channelcoding信道编码英文知识.ppt
1,Error Control Coding,Error Control Strategies,Performance Measures,Outline,Error Control StrategiesForward Error Correction(FEC)Automatic Repeat Request(ARQ)Hybrid ARQPerformance Measures Probability of decoding error(error probability)Coding gainShannon Limit,2,Forward Error Correction(FEC)One-way system,even not strictly one-wayDigital storage systems,deep-space communication systems and most of the coded systems in use today Automatic Repeat Request(ARQ)Two-way systemsome data networks and satellite communication systems,Error Control Strategies,3,FEC,Employ error-correcting codesAutomatically correct errors detected at receiverThis book is devoted mostly to the analysis and design of FEC system,4,ARQ,Stop-and-wait ARQContinuous ARQGo back NSelective reject(selective retransmission),5,Stop-and-wait ARQ VS Continuous ARQ,Continuous ARQ is more efficientContinuous ARQ is more expensiveStop-and-wait ARQ designed for half-duplex channelsContinuous ARQ designed for full-duplex channels,6,ARQ VS FEC,ARQ requires much simpler decoding equipment(Actually only detecting no correcting)ARQ is adaptive in the sense that information is retransmitted only when errors occurWhen error rate is high,the system throughput is lowered by using ARQA hybrid combination of FEC is more efficient than ARQ alone,7,How to measure the performance of a coded system?(A curve of BER vs)Probability of decoding error(error probability)-Y directionCoding gain-X direction,Performance Measures,8,Error probability(WER and BER)Probability of word(or block)error-WER-Word error rateProbability of bit error-BER-Bit error rateThese two error probabilities should be kept as low as possible,Error Probability,9,Error Probability(Relation between Eb and Es),The energy-per-information bit:energy per transmitted symbolR:code rate,R=k/n commonly used to express error probability:one-sided PSD(Power Spectrum Density)of the channel noise,10,Error Probability-Y direction,11,Coding gain,The reduction in the Eb/No required to achieve a specific error probability(BER or WER)Coding gain is important in communication applications Coding threshold is the SNR at which the coding gain actually becomes negativeAsymptotic coding gain the coding gain for large SNR,12,Coding gain,13,Coding threshold,14,Shannon Limit,Definition:the minimum SNR required-error-free-coded system-code rate RMeaning:SNR should exceeds Shannon limitUse:measure the maximum achievable coding gain,15,-error-free-AWGN-BPSK-coded system-R=0.5-Shannon limit:0.188dB,Figure of Shannon limit,16,R:00.99Eb/No:Shannon limitExample:-AWGN channel-BPSK-10-5-uncoded system:9.65dB-coded R=0.5:0.188dB-(coding gain)max:9.462dB(So you can compare different coding schemes to see which one is better),Table of Shannon limit(Ref.Shu Lin),BER Performance of Convolutional Code,17,Soft-decision MLD,R=0.5,m=6,18,Improvement gap:0.5dB,4.4dB,3.91dB,(65520,61425),R=15/16,BER Performance of parity-check code,19,Thank you！,
