystem BTS, BSC Operation Support Subsystem.

GSM

Mobile Computing

IT644

GSM System Architecture

Network Subsystem MSC ??

Radio Subsystem BTS, BSC

Operation Support Subsystem

GSM System Hieararchy

Location Area

MSC R......

GSM Network

MSC Region

Location Area

BSC

BSCLocation Area

.

.

.

.

.

..

MSC R.

Operations and Maintenance Centre

BSC MSC

G MSC

BSC

BTS

BTS

MS BTS

MS

OMC

HLR

VLR

EIR

AVC

Home Location Register

Visitor Location Register

Equipment Identity Register

Authentication centre

The Mobile Station (MS)

Comprises :– Mobile Equipment

– SIM (Subscriber Identity Module ??)

Other Identification :• International Mobile Station Equipment Identity (IMEI)• International Mobile Subscriber Identity (IMSI)• Mobile Subscriber ISDN Number (Mobile Telephone No.)

Radio Network

Base Subsystem Controller ? (BSC)

- Handles essential control and protocol intelligence.

- Handover is executed at the BSC.

Base Transceiver System ? (BTS)

- Is a high frequency Transmitter/Receiver.

- Handles error protection; coding/decoding for the radio channel.

Frequency Channels in GSM

• Uplink frequency band : 890 - 915 MHz• Downlink frequency band : 935 - 960 MHz• 124 channels (of 200 kHz each) in each band. ?? • Each channels has a TDMA structure with 8 timeslots. ( => upto 8 users per freq. channel )

•

124

•

•

•

1

2

1 2 73 4 5 6 8

TDMA structure of each channel

935 Mhz

960 Mhz

1 2 73 4 5 6 8

1 2 73 4 5 6 8

Delay

Downlink

Uplink

So the MS does not have to Transmit and Receive at the same time instance!

Logical Channels

Traffic Channels (TCH)

Signaling Channels • Broadcast Channel (BCH)• Common Control Channel (CCH)• Dedicated/Associated Control Channel (DCCH/ACCH)

Note: These logical channels are then mapped onto Physical channels.

A GSM Physical channel comprises a particular timeslot on a given freq. Channel.

Note: These logical channels are then mapped onto Physical channels.

A GSM Physical channel comprises a particular timeslot on a given freq. Channel.

BCH :• Broadcast Control Channel (BCCH)• Frequency Correction Channel (FCCH)• Synchronization Channel (SCH)

CCH :• Random Access Channel (RACH)• Paging Channel (PCH)

D/ACCH• Stand-alone Dedicated Control Channel (SDCCH)• Slow Associated Control Channel (SACCH)

Signalling channel contd. ....

Traffic channel structure for full rate coding

1 2 73 4 5 6 8 1 2 73 4 5 6 8 1

1 2 73 4 5 6 8 9 10 • 11 12 13 14 • 26

Traffic

Signaling (S)

TDMA slots

TDMA Frameson a given channel

S-contains information about the signal strength in neighboring cells

Traffic channel structure for half rate coding

1 2 73 4 5 6 8 1 2 73 4 5 6 8 1

1 2 73 4 5 6 8 • • • • • • 26

1 2 73 4 5 6 8 9 10 11 12 13 14 • • 26

• •

Burst for one user

Burst for another user

Why 26 burst Traffic Channel?

• Mobile needs to send information about signal strength from surrounding cells to the network.

• Capacity required to send measurement info is 1/24 the capacity required to send voice!

Signaling Channel – Control Channel

Associated Control Channel

Slow Associated Control Channel

Adaptive Frame Synchronization

Timing Advance:

• MS advances its burst transmission by a time corresponding to round trip time.• The delay is quantiled as a 6 bit number. => 64 steps (0-63); each step advances the Timing by one bit duration ie 3.7 s.• 64 steps allows compensation over a maximum propagation time of 31.5 bit periods ie 113.3 s ( => a maximum distance of ~ 35 km)

Timing Advance : How it works.

1 2 3 4 5 6 7

(Sent by BS on down link)

1 2 3 4 5 6 7

8

(received by BS on up link)

(received by MS on down link)

(Sent by MS on up link)

|||||

8

1 2 3 4 5 6 7

8

1 2 3 4 5 6 7 8

||||

One wayPropagationdelay

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|

|

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Two way propagation delay

Transmission Bursts

• Normal Burst• Synchronization Burst• Frequency Correction Burst• Dummy Burst• Access Burst

Control Channel

Functions:• To help the MS find the control channels. ??• To provide information about

- voice and control channel repetition cycle. - parameters in the cell.

- surrounding cells.- paging.

• To allow random access attempts by the MS.

The Downlink Control Channel

• 51 cycle burst (2x26 -1)• Third burst on the control channel provides content information.• Basic structure of control channel : FSxxx xxxxx FSxxx xxxxx FSxxx xxxx .... F: Frequency Correction Channel

(occurs every 10th burst) S: Synchronization Channel

TDMA Bursts in GSM

142 fixed bits 33

339

data39

data3

64 bitTraining seq

8.25

3 326 bit

Training seq8.25

8.25

841 bit

Training seq36 data 3 68.25

FB

SB

DummyBurst

Access

Burst

Fig.

Normal Burst

Tail bit

Stealing Flags

357

Data bits26 bit

Training seq57

Data bits3

8.25Bit GP

F S B B B

F S B B B

F S B B B

Frequency synchronization

Time synchronizatio

nBCCH

R R R RRequest

(Random access)

C C C C Grant

D D D

Full Rate Speech Coding

• Coder for 20ms segments - 120 bits at the

output.

• 13 Kbps.

•Unequal error protection:

Out of 260 bits

- 182 bits are protected.

- 78 bits are not protected.

Error Coding

Class 1a - 3 parity bits from 50 bits.

Class 1b - 132 bits are not parity checked,

but fed to convolutional encoder.

Class 2 - 78 bits are not protected.

18278

A Block Encoder (53, 50)

D D D+ +

G(D) = 1 + D+ D3

1 - 50 clock cycles, SW closed51 - 53 clock cycles, SW open

SW

data

Last 3 parity bits

First 50 bits

Convolutional Encoder

K- constraint length k=5R- rate of code r=2/1

K – storage location

G0 = d4 + d3 + 1G1 = d4 + d3 + d + 1

R=1/2 k=5; 378 78

456 bits

50 3 4132 78

Class 1a50 bits

Class 1b132 bits

Class 278 bits

456 bits in 20 ms ~ 456/0.02 = 22.8 kbps

Tail bits

Paritybits

Interleaving

1. Block Interleaving:Code words are written line by line to a matrix and read column by column.??8 (col) * 57 (row) matrix

2. Diagonal Interleaving

1 2 3 4

5 6 7 8

Discontinuous Transmission

• On an average speech actually lasts only 50% of the time.

• So transmitter is kept off whenever there is no speech.

• This reduces co-channel interference and saves battery power.

• Voice Activity Detector (VAD) is used at the transmitter, and Comfort Noise Generation (CNG)

is used at the receiver.

VAD

• Background noise is stationary over relatively long periods.• Measure the deviations from the spectral characteristics of the background noise.

CNG

• Comfort noise characteristics are matched to the transmitted noise.