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GSM CHANNELSGSM CHANNELS

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• Physical channelPhysical channel - Each timeslot on a carrier is referred to as a physical channel. Per carrier there are 8 physical channels.

• Logical channelLogical channel - Variety of information is transmitted between the MS and BTS. There are different logical channels depending on the information sent. The logical channels are of two types

– Traffic channel

– Control channel

Downlink

Uplink

CHANNELSCHANNELS

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GSM Traffic ChannelsGSM Traffic Channels•

Traffic Channels

TCH/FFull rate 22.8kbits/s

TCH/HHalf rate 11.4 kbits/s

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GSM Control ChannelsGSM Control Channels

BCH ( Broadcast channels )Downlink only

Control Channels

DCCH(Dedicated Channels)Downlink & Uplink

CCCH(Common Control Chan)Downlink & Uplink

Synch.

ChannelsRACH

Random Access Channel

CBCHCell Broadcast

Channel

SDCCHStandalone dedicated

control channel

ACCHAssociated

Control Channels

SACCHSlow associated Control Channel

FACCHFast Associated

Control Channel

PCH/AGCH

Paging/Access grant

FCCHFrequency

Correction channel

SCHSynchronisation

channel

BCCHBroadcast

control channel

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BCH ChannelsBCH Channels• BCCH( Broadcast Control Channel )

– Downlink only

– Broadcasts general information of the serving cell called System Information

– BCCH is transmitted on timeslot zero of BCCH carrier

– Read only by idle mobile at least once every 30 secs.

• SCH( Synchronisation Channel )

– Downlink only

– Carries information for frame synchronisation. Contains TDMA frame number and BSIC

• FCCH( Frequency Correction Channel )

– Downlink only.

– Enables MS to synchronise to the frequency.

– Also helps mobiles of the ncells to locate TS 0 of BCCH carrier.

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CCCH ChannelsCCCH Channels• RACH( Random Access Channel )

– Uplink only

– Used by the MS to access the Network.

• AGCH( Access Grant Channel )

– Downlink only

– Used by the network to assign a signalling channel upon succesfull decoding of access bursts.

• PCH( Paging Channel )

– Downlink only.

– Used by the Network to contact the MS.

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DCCH ChannelsDCCH Channels• SDCCH( Standalone Dedicated Control Channel )

– Uplink and Downlink

– Used for call setup, location update and SMS.

• SACCH( Slow Associated Control Channel )

– Used on Uplink and Downlink only in dedicated mode.

– Uplink SACCH messages - Measurement reports.

– Downlink SACCH messages - control info.

• FACCH( Fast Associated Control Channel )

– Uplink and Downlink.

– Associated with TCH only.

– Is used to send fast messages like handover messages.

– Works by stealing traffic bursts.

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Normal BurstNormal Burst

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

57 bits 57 bits26 bits 33

FRAME1(4.615ms) FRAME2

Training sequence

Data DataTailBits

TailBits

FlagBit

FlagBit

GuardPeriod

GuardPeriod

0.546ms0.577ms

• Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH, SACCH and FACCH.

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DataData - Two blocks of 57 bits each. Carries speech, data or control info.

Tail bitsTail bits - Used to indicate the start and end of each burst. Three bits always 000.

Guard periodGuard period - 8.25 bits long. The receiver can only receive and decode if the burst is received within the timeslot designated for it.Since the MS are moving. Exact synchronization of burst is not possible practically. Hence 8.25bits corresponding to about 30us is available as guard period for a small margin of error.

Flag bitsFlag bits - This bit is used to indicate if the 57 bits data block is used as FACCH.

Training SequenceTraining Sequence - This is a set sequence of bits known by both the transmitter and the receiver( BCC of BSIC). When a burst of information is received the equalizer searches for the training sequence code. The receiver measures and then mimics the distortion which the signal has been subjected to. The receiver then compares the received data with the distorted possible transmitted sequence and chooses the most likely one.

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Frequency correction BurstFrequency correction Burst

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

142 bits 33

FRAME1(4.615ms) FRAME2

Fixed DataTailBits

TailBits

GuardPeriod

GuardPeriod

0.546ms0.577ms

• Carries FCCH channel.

• Made up of 142 consecutive zeros.

• Enables MS to correct its local oscillator locking it to that of the BTS.

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Synchronization BurstSynchronization Burst

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

39 bits 33

FRAME1(4.615ms) FRAME2

SynchronizationSequence

TailBits

TailBits

GuardPeriod

GuardPeriod

0.546ms0.577ms

64 bits 39 bits

EncryptedBits

EncryptedBits

• Carries SCH channel.

• Enables MS to synchronize its timings with the BTS.

• Contains BSIC and TDMA Frame number.

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Dummy BurstDummy Burst

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

57 bits 57 bits26 bits 33

FRAME1(4.615ms) FRAME2

Training sequence

Data DataTailBits

TailBits

FlagBit

FlagBit

GuardPeriod

GuardPeriod

0.546ms0.577ms

• Transmitted on the unused timeslots of the BCCH carrier in the downlink.

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Access BurstAccess Burst

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

41 bits 68.25 bits8

FRAME1(4.615ms) FRAME2

TailBits

TailBits

GuardPeriod

0.577ms

36 bits

SynchronisationSequence

EncryptedBits

3

• Carries RACH.

• Has a bigger guard period since it is used during initial access and the MS does not know how far it is actually from the BTS.

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Need for Timeslot offsetNeed for Timeslot offset

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

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

If Uplink and Downlink are aligned exactly, then MS will have to transmit and receive at the same

time. To overcome this problem a offset of 3 timeslots is provided between downlink and uplink

BSS Downlink

MS Uplink

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Need for Timeslot offsetNeed for Timeslot offset

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

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

As seen the MS does not have to transmit and receive at the same time. This simplifies the MS

design which can now use only one synthesizer.

BSS Downlink

MS Uplink

5

0

3 timeslotoffset

16

T15

T5

T9

T10

T11

S12

T13

T14

T6

T7

T8

T0

T1

T2

T3

T4

T16

T17

T18

T19

T20

T21

T22

T23

T24

I25

00 11 22 33 44 55 66 77 00 11 22 33 44 55 66 77 00 11 22 33 44 55 66 77

120 msec

4.615 msec26 Frame Multiframe Structure26 Frame Multiframe Structure

• MS on dedicated mode on a TCH uses a 26-frame multiframe structure.

• Frame 0-11 and 13-24 used to carry traffic.

• Frame 12 used as SACCH to carry control information from and to MS to BTS.

• Frame 25 is idle and is used by mobile to decode the BSIC of neighbor cells.

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00 11

00 11 22 20452045 20462046 20472047

1 Hyperframe = 2048 superframes = 2,715,648 TDMA frames3h 28min 53s 760ms

1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr) multiframes

11 2 33 494847 50

00 11 2424 2525

00 11 22 2323 2424 2525 00 484811 22 4949 5050

22 33 44 55 66 77

6.12s

0

235.38ms120ms

Control 51 - Frame MultiframeTraffic 26 - Frame Multiframe

4.615ms

TDMA Frame

Hyperframe and Superframe structureHyperframe and Superframe structure

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SPEECHCODING

CHANNELCODING

INTERLEAVING

BURSTASSEMBLING

CIPHERING

MODULATION DEMODULATION

DECIPHERING

BURSTDISASSEMBLING

DEINTERLEAVING

CHANNELDECODING

SPEECHDECODING

Transmission

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The transmission of speech is one of the most important service of a mobile cellular system. The GSM speech codec, which will transform the analog signal(voice) into a digital representation, has to meet the following criterias

A good speech quality, at least as good as the one obtained with previous cellular systems.

To reduce the redundancy in the sounds of the voice. This reduction is essential due to the limited capacity of transmission of a radio channel.

The speech codec must not be very complex because complexity is equivalent to high costs.

The final choice for the GSM speech codec is a codec named RPE-LTP (Regular Pulse Excitation Long-Term Prediction). This codec uses the information from previous samples (this information does not change very quickly) in order to predict the current sample. The speech signal is divided into blocks of 20 ms. These blocks are then passed to the speech codec, which has a rate of 13 kbps, in order to obtain blocks of 260 bits.

SPEECH CODINGSPEECH CODING

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CHANNEL CODINGCHANNEL CODING• Channel coding adds redundancy bits to the original information in

order to detect and correct, if possible, errors ocurred during the transmission.

• The channel coding is performed using two codes: a block code and a convolutional code.

• The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits.

• A convolutional code adds redundancy bits in order to protect the information. A convolutional encoder contains memory. This property differentiates a convolutional code from a block code.

• A convolutional code can be defined by three variables : n, k and K.

• The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder.

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CHANNEL CODING ( Cont )CHANNEL CODING ( Cont )• The ratio, R, of the code is defined as R = k/n.

Example - Let's consider a convolutional code with the following values: k is equal to 1, n to 2 and K to 5. This convolutional code uses then a rate of R = 1/2 and a

delay of K = 5, which means that it will add a redundant bit for each input bit. The convolutional code uses 5

consecutive bits in order to compute the redundancy bit. As the convolutional code is a 1/2 rate convolutional code, a

block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted :

C (11 + 15 j) for j = 0, 1, ..., 31

• The block of 456 bits produced by the convolutional code is then passed to the interleaver

Convolution code R = k/n = 1/2k=1

1 bit inputn=2

2 bit input

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CHANNEL CODING FOR GSM SPEECH CHANNELSCHANNEL CODING FOR GSM SPEECH CHANNELS• Before applying the channel coding, the 260 bits of a GSM speech frame

are divided in three different classes according to their function and importance.

• The most important class is the class 1a containing 50 bits.Next important is the class 1b, which contains 132 bits.The least important is the class 2, which contains the remaining 78 bits.

• The different classes are coded differently.

• First of all, the class 1a bits are block-coded. Three parity bits, used for error detection, are added to the 50 class 1a bits.The resultant 53 bits are added to the class 1b bits.

• Four zero bits are added to this block of 185 bits (50+3+132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits.

• The class 2 bits are added, without any protection, to the output block of

the convolutional coder. An output block of 456 bits is finally obtained.

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Speech Channel CodingSpeech Channel Coding

Class 1a50 bits

Class 1b132 bits

Class 1a50 bits

Class 1b132 bits

378 bits

Class 278 bits

3 4

Paritycheck

Tailbits

260 bits

456 bits

Convolution coding

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CHANNEL CODING FOR CONTROL CHANNELSCHANNEL CODING FOR CONTROL CHANNELS• In GSM the signalling information is just contained in 184 bits.

• Forty parity bits, obtained using a fire code, and four zero bits are added to the 184 bits before applying the convolutional code (r = 1/2 and K = 5). The output of the convolutional code is then a block of 456

bits, which does not need to be punctured. 184 bits

184 bits

456 bits

Firecode

Tailbits

Convolution coding

40 bits 4

Paritybits

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CHANNEL CODING FOR DATA CHANNELSCHANNEL CODING FOR DATA CHANNELS• In data information is contained in 240 bits.

• Four tails bits are added to the 240 bits before applying the convolutional code (r = 1/2 and K = 5). The output of the convolutional code is then a block of 488 bits which when punctuated yields 456 bits.

240 bits

240 bits

488 bits

Tailbits

Convolution coding

4

456 bits

Punctuate

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INTERLEAVINGINTERLEAVING• An interleaving rearranges a group of bits in a particular way.

• It is used in combination with FEC codes( Forward Error Correction Codes ) in order to improve the performance of the error correction mechanisms.

• The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors.

• As the errors are less concentrated, it is then easier to correct them.

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GSM SPEECH CHANNEL INTERLEAVINGGSM SPEECH CHANNEL INTERLEAVING• A burst in GSM transmits two blocks of 57 data bits each.

• Therefore the 456 bits corresponding to the output of the channel coder fit into 8 ‘57 data’ bits (8 * 57 = 456). The 456 bits are divided into eight blocks of 57 bits.

• The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the second one the bit numbers (1, 9, 17, .....449), etc.

• The last block of 57 bits will then contain the bit numbers (7, 15, .....455).

• The first four blocks of 57 bits are placed in the even-numbered bits of four consecutive bursts.

• The other four blocks of 57 bits are placed in the odd-numbered bits of the next four bursts.

• The interleaving depth of the GSM interleaving for speech channels is eight.

• A new data block also starts every four bursts. The interleaver for speech

channels is called a block diagonal interleaver.

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4

GSM SPEECH CHANNEL INTERLEAVING ( Diagram )GSM SPEECH CHANNEL INTERLEAVING ( Diagram )

1 2 3 5 6

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

4456 bits

5456 bits

6456 bits

Full rate encoded speech blocks from a single conversation

Bursts

TDMAFrames Frame 1 Frame 2 Frame 3 Frame 4

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CONTROL CHANNEL INTERLEAVINGCONTROL CHANNEL INTERLEAVING• A burst in GSM transmits two blocks of 57 data bits each.

• Therefore the 456 bits corresponding to the output of the channel coder fit into four bursts (4*114 = 456).

• The 456 bits are divided into eight blocks of 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the second one the bit numbers (1, 9, 17, .....449), etc. The last block of 57 bits will then contain the bit numbers (7, 15, .....455).

• The first four blocks of 57 bits are placed in the even-numbered bits of four bursts.

• The other four blocks of 57 bits are placed in the odd-numbered bits of the same four bursts.

• Therefore the interleaving depth of the GSM interleaving for control channels is four and a new data block starts every four bursts.

• The interleaver for control channels is called a block rectangular

interleaver.

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DATA INTERLEAVINGDATA INTERLEAVING• A particular interleaving scheme, with an interleaving depth equal to 22,

is applied to the block of 456 bits obtained after the channel coding.

• The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits each, 2 blocks of 12 bits each and 2 blocks of 6 bits each.

• It is spread over 22 bursts in the following way :

– the first and the twenty-second bursts carry one block of 6 bits each

– the second and the twenty-first bursts carry one block of 12 bits each

– the third and the twentieth bursts carry one block of 18 bits each

– from the fourth to the nineteenth burst, a block of 24 bits is placed in each burst

• A burst will then carry information from five or six consecutive data blocks. The data blocks are said to be interleaved diagonally.

• A new data block starts every four bursts.

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CIPHERINGCIPHERING• Ciphering is used to protect signaling and user data.

• A ciphering key is computed using the algorithm A8 stored on the SIM card, the subscriber key and a random number delivered by the network (this random number is the same as the one used for the authentication procedure).

• A 114 bit sequence is produced using the ciphering key, an algorithm called A5 and the burst numbers.

• This bit sequence is then XORed with the two 57 bit blocks of data included in a normal burst.

• In order to decipher correctly, the receiver has to use the same algorithm

A5 for the deciphering procedure.

MODULATIONMODULATION• Modulation is done using 0.3 GMSK