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GSM Digital Mobile System
- 1. Chapter 4 Digital Mobile System Eng. Ahmed Khaled
- 2. GSM Transmission & Reception Chain: Speech Encoder Channel Encoder Interleaver Ciphering Burst Assembling & Multiplexing Modulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Channel Microphone Digital Speech Signal Analog Electrical Speech Signal Acoustic Signal Speech Decoder Channel Decoder De-interleaver Deciphering Burst de-assembling & De- multiplexing Demodulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Received Speech
- 3. GSM Reception Chain: Channel Speech Decoder Channel Decoder De-interleaver Deciphering Burst de-assembling & De-multiplexing Demodulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Received Speech
- 4. GSM Transmission Chain (MSC IC BMC): Speech Encoder Channel Encoder Interleaver Ciphering Burst Assembling & Multiplexing Modulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Channel Microphone Digital Speech Signal Analog Electrical Speech Signal Acoustic Signal
- 5. GSM Coding: 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
- 6. GSM Coding: First step A/D converter 1 10 A/D
- 7. GSM Coding: A/D is performed by PCM PCM involves 4 steps: 1. Band-limiting 2. Sampling 3. Quantizing 4. Encoding
- 8. 1. Band-limiting: Low-pass Filter 𝑯(𝒇) Band-limited Signal 𝑭(𝒇) ff Baseband Signal 𝑭(𝒇) f 𝒇 𝒙
- 9. 2. Sampling & Quantizing: 𝒇 𝒔 = 𝟐 × 𝒇 𝒎𝒂𝒙 = 𝟐 × 𝟒 × 𝟏𝟎 𝟑 = 𝟖𝟎𝟎𝟎 𝒔𝒂𝒎𝒑𝒍𝒆/𝒔𝒆𝒄 Number of levels = 𝟐 𝟏𝟑=8192 levels
- 10. Coding: Encoding rate = 8000 x 13 = 104 kb/sec. Encoder 0 1 0 1 1 0 0 1 Quantized Samples Binary Stream of bits
- 11. Segmentation: 101010001001111101010010100110010010010100100100110 01000101111010 20 msec = 2080 bit 8 : 1 Compression 260 bit 13 kbps 104 kbps
- 12. Discontinuous TX: DTX Discontinuous Transmission VAD Voice Activity Detection Speech Coder Comfort Noise Synthesizer VAD Voice Frame Voice13 bit 8000 sample / sec SID
- 14. Predictive Decoding: DTX Discontinuous Transmission Frame Replacement Voice Decoder Comfort Noise Synthesizer Voice, 13 bit 8000 sample/sec FI Voice Frame SID Frame
- 15. GSM Transmission Chain: Speech Encoder Channel Encoder Interleaver Ciphering Burst Assembling & Multiplexing Modulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Channel Microphone Digital Speech Signal Analog Electrical Speech Signal Acoustic Signal
- 16. Channel coding: 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. GSM uses: 1. Block code 2. Convolutional code 1 1 0 1 0 1 0 1 Channel Coding Bits Speech Bits
- 17. Convolutional code: 1:2 Convolution al Coder 53 bBlock Coder 50 very Important bits 378 b 132 Important bits 0000 78 Not So Important 378 b 456 bits
- 18. Convolutional code: 𝒎𝒋−𝟐 𝒎𝒋−𝟏 𝒎𝒋 State input Input rate 𝒓 𝒃 𝒙𝒋 ′ 𝒙𝒋 ′′ Output rate 𝟐𝒓 𝒃
- 19. Channel Coding for TCH/FR: 260 bits / 20 msec Tail bits 378 78 Class 2 78 bits 50 3 132 4 Parity Check 456 bits / 20 msec 1 : 2 Convolutional Coder Class 1a 50 bits Class 1b 132 bits
- 20. GSM Transmission Chain: Speech Encoder Channel Encoder Interleaver Ciphering Burst Assembling & Multiplexing Modulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Channel Microphone Digital Speech Signal Analog Electrical Speech Signal Acoustic Signal
- 21. Interleaving: First level: Block interleaving 57 bits 1 9 17 25 . . . 449 2 10 18 26 . . . 450 3 11 19 27 . . . 451 4 12 20 28 . . . 452 5 13 21 29 . . . 453 6 14 22 30 . . . 454 7 15 23 31 . . . 455 8 16 24 32 . . . 456
- 22. GSM Transmission Chain: Speech Encoder Channel Encoder Interleaver Ciphering Burst Assembling & Multiplexing Modulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Channel Microphone Digital Speech Signal Analog Electrical Speech Signal Acoustic Signal
- 24. GSM Security: GSM PLMN AUC IMSI, Ki, A3, A8 HLR EIR IMEI VLR MSC A5 SIM: (IMSI, Ki, A3, A8) ME: (IMEI & A5) BTS
- 25. Authentication Center (AUC): It is a processor system that performs the authentication function RAND Database IMSI, KI 𝑨 𝟑 𝑨 𝟖 RAND SRES 𝑲 𝒄 Triplet Triplet Request for Triplet from HLR, VLR IMSI Ki AUC
- 26. Authentication: =/ ≠ VLR 𝑨 𝟑 𝑨 𝟑 Ki KiRAND IMSI HLR/AUCMS SRES SRESSRES Yes No Continue Ciphering “Authentication Refused” Message
- 27. IMEI check: MSC/VLR Initiate IMEI Request EIR Check IMEI White ListGray List Black List MS BSS MSC/VLR IMEI Request IMEI Response
- 28. Ciphering: NetworkMS 𝑨 𝟖 𝑨 𝟖 Ki KiRAND 𝑲 𝒄 𝑲 𝒄
- 29. Ciphering: VLR MSC Decryption Process Using A5 Encryption Process Using A5 1. CMC + 𝑲 𝒄 2. CMC MS 𝑲 𝒄 BTS 4. Encrypted CMC 5. Decrypted CMC Successful? 6. Ciphering Mode Completed Cipher Mode Command 3. Encrypt CMC 𝑲 𝒄CMC If Yes
- 30. Ciphering: A5 A5 1 0 1 1 0 0 1 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 SYNC. Cipher Sequence Key Flow Cipher Sequence Key Flow 𝑲 𝒄 User data Flow Ciphered Text Ciphering Process Deciphering Process User data Flow FN 𝑲 𝒄 F N
- 31. GSM Transmission Chain: Speech Encoder Channel Encoder Interleaver Ciphering Burst Assembling & Multiplexing Modulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Channel Microphone Digital Speech Signal Analog Electrical Speech Signal Acoustic Signal
- 32. Interleaving: 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 data bits 57 data bits57 data bits
- 33. GSM Transmission Chain: Speech Encoder Channel Encoder Interleaver Ciphering Burst Assembling & Multiplexing Modulator 13 Kbps 22.8 Kbps 22.8 Kbps 22.8 Kbps 33.85 Kbps Channel Microphone Digital Speech Signal Analog Electrical Speech Signal Acoustic Signal
- 34. Modulation: High bandwidth. Low power drain. Good BER performance. Low implementation cost. “GMSK is the modulation scheme of the GSM system”