Lesson 8 Speech coding

Transcription

1 Lesson 8 coding Encoding Information Transmitter Antenna Interleaving Among Frames De-Interleaving Antenna Transmission Line Decoding Transmission Line Receiver Information Lesson 8 Outline How information is transmitted over radio links Coding in Wireless Systems ( 語音編碼 ) A/D, PCM Source coding Vocoders of Major Standards Error Control (Error Control coding) 錯誤更正 Error Detection Error Correction - Forward Error Correction (FEC) Interleaving ( 交錯 ) Multiple Access Methods Structure of Individual TDMA slots Modulation & Access Schemes used in Wireless Telephony Baseband ( 基頻 ) and RF band Circuit Handset Circuits Baseband Digital and Baseband Analog Lesson 8 coding Dr. Sheng-Chou Lin Page

2 Coding in Wireless Ststems Goal is to digitize and compress speech and transmit it with the best quality possible and the smallest number of bits Three categories of speech coders Waveform coders PCM, ADPCM, DM Vocoders LPC, Phase, Formant Hybrid coders RPE-LTP, CELP, VSELP Hybrid coders are used in the major wireless systems, where bandwidth is scarce, but quality is important. Low-tier PCS mostly uses ADPCM. speech Source coder Channel coder Intereaver Coder Decoding Modulator Encoding Interleaving Among Frames De-Interleaving Multiplexing Lesson 8 coding 2 Dr. Sheng-Chou Lin Vocoders of Major Standards (GSM) GSM RPE-LTP Coding: 6 A/D samples every 2ms 3kb/s Analog Bandpass Filter 3-3,4Hz A/D 8kHz 3-bit 4kb/s Pre-Emphasis Filter LPC LTP RPE RPE-LTP Encoder To channel coder 3-bit = 2 3 = 8,92 Quantization levels VAD Blocks of 26 Bits every 2ms Note: Decoder includes interpolation, A lowpass filter, 3-8 bit converter, and does not include VAD RPE Regular Pulse Excitation LTP Long Term Prediction VAD Voice Activity Detection Lesson 8 coding 3 Dr. Sheng-Chou Lin Page 2

3 Vocoders of Major Standards (TDMA) TIA/EIA-36 TDMA Use Vector Sum Excited Linear Prediction (VSELP) at rat of 7.95 kb/s (typically referred to as 8k vocoder) Analog speech 64 kb/s speech A/D converter Log/Linear Converter switch 2 ms store 6 samples 7.95 kb/s VSELP Vocoder Filter coefficients: 38 bits Frame Energy: 5 bits Pitch lag: 28 bits Codewords: 56 bits Gain values: 32 bits 77 class bits 82 class 2 bits To channel corder Now uses much improved Algebraic Code Excited Linear Prediction (ACELP), or IS-64. ACELP uses a slightly lower rate of 7.4 kb/s giving up more bandwidth for channel coding. Lesson 8 coding 4 Dr. Sheng-Chou Lin TDMA Error Control IS-54 TDMA uses several techniques to protect the transmitted data in the hostile, error-prone RF transmission environment. After speech coding, the following steps occur: Encoding the data is combined with an error-recovery code and expanded Interleaving each block of coded speech is spread across two frames RF Transmission De-Interleaving blocks of speech are reassembled Decoding most errors are corrected using the convolutional code Encoding Interleaving Among Frames Decoding De-Interleaving Lesson 8 coding 5 Dr. Sheng-Chou Lin Page 3

4 Channel Coding ( 頻道編碼 ) Purpose is to protect the source information in an ofter volatile environment, by adding redundancy Shannon taught us that we can make the BER ifinitely small by adding redundancy to the data Redundancy = more bits Two categories of channel coding Error Detection Error Correction, namely Forward Error Correction (FEC) Coding Gain is the difference in Signal-to-Noise Ratio (SNR) provided by a coded signal versus an un-coded signal, for a fixed Bit Error Rate (BER) Lesson 8 coding 6 Dr. Sheng-Chou Lin Channel Coding Process Example (GSM) GSM (like most standards) uses a combination of Block and Coding Pre-LTP Coder 78 least important bits 32 important bits Blocks of 26 bits 5 essential bits Calculate 3 Parity Bits 53 bit Rate ½ Code 456 bits to Multiplexing and Interleaving, Arranged as data Fields over 4 time slots 4 tail bits Lesson 8 coding 7 Dr. Sheng-Chou Lin Page 4

5 Interleaving ( 交錯 ) Interleaving is a means of adding time diversity to a signal without adding any redundant bits Bit errors typically occur together By jumbling the bits, errors are diversified Channel coding cam make up for a small number of bit errors Coded Data Interleaved Coded Data Transmission De-interleaved Coded Data A block of bits is lost due to a channel impairment The large block Of lost bits is Distributed over A large range Lesson 8 coding 8 Dr. Sheng-Chou Lin Interleaving (cont.) Interleaving is often done in a matrix form: Row Read in by column Read out by row Column Lesson 8 coding 9 Dr. Sheng-Chou Lin Page 5

6 Modulation & Access Schemes Used in Wireless Telephony System AMPs Analog Cellular Voice Channels Control & Messaging IS-54 TDMA NADC Cellular (voice channels) GSM-9 DCS-8 PCS-9 GSM-Based IS-95 CDMA ESMR MIRS Access Strategy FDMA FDMA FDMA + TDMA FDMA + TDMA TDMA + CDMA FDMA + TDMA Modulation Type RF Channel Width C/I Req FM 3 khz 7 db FSK PSK: /4 DQPSK 3 khz 3 khz 7 db 7 db FSK: GMSK OQPSK (bs) QPSK (ms) 2 khz.23 MHz 7-9 db 6 db M6QAM 25 khz 8 db Users per RF channel n/a (analog) * n/a 3 (future 6?) VSELP 8kb 8 (future 6?) RELP-LTP 3kb <8 2 practical Var. Rate 8kb, 3kb 6 8kb Lesson 8 coding Dr. Sheng-Chou Lin Cellular TDMA Format One Frame Slots One Frame (frames repeat continuously) A TDMA signal is a continuous stream of bits divided into: Frames 25 frames per second -- frame duration: 4 milliseconds Slots in each frame, six timeslots numbered through 5 each timeslot contains 324 bits of information Individual communication bursts occupy different timeslots Current implementation using the Full rate speech coder serves three users, each one occupying two timeslots per frame (i.e., & 3, & 4, or 2 & 5) Half-rate coder planned to serve six users, one timeslot each Lesson 8 coding Dr. Sheng-Chou Lin Page 6

7 Structure of Individual TDMA slots A timeslot is 6.66 msec long and contains 324 bits of information slots contain various fields of data and signalling information The timeslot structures on forward and reverse paths differ slightly forward path is continuously transmitted by base station reverse path is transmitted intermittently by users Cell Downlink orward Path 324 bits 6.66 ms SYNC SACCH DATA CDVCC DATA RSV Bits: Uplink everse Path 324 bits 6.66 ms G R DATA SYNC DATA SACCH CDVCC DATA Lesson 8 coding 2 Dr. Sheng-Chou Lin Handset Design Antenna Modulator / Demodulator A/D D/A RF Circuit CPU I/O SRAM DSP Flash Lesson 8 coding 3 Dr. Sheng-Chou Lin Page 7

8 Antenna T/R switch SAW Power Amplifier RF Circuit Diagram LNA VCO RF Mixer Phase Detector SAW VCO Mixer VCO IF Mixer /2 I/Q Mod I/Q Demo Synthesizer Synthesizer I Q I Q BASEBAND Lesson 8 coding 4 Dr. Sheng-Chou Lin RF Module and Circuit Design Antenna T/R Switch (Duplexer) Mixer VCO Synthesizer SAW filter Power Amplifier (PA) Low Noise Amplifier (LNA) Antenna Radiation Pattern, Gain Filter Bandwidth, Quality (Q), Stability Circuit Matching S parameters, Smith Chart Performance Sensitivity, S/N, BER Lesson 8 coding 5 Dr. Sheng-Chou Lin Page 8

9 Baseband Digital and Baseband Analog System Control - CPU SRAM, Flash, Serial Port, I/O, Main program Software execution LCD, LED, keypad, SIM card interface Control Power Saving Management Digital Signal Processing - DSP Modulation/Demodulation Voice Compression/Decompression Analog to Digital conversion I/Q signal from RF module Microphone Battery Charger Digital to Analog Conversion I/Q signal to RF module Speaker Buzzer Lesson 8 coding 6 Dr. Sheng-Chou Lin Lesson 4 Complete Lesson 8 coding 7 Dr. Sheng-Chou Lin Page 9