Ch4 digital mobile systems
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This document discusses the key elements of digital mobile communication systems like GSM. It describes the end-to-end transmission chain including speech encoding, channel encoding, interleaving, burst assembly, ciphering, modulation, and the reception chain in reverse order. It also covers predictive coding, discontinuous transmission, channel coding, interleaving techniques, security aspects like authentication and ciphering, and Gaussian minimum shift keying modulation used in GSM.
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Ch4 digital mobile systems
- 1. ELEMENTS OF DIGITAL MOBILE By: Eng WALEED EL SHAFAI
- 2. 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 ELEMENTS OF DIGITAL MOBILE
- 3. 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 ELEMENTS OF DIGITAL MOBILE
- 4. • 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
- 5. • First step A/D converter ELEMENTS OF DIGITAL MOBILE
- 6. A/D is performed by PCM. PCM involves 4 steps: 1- Bandlimiting 2- Sampling 3- Quantizing 4- Encoding H(f) Low-pass filter ff F(f) Baseband signal f F(f) Bandlimited signal fX -Bandlimiting ELEMENTS OF DIGITAL MOBILE
- 7. -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
- 8. 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 ELEMENTS OF DIGITAL MOBILE
- 9. 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
- 10. Encoding 0 1 0 1 1 0 0 1 Binary stream of bitsQuantized Samples o Encoding rate = 8000 x 13 = 104 kb/sec. Encoder ELEMENTS OF DIGITAL MOBILE
- 11. ELEMENTS OF DIGITAL MOBILE Segmentation 101010001001111101010010100110010010010100100100110 01000101111010 20 msec = 2080 bit 260 bit 8 : 1 compression 104 kb/sec 13 kb/sec
- 12. • 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 ELEMENTS OF DIGITAL MOBILE
- 13. • DTX ELEMENTS OF DIGITAL MOBILE
- 14. • Predictive decoding ELEMENTS OF DIGITAL MOBILE
- 15. • 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 ELEMENTS OF DIGITAL MOBILE
- 16. 50 Very important bits 53 b 132 Important bits 4 tail bits 456 bits 378 b378 b Block coder 1:2 Convolutional coder • Convolutional code ELEMENTS OF DIGITAL MOBILE 78 Not so important bits
- 17. • Convolutional code ELEMENTS OF DIGITAL MOBILE
- 18. ELEMENTS OF DIGITAL MOBILE 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
- 19. • 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 ELEMENTS OF DIGITAL MOBILE
- 20. 57 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 speech 8x57=456 bits Normal Burst 20 msec speech 8x57=456 bits 20 msec speech 8x57=456 bits ELEMENTS OF DIGITAL MOBILE
- 21. ELEMENTS OF DIGITAL MOBILE security Security Attacks Passive attacks Active attacks
- 22. ELEMENTS OF DIGITAL MOBILE A B NORMAL FLOW A B A B A BA B C C C C
- 23. ELEMENTS OF DIGITAL MOBILE GSM security • TMSI , IMSI • Authentication • IMEI check • ciphering
- 24. SIM Ciphered digital data EIR IMEI BTS VLR MSC GSM PLMN AUC IMSI, Ki, A3, A8 HLR A5 ELEMENTS OF DIGITAL MOBILE
- 25. Authentication - It is a processor system that performs the authentication function AUC RAND Database IMSI,Ki A8A3 RAND SRES Kc Triplet Ki IMSI Triplet Request for Triplet from HLR,VLR ELEMENTS OF DIGITAL MOBILE
- 26. 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 ELEMENTS OF DIGITAL MOBILE IMEI check
- 27. • ciphering A8 A8 Kc Kc Ki KiRAND Network Ms ELEMENTS OF DIGITAL MOBILE
- 28. • 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
- 29. • 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
- 30. modulator - Gaussian minimum shift keying ELEMENTS OF DIGITAL MOBILE