25513738 speech transmission

7/31/2019 25513738 Speech Transmission

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CONTENTS

SPEECH TO RADIO TRANSMISSION

CHANNEL CODING INTERLEAVING

BURST FORMATTING

CIPHERING

MODULATION


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SPEECH TRANSMISSIONSpeech

Source

decoding

Channeldecoding

De-interleaving

Burst deformatting

Deciphering

Demodulation

equalization

Speech

Digitizing and

source coding

Channel

coding

Modulation

Ciphering

Burst formatting

Interleaving

Step 1

Step 2

Step 3

Step 4

Transmission

Step 5

Step 6 Diversity


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SPEECH TRANSMISSION

From speech to Radio Transmission, several operations are performed,main operations are the following:

DIGITIZING-SPEECH CODING

CHANNEL CODING

INTERLEAVING

BURST FORMATTING

CIPHERING

MODULATION

DIGITIZING SPEECH CODING: The GSM Speech Coderis based on the ResiduallyExcited Linear Predictive Coder[RELP], which is enhanced by a LongTerm Predictor [LTP]. The

coder provides 260 bits for each 20ms blocks of speech, whichyields a bit rate of 13kbps.


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CHANNEL CODING

CHANNEL CODING: The output bits of the speech coder

are ordered in to groups for error protection, based upon theirsignificance in contributing to speech quality.Out of the total260bits the most important 50bits, called type Ia bits, have 3parity check [CRC] bits added to them.The next 132 bits along

with 53 are re-ordered by four trailing bits thus providing a totalof 189bits. This block is then encoded for error protection usinga convolutional encoder providing a sequence of378bits.The least important 78 bits do not have any error

protection and are added to the existing sequence of378 bits toform a block of456bits in a 20ms frame.The error protectioncoding scheme increases the Gross Rate of GSM Speech signal,

with channel coding to 22.8kbps.


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INTERLEAVING

INTERLEAVING: In order to minimize the effect of sudden

fades on the received data, the total of 456 encoded bits withineach 20ms speech frame are broken into eight 57bit sub-blocks.These 8 sub-blocks which make up a single speech frame arespread over eight consecutive TCH timeslots. If a burst is lostdue to interference or fading, channel coding ensures that

enough bits will still be received correctly to allow the errorcorrection to work. Each TCH time slot carries two 57bit blocksof data from two different 20ms [456bits] speech segments.


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INTERLEAVING

C

20 ms20 ms

A

A8

A7

A6

A5

B4

B3

B2

B1

B8

B7

B6

B5

C4

C3

C2

C1

57 bits

Information

1 1

CRL CRL

3 3

Tail Tail

26 bits

Training

8 Bursts

8 Sub blocks

of 57 bits

Source coding

Channel coding

Interleaving

Normalburst

20 ms

B

456 bitsA

456 bitsB

456 bitsC

57 bits

Information

A8B4

A7B3

A6B2

A5B1

B8C4

B7C3

B6C2

B5C1

Speech blocks

260 bits 260 bits260 bits


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INTERLEAVING

0 1 2 3 4 5 6 7 8 ... ... 452 453 454 455

0 1 2 3 4 5 6 78 9 10 11 12 13 14 15

448 449 450 451 452 453 454 455

57Rows

Divide 456 bits in 8 sub-blocks

765432107654 0 1 2 3

reordering&

partitioningout

diagonal

interleaving

456coded bits

burst

b0 b1 b56 b1 b56b0

bitinterleaving


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BURST FORMATTING

BURST FORMATTING: A basic unit of measure intransmission on a radio path is BURST. A series of 114 modulatedbits of information.Bursts have a finite duration and occupy a finitepart of the radio spectrum.Bursts are sent in time and frequencywindows called slots.A normal burst is made of:

Tail Bits Three 0 bits at the beginning and end to help avoid lossof synchronization.

Information Speech, Data or Signaling two blocks of 57bits.

Training Sequence A list of bits [26] known by the receiverallowing it to demodulate the burst.

Stealing Flags Indicate if information is either users data orsignaling data for call in state [2 in total].

Guard Band Bits [8.25] where nothing is transmitted to allow foroverlap due to the variable distance from the mobile phone to BaseStation. This is necessary if the Timing Advance is not exactly right.

Normal Burst bears traffic channels, its associated channels [Fast orSlow], Standalone and the Broadcast Control Channels.


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BURST FORMATTING

Guard

157 126 57

156.25 bits

0.577 ms

1 frame:

4.615 ms

0 1 2 3 4 5 6 7

33 8.25

S SDATA DATAGuard

BandTraining

sequence

Burst

148 bits


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BURST FORMATS

A FrequencyCorrection Burst contains 142 bits for the frequencycorrection and 3 tail bits at the beginning and end. This burst in

used on the FCCH in order to enable the mobile to find anddemodulate a synchronization burst in same cell. No information is

transmitted in this burst as all bits are 0.

A Synchronization Burst contains 64 bits for the training sequence,twice 39 for the information & 3 tail bits at the beginning and end.It is used in downlink direction, for time synchronization of themobile station. It has larger training sequence when compared withother bursts as this is the first burst mobile needs to be able to

demodulate.


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Synchronization Burst

(SCH)

Tail Data Extended Training Sequence Data Tail

156.25 bits (0.577 ms)

3 bits 39 encrypted bits 64 synchronization bits 39 bits 3 bits 8.25 bits

Guard

Period

Frequency Correction Burst

(FCCH)

Tail Data Tail

156.25 bits (0.577 ms)

3 bits 142 fixed bits (0) 3 bits 8.25 bits

Guard

Period

BURST FORMATS


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BURST FORMATS

A DummyBurst structure is similar to the Normal Burst but

information bits are replaced by mixed bits: this burst is used toreplace data if there is nothing to transmit. It is the case of TCHfilling when they are transmitted on beacon frequency.

An Access Burst contains 41bits for the training sequence, 36 bits

for the information 8 and 3 tail bits at respectively the beginningand the end of the burst. The Guard Period is of 68.25bits. It isused on RACH, in the uplink direction during initial phase of

transmission when the propagation delay between the mobile

station and the BTS is not yet known.


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Access Burst

TailTraining

Sequence Guard PeriodTail

156.25 bits (0.577 ms)

8 bits 36 encrypted bits 68.25 bits3 bits41 synch bits

Data

Dummy Burst

Tail Dummy Sequence Training Sequence Dummy SequenceTail

3 bits 58 mixed bits 28 midamble bits 58 mixed bits 3 bits 8.25 bits

GuardPeriod

156.25 bits (0.577 ms)

156.25 bits (0.577 ms)

13 bits 57 encrypted bits 1 26 bits 57 encrypted bits 3 bits 8.25 bits

Tail Data Training Sequence Data Tail Guard

Period

BURST FORMATS


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CIPHERING

Ciphering or encryption, is a procedure that provides additionalsecurity for the subscriber. Ciphering is only done on two Data

segments of Normal Burst. Ciphering is achieved by performingan XOR function between a psuedo random bit sequence and

the 114 useful bits of a Normal Burst.

De-ciphering in turn applies the same operation, since XORtwice with the same data leads back to the original value.

GSM uses two A5 types of algorithm for Ciphering:

Encryption Algorithm A5/1 which contains European

and US technical software that could not directly orindirectly exported to any country.

Encryption Algorithm A5/2 which contains softwarethat do not require license or approval.


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CIPHERING

Plain data: 0 1 1 1 0 0 1 0.....

Ciphering sequence: 0 0 0 1 1 0 1 0.....

XOR:Ciphered data (transmitted): 0 1 1 0 1 0 0 0.....

Ciphered sequence: 0 0 0 1 1 0 1 0.....

XOR:

Recovered data: 0 1 1 1 0 0 1 0.....

Data S S DataBurst to be

transmitted

Data S S DataTrainingsequence

Receivedburst


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MODULATION

Modulation transforms the binary signal into an analog signalusingGaussian Minimum Shift Keying [GMSK].

The modulation scheme used by GSM is 0.3GMSK, where 0.3describes the 3dB bandwidth of the Gaussian pulse ShapingFilter with relation to the bit rate. GMSK is a special type ofdigital FM modulation. The MSK modulated signal is passedthrough a Gaussian Filter to smooth the rapid frequencytransitions which would otherwise spread energy into adjacentcells.

Reason for GMSK chosen for modulation and transmission ofGSM Signal is that it filters the side bands and consequentlyhelps in reducing Adjacent Channel Interference.


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THANK YOU!

25513738 speech transmission

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