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MAP1
MAP2
Received systematics
Interleave
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A priori
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A priori
Received systematics
Received parities
Received parities A’ & B ’ symbols
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decisions
calculation
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X’ symbols
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Overview
usesthecorrespondingestimatesfromtheotherdecoderasapriorilikelihood.Theaprioriinformation
isseenasbeforehandknowledge,meaningthatsomemessagesaremorelikelytooccurthanothers.
Aposterioriinformationaddstotheaprioriinformationtheknowledgegainedbythedecoding.
Theuncodedinformationbits(corruptedbythenoisychannel)areavailabletoeachdecoderto
initializetheapriorilikelihoods.ThedecodersusetheMaximumaPosteriori(MAP)bitwisedecoding
algorithmthatrequiresthesamenumberofstatesasthewellknownViterbialgorithm.Theturbo
decoderiteratesbetweentheoutputsofthetwoconstituentdecodersuntilitreachessatisfactory
convergence.Thefinaloutputisahard-quantizedversionofthelikelihoodestimatesofthedecoders.
Figure2.3GPPandIS2000Turbo-DecoderBlockDiagram
TheDSPcontrolstheoperationoftheTCP2(Figure3)usingmemory-mappedregisters.TheDSP
typicallysendsandreceivesdatausingsynchronizedEDMA3transfersthroughthe64-bitEDMA3bus.
TheTCP2sendstwosynchronizationeventstotheEDMA3:areceiveevent(TCPREVT)andatransmit
event(TCPXEVT).
TheprocessingunitcanimplementtheMax*-Log-MAPorMax-Log-MAPapproximationsoftheBCJR
algorithmandisselectedwiththeE_MAX_STARbitoftheTCPIC3register.(SeeL.R.Bahl,J.Cocke,F.
Jelinek,andJ.Raviv,"Optimaldecodingoflinearcodesforminimizingsymbolerrorrate",.IEEETrans.
Inform.Theory,vol.IT.20,pp.284.287,Mar.1974andP.Robertson,E.Villebrun,andP.Hoeher,"A
comparisonofoptimalandsub-optimalMAPdecodingalgorithmsoperatinginthelogdomain",inProc.
1996IEEEInt.Conf.onCommunications(Seattle,WA),June1995,vol.2,pp.1009-1013.)
TheTCP2hastwofundamentalmodes:standalone(SA)andsharedprocessing(SP).
InSAmode,theTCP2iteratesagivennumberoftimesandoutputsharddecisions.InSPmode,the
TCP2executesasingleMAPdecodeandoutputsextrinsicinformation(softinformation).SAmodeis
typicallyusedforframesizesupto20730.SPmodemustbeusedforframesstrictlylargerthan20730.
Table1describeswhichmodetousedependingontheframesize.
TheTCP2inputdatacorrespondstochannellog-likelihoodratiosscaledon6bits,whiletheTCP2output
datatohard-decisions(SAmode)orextrinsics(SPmode)scaledon7bits.
SPRUGK1–March2009TMS320C6457Turbo-DecoderCoprocessor211
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