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1、Wireless Mobile Communication and Transmission Lab.ErrorControlCodingErrorControlStrategies,PerformanceMeasures1Outline Error Control StrategiesnForward Error Correction (FEC)nAutomatic Repeat Request (ARQ)nHybrid ARQPerformance Measuresn n Probability of decoding error (error probability)nCoding ga
2、inShannon Limit 2Forward Error Correction(FEC)nOne-way system, even not strictly one-waynDigital storage systems, deep-space communication systems and most of the coded systems in use today Automatic Repeat Request(ARQ)nTwo-way systemnsome data networks and satellite communication systemsError Contr
3、ol Strategies3FECEmploy error-correcting codesAutomatically correct errors detected at receiverThis book is devoted mostly to the analysis and design of FEC system4ARQStop-and-wait ARQContinuous ARQnGo back NnSelective reject (selective retransmission)5Stop-and-wait ARQ VS Continuous ARQContinuous A
4、RQ is more efficientContinuous ARQ is more expensiveStop-and-wait ARQ designed for half-duplex channelsContinuous ARQ designed for full-duplex channels6ARQ VS FECARQ requires much simpler decoding equipment (Actually only detecting no correcting) ARQ is adaptive in the sense that information is retr
5、ansmitted 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 alone7How to measure the performance of a coded system? (A curve of BER vs ) nProbability of decoding error (error probability) - Y directionnC
6、oding gain - X directionPerformance Measures 8Error probability (WER and BER)nProbability of word (or block) error-WER-Word error ratenProbability of bit error-BER-Bit error rateThese two error probabilities should be kept as low as possibleError Probability9Error Probability(Relation between Eb and
7、 Es)The energy-per-information bitn : energy per transmitted symbolnR: code rate, R=k/n commonly used to express error probabilityn : one-sided PSD (Power Spectrum Density) of the channel noise10Error Probability - Y direction3.7dB3.7dB11Coding gainThe reduction in the Eb/No required to achieve a sp
8、ecific error probability (BER or WER)Coding gain is important in communication applications Codingthresholdis the SNR at which the coding gain actually becomes negativeAsymptoticcodinggain the coding gain for large SNR12Coding gainCodingthresholdCodingthresholdCodinggainCodinggain13Wireless Mobile C
9、ommunication and Transmission Lab.Shannon Limit Definition: the minimum SNR required - error-free - coded system - code rate RMeaning: SNR should exceeds Shannon limitUse : measure the maximum achievable coding gain14 - -error-free - -AWGN - -BPSK - -codedsystem - -R=0.5-Shannon limit:-Shannon limit
10、:0.188dBFigure of Shannon limit0.188dB0.188dBR=0.5R=0.515Wireless Mobile Communication and Transmission Lab.R:00.99Eb/No:ShannonlimitExample: - -AWGNchannel - -BPSK-10-5 -uncoded system: -uncoded system:9.65dB- -coded R=0.5:coded R=0.5:0.188dB - (coding gain) - (coding gain)maxmax: : 9.462dB(So you
11、can compare different coding schemes to see which one is better)Table of Shannon limit (Ref. Shu Lin)16BER Performance of Convolutional CodeCodinggain:Codinggain:4.15dB4.15dB(Codinggain)max:(Codinggain)max:9.462dB9.462dBImprovementgap:3.962dBImprovementgap:3.962dBShannonlimit:0.188dBShannonlimit:0.188dB4.15dB4.15dB9.65dB9.65dBSoft-decisionMLD,R=0.5,m=6Soft-decisionMLD,R=0.5,m=617Improvementgap:0.5dBImprovementgap:0.5dB4.4dB4.4dB3.91d3.91dB B(65520,61425),R=15/16,(65520,61425),R=15/16,BER Performance of parity-check code 18Thank youThank you！19个人观点供参考，欢迎讨论
