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ELEMENTS OF DIGITAL MOBILE

By: Eng\ WALEED ELSAFOURY Copyright © 2013 Waleed Elsafoury


ELEMENTS OF DIGITAL MOBILE

GSM Transmission Chain Speech encoder

13 kbps

Channel encoder

Microphone Acoustic signal

22.8 kbps Analog electrical speech signal

Digital speech signal

Interleaver 22.8 kbps Ciphering 22.8 kbps Burst assembling and multiplexing 33.85 kbps Modulator

Channel


ELEMENTS OF DIGITAL MOBILE

GSM Reception Chain Channel

Demodulator 33.85 kbps Burst Deassembling and demultiplexing 22.8 kbps Deciphering 22.8 kbps De-Interleaver Recovered speech

22.8 kbps Speech decoder

13 kbps

Channel decoder


ELEMENTS OF DIGITAL MOBILE •

speech coding must give : 1. Low bit rate 2. Preserving the essential elements of speech quality 3. An acceptable cost of the equipment

- GSM sends information about speech not the speech itself


ELEMENTS OF DIGITAL MOBILE • First step A/D converter


ELEMENTS OF DIGITAL MOBILE  A/D is performed by PCM.  PCM involves 4 steps: 1- Bandlimiting 3- Quantizing

2- Sampling 4- Encoding

-Bandlimiting F(f)

H(f)

Baseband signal

f

F(f)

f Low-pass filter

fX

Bandlimited signal

f


ELEMENTS OF DIGITAL MOBILE -Sampling

fs(t)

f(t)

t

Analog signal

fs(t)

Ts

t

t Sampling

fs = 2 fx sample/sec

Sampled signal


ELEMENTS OF DIGITAL MOBILE Bandlimiting + Sampling fs(t)

Ts

t f(t)

f(t)

t

Analog signal

Sampling

Low Pass Filter 3.4 kHz

t Bandlimited to 3.4 kHz

Electronic Switch

fs= 8 ksample/s

fs(t ) t PAM sampled signal


ELEMENTS OF DIGITAL MOBILE Quantizing q7 q6 q5 q4 q3

Sampled value

Quantized value

Quantized error

time

q2 q1 q0

o Number of levels = 2^13=8192 levels. o This gives 13 bit/sample.


ELEMENTS OF DIGITAL MOBILE Encoding

Encoder 0 1 0 1 1 0 0 1

Quantized Samples

Binary stream of bits

o Encoding rate = 8000 x 13 = 104 kb/sec.


ELEMENTS OF DIGITAL MOBILE Segmentation 20 msec = 2080 bit 104 kb/sec 101010001001111101010010100110010010010100100100110

8 : 1 compression

13 kb/sec

260 bit 01000101111010


ELEMENTS OF DIGITAL MOBILE

• Discontinuous TX

Voice activity detection

Voice 8000 s/sec 13 b/sample

VAD

Discontinuous Speech coder

Voice frame

Comfort noise

Noise frame

transmission


ELEMENTS OF DIGITAL MOBILE • DTX Click to edit Master text styles Second level ● Third level ● Fourth level ● Fifth level


ELEMENTS OF DIGITAL MOBILE • Predictive decoding Click to edit Master text styles Second level ● Third level ● Fourth level ● Fifth level


ELEMENTS OF DIGITAL MOBILE • Channel coding o o o o

Mobile channel is error-prone. Compressed speech is sensitive to errors. Error correction is necessary. Channel coding adds extra bits to help in error correction.

o GSM uses: 1. Block code 2. Convolutional code


ELEMENTS OF DIGITAL MOBILE • Convolutional code 50 Ver y impor tant bits

Block coder

53 b

378 b

378 b

1:2 Convolutional coder

132 Important bits

456 bits 0000

78 Not so impor tant bits


ELEMENTS OF DIGITAL MOBILE • Convolutional code Click to edit Master text styles Second level ● Third level ● Fourth level ● Fifth level


ELEMENTS OF DIGITAL MOBILE

Channel Coding for TCH/FR 260 bits/20 msec Class 1a 50 bits

50 Parity check

3

Class 1b 132 bits

132

Class 2 78 bits

4 Tail bits

1:2 Convolutional coder 378

456 bits/20 msec

78


ELEMENTS OF DIGITAL MOBILE •

Interleaving

- First level :Block interleaving

57 bits

1 9 17 25

2 10 18 26

3 11 19 27

4 12 20 28

5 13 21 29

6 14 22 30

7 15 23 31

8 16 24 32

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

449

450

451

452

453

45 4

455

456


ELEMENTS OF DIGITAL MOBILE - second level :Burst interleaving - Normal Burst has 2 of 57b blocks - If Burst is lost BER=25% - To reduce it to 12.5% we put 2 blocks of two 20 msec

20 msec speech 8x57=456 bits

20 msec speech 8x57=456 bits

20 msec speech 8x57=456 bits

57 bits

57 bits Normal Burst


ELEMENTS OF DIGITAL MOBILE

GSM security ●

TMSI

Authentication

IMEI check

ciphering


ELEMENTS OF DIGITAL MOBILE Ciphered digital data

A5

AUC HLR

IMSI, Ki, A3, A8

GSM PLMN

EIR IMEI

A5 BTS

VLR

MSC

IMEI

IMSI, Ki, A3, A8

SIM


ELEMENTS OF DIGITAL MOBILE Authentication - It is a processor system that performs the authentication function

AUC IMSI

Ki

Database IMSI,Ki

RAND A3

RAND

SRES Triplet

A8

Kc

Request for Triplet from HLR,VLR

Triplet


ELEMENTS OF DIGITAL MOBILE MS

HLR/AUC IMSI RAND

Ki

A3

Ki

A3 SRES

SRES

VLR SRES =?

Yes

No

“Authentication Refused" message

Continue ciphering


ELEMENTS OF DIGITAL MOBILE IMEI check Authentication Ciphering

MSC/VLR Initiate  Authentication  Ciphering  IMEI request

IMEI Request ME IMEI MS

IMEI Response

BSS

MSC/VLR

EIR Check IMEI

Black List Gray List

White List


ELEMENTS OF DIGITAL MOBILE • ciphering Ms

Network

RAND

Ki

A8

Ki

A8 Kc

Kc


ELEMENTS OF DIGITAL MOBILE 1. CMC + Kc VLR

MSC

2. CMC

Cipher mode command

MS

BTS 6. Ciphering mode complete

If yes 5. Decryption of CMC successful?

Kc

CMC

FN

Decryption process using A5

4.Encrypted CMC

Kc FN

Encryption process using A5

3.Encrypt CMC


ELEMENTS OF DIGITAL MOBILE • ciphering SYNC.

0 1 1 0 0 1 0

0 1 1 0 0 1 0

Kc

Kc FN

A5

1 0 1 1 0 0 1

Cipher sequence key flow

User data flow

Cipher sequence key flow

User data flow

A5

1 0 1 1 0 0 1

1 1 0 1 0 1 1 Ciphered text

Ciphering Process

FN

Deciphering Process


ELEMENTS OF DIGITAL MOBILE •

Modulation 1. High bandwidth efficiency i.e. high bit rate per Hz 2. Low power drain 3. Good BER performance 4. Low implementation cost

GMSK is the modulation scheme of the GSM system


ELEMENTS OF DIGITAL MOBILE - Gaussian minimum shift keying

modulator


Ch4 digital mobile systems