Ch4 digital mobile systems

ELEMENTS OF DIGITAL MOBILE

By: Eng\ WALEED ELSAFOURY

Copyright © 2013 Waleed Elsafoury

GSM Transmission ChainSpeech encode

r

Channel encoder

Interleaver

Burst assembling

and multiplexing

Ciphering

Modulator

Channel

Acoustic signal

Analog electrical speech signal

Digital speech signal

13 kbps22.8 kbps

22.8 kbps

22.8 kbps

33.85 kbps

Microphone


GSM Reception ChainChannel

Recovered speech

De-Interleaver

Deciphering

Demodulator

Burst Deassemblingand demultiplexing

Speech decode

rChannel 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- Sampling3- Quantizing 4- Encoding

H(f)

Low-pass filter

ff

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)

LowPassFilter

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


ELEMENTS OF DIGITAL MOBILESegmentation

101010001001111101010010100110010010010100100100110

01000101111010

20 msec = 2080 bit

260 bit

8 : 1 compression

104 kb/sec

13 kb/sec

• Discontinuous TX

Voice frame

VADVoice activity

detection

Speech coder

Comfort noise Noise frame

Voice 8000 s/sec

13 b/sample

Discontinuoustransmission


• 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 code2. Convolutional code


50 Very important bits

53 b

132 Important bits

0000

456 bits

378 b 378 bBlockcoder

1:2Convolutional

coder

• Convolutional code


78 Not so important bits

• Convolutional code



Channel Coding for TCH/FR

Class 1a

50 bits

Class 1b132 bits

Class 278 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

19

1725...

449

2101826...

450

3111927...

451

8162432...

456

4122028...

452

5132129...

453

6142230...

454

7152331...

455

57 bits


57 bits57 bits57 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 speech8x57=456 bits

Normal Burst





GSM security• TMSI• Authentication• IMEI check• ciphering

IMSI, Ki, A3, A8

SIM

A5

IMEI

Ciphered digital data

EIRIMEI

BTSVLR

MSC

GSM PLMN

AUCIMSI, Ki,A3, A8

HLR

A5


Authentication - It is a processor system that performs the authentication

function

AUC

RAND

DatabaseIMSI,Ki

A8A3

RAND SRES Kc

Triplet

KiIMSI

Triplet

Request for Triplet from HLR,VLR


A3

Ki

A3

KiRAND

VLR

SRES

=?

SRES

SRES

HLR/AUCMS

YesNo

“Authentication Refused" message

Continue ciphering

IMSI


EIRCheck IMEI

MS

AuthenticationCiphering

IMEI Request

MSC/VLR

ME

IMEI

BSS

IMEI Response

Gray List

Black List

White List

MSC/VLRInitiateAuthenticationCipheringIMEI request


IMEI check

• ciphering

A8 A8

Kc Kc

Ki KiRAND

NetworkMs


6. Ciphering mode

complete

BTS

1. CMC + Kc 2. CMC

Kc FN

3.Encrypt CMC

4.Encrypted CMC

If yes

MSCipher mode command

VLR MSC

5. Decryption of

CMC successful?

Encryption processusing A5

CMC Kc FN

Decryption processusing A5


• ciphering

A5 A5

1 0 1 1 0 0 1

1 1 0 1 0 1 1

0 1 1 0 0 1 00 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


Documents

Ch4 digital mobile systems