Ch4 digital mobile systems

ELEMENTS OF DIGITAL MOBILE

By:

Eng\ WALEED EL SHAFAI

GSM Transmission Chain

Speech encoder

Channel encoder

Interleaver

Burst assembling and multiplexing

Ciphering

Modulator

Channel

Acoustic signal

Analog electrical speech signal

Digital speech signal

13 kbps

22.8 kbps

22.8 kbps

22.8 kbps

33.85 kbps

Microphone


GSM Reception Chain Channel

Recovered speech

De-Interleaver

Deciphering

Demodulator

Burst Deassembling and demultiplexing

Speech decoder

Channel decoder

33.85 kbps

22.8 kbps

22.8 kbps

22.8 kbps

13 kbps


• 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


• First step A/D converter


A/D is performed by PCM. PCM involves 4 steps:

1- Bandlimiting 2- Sampling 3- Quantizing 4- Encoding

H(f)

Low-pass filter

f f

F(f)

Baseband signal

f

F(f)

Bandlimited signal fX

-Bandlimiting


-Sampling

Sampled signal

t

fs(t) Ts

t

Sampling

fs(t)

Analog signal

t

f(t)

fs = 2 fx sample/sec


Bandlimiting + Sampling

PAM sampled signal

t

fs(t)

Ts

t

Sampling

fs(t)

Analog signal

t

f(t)

Low

Pass

Filter

3.4 kHz Bandlimited to 3.4 kHz

t

f(t)

Electronic

Switch

fs= 8 ksample/s


Quantizing

q7 q6 q5 q4

q3

q2

q1

q0

Sampled value

Quantized value

time

Quantized error

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


Encoding

0 1 0 1 1 0 0 1

Binary stream of bits Quantized Samples

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

Encoder



Segmentation

101010001001111101010010100110010010010100100100110

01000101111010

20 msec = 2080 bit

260 bit

8 : 1 compression

104 kb/sec

13 kb/sec

• Predictive coding

1. We send information about noise not the voice itself (pitch, tone,….etc)

2. Less power and less co-channel interference

Voice frame

VAD Voice activity

detection

Speech coder

Comfort noise Noise frame

Voice 8000 s/sec

13 b/sample

Discontinuous

transmission


• DTX


• Predictive decoding


• Channel coding

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

1. Block code 2. Convolutional code


50 Very important bits

53 b

132 Important bits

4 tail bits

456 bits

378 b 378 b Block coder

1:2 Convolutional

coder

• Convolutional code


78 Not so important bits

• Convolutional code



Channel Coding for TCH/FR

Class 1a 50 bits

Class 1b 132 bits

Class 2 78 bits

50 3 132 4

378

260 bits/20 msec

456 bits/20 msec

Parity check

Tail bits

1:2 Convolutional coder

78

• Interleaving

- First level :Block interleaving

1

9

17

25

.

.

.

449

2

10

18

26

.

.

.

450

3

11

19

27

.

.

.

451

8

16

24

32

.

.

.

456

4

12

20

28

.

.

.

452

5

13

21

29

.

.

.

453

6

14

22

30

.

.

.

454

7

15

23

31

.

.

.

455

57 bits


57 bits 57 bits

- 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

Normal Burst

20 msec speech

8x57=456 bits

20 msec speech

8x57=456 bits



security

Security Attacks

Passive attacks Active attacks


A B

NORMAL FLOW

A B A B

A B A B

C

C C

C


GSM security

• TMSI , IMSI

• Authentication

• IMEI check

• ciphering

SIM

Ciphered digital data

EIR IMEI

BTS

VLR MSC

GSM PLMN

AUC

IMSI, Ki, A3, A8

HLR

A5


Authentication

- It is a processor system that performs the authentication function

AUC

RAND

Database

IMSI,Ki

A8 A3

RAND SRES Kc

Triplet

Ki IMSI

Triplet

Request for

Triplet from

HLR,VLR


EIR

Check IMEI

MS

Authentication Ciphering

IMEI Request

MSC/VLR

ME

IMEI

BSS

IMEI Response

Gray List

Black List

MSC/VLR

Initiate

Authentication

Ciphering

IMEI request


IMEI check

• ciphering

A8 A8

Kc Kc

Ki Ki RAND

Network Ms


• ciphering

A5 A5

1 0 1 1 0 0 1

1 1 0 1 0 1 1

0 1 1 0 0 1 0 0 1 1 0 0 1 0

1 0 1 1 0 0 1

Kc

FN

Kc

FN

SYNC.

Ciphered text

User data flow

User data flow

Ciphering Process Deciphering Process

Cipher sequence key flow

Cipher sequence key flow


• 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


modulator

- Gaussian minimum shift keying


Engineering

Ch4 digital mobile systems