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4.1.2InterleaverIndexes
4.2OutputDecisionDataFormat
4.3StoppingCriteria
Standalone(SA)Mode
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Figure15.Rate3/4EN=0(Big-EndianMode)Rate=3/4
WordWord
NN+1
SP4SP3SP2SP1SP0SP9SP8SP7SP6SP5
000A0X00000X1
WordWord
N+2N+3
SP4SP3SP2SP1SP0SP9SP8SP7SP6SP5
0000X20A3'00X3
WordWord
N+4N+5
SP4SP3SP2SP1SP0SP9SP8SP7SP6SP5
0000X40000X5
Eachindexisa15-bitvaluebeingeffectivelysavedas16bitsright-justified.Givenanindexj,an
interleavertablet,andadatax,theinterleaveddataxisgivenasx'=x[t(j)].Table2showshowdata
mustbeorganizedinthememory.Thebaseaddressmustbedouble-wordaligned.Forbig-endian
configurations,seetheTCP2endianregister(TCPEND)inSection6.22.
Table2.InterleaverData
Little_big_endianEndian_intrDescription(MSBtoLSB)
001,0,3,2⇒3,2,1,0(halfword)
010,1,2,3⇒3,2,1,0(halfword)
10Endiannessmanagerhasnoeffect
3,2,1,0⇒3,2,1,0(halfword)
11Endiannessmanagerhasnoeffect
3,2,1,0⇒3,2,1,0(halfword)
HarddecisionsforTCP2are32-bitword-packed.Thebitorderingwithinthe32-bithard-decisionwordis
programmable,suchthattheoldestbitcanbeeitherintheMSBortheLSBposition.Theirdestination
storagebaseaddressmustbedouble-wordaligned.Moreover,thebufferlengthmustcontainaneven
numberofwords.
Theturbodecoderhasaniterativestructure,andthenumberofiterationsthatareperformedforeach
frameiseitheradeterministicnumberoritdependsonatestperformedontheturbodecoderoutputafter
eachiteration.Inthefirstcase,youdecidehowmanyiterationsshouldbeperformedpriortodecodinga
frame.Inthesecondcase,theturbodecoderperformstestsaftereachiterationtodeterminewhetherthe
iterativeprocessshouldcontinue.Inthiscase,theboundaryconditionsareprogrammed(forexample,the
minimumandthemaximumnumberofiterationsthatshouldbeperformed).Thetestsperformedarethe
SNRstoppingcriterionandcyclicredundancycheck(CRC)iterationspassed.
ThisSNRstoppingcriteriononTCP2canbeusedbysettingtheSNRthresholdfrom1to100(0disables
theSNRthresholdcheck).ThestoppingcriteriaismetandaTCPREVTisgeneratedwhentheSNR
thresholdismet,theminimumiterationshavebeenprocessed,andsufficientCRCiterationshavepassed,
ifCRCisenabled.ThisindicatesthatdecisionsarereadyfortheEDMA3toaccess.
Largerthresholdsimprovebit-errorrate(BER)performance,butrequiremoreiterations.Smaller
thresholdsrequirefeweriterations,butmayyieldpoorerBERperformance.Theactualnumberof
iterationsruncanbereadfromtheoutputparameters.
16TMS320C6457Turbo-DecoderCoprocessor2SPRUGK1–March2009
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