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