ch4 digital mobile systems
DESCRIPTION
ÂTRANSCRIPT
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
ELEMENTS OF DIGITAL MOBILE
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
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 2- Sampling3- Quantizing 4- Encoding
H(f)
Low-pass filter
ff
F(f)
Baseband signal
f
F(f)
Bandlimited signal
fX
-Bandlimiting
ELEMENTS OF DIGITAL MOBILE
-Sampling
Sampled signal
t
fs(t)Ts
t
Sampling
fs(t)
Analog signal
t
f(t)
fs = 2 fx sample/sec
ELEMENTS OF DIGITAL MOBILE
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
ELEMENTS OF DIGITAL MOBILE
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.
ELEMENTS OF DIGITAL MOBILE
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 MOBILE
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
ELEMENTS OF DIGITAL MOBILE
• DTX
ELEMENTS OF DIGITAL MOBILE
• Predictive decoding
ELEMENTS OF DIGITAL MOBILE
• 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
ELEMENTS OF DIGITAL MOBILE
50 Very important bits
53 b
132 Important bits
0000
456 bits
378 b 378 bBlockcoder
1:2Convolutional
coder
• Convolutional code
ELEMENTS OF DIGITAL MOBILE
78 Not so important bits
• Convolutional code
ELEMENTS OF DIGITAL MOBILE
ELEMENTS OF DIGITAL MOBILE
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
ELEMENTS OF DIGITAL MOBILE
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
20 msec speech8x57=456 bits
20 msec speech8x57=456 bits
ELEMENTS OF DIGITAL MOBILE
ELEMENTS OF DIGITAL MOBILE
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
ELEMENTS OF DIGITAL MOBILE
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
ELEMENTS OF DIGITAL MOBILE
A3
Ki
A3
KiRAND
VLR
SRES
=?
SRES
SRES
HLR/AUCMS
YesNo
“Authentication Refused" message
Continue ciphering
IMSI
ELEMENTS OF DIGITAL MOBILE
EIRCheck IMEI
MS
AuthenticationCiphering
IMEI Request
MSC/VLR
ME
IMEI
BSS
IMEI Response
Gray List
Black List
White List
MSC/VLRInitiateAuthenticationCipheringIMEI request
ELEMENTS OF DIGITAL MOBILE
IMEI check
• ciphering
A8 A8
Kc Kc
Ki KiRAND
NetworkMs
ELEMENTS OF DIGITAL MOBILE
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
ELEMENTS OF DIGITAL MOBILE
• 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
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
modulator
- Gaussian minimum shift keying
ELEMENTS OF DIGITAL MOBILE