8.4.2InputSign
8.4.3LogEquation
8.4.4Re-Encode
9Programming
Programming
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TheTCPassumesthattheencodedbitsareconvertedintosignedbinarysymbolsusingthefollowing
mapping:0→-1,1→+1andscaledby-2*a/Σ
2
whereaisthefadingfactorandΣisthenoisevariance.
Manyreceiversmayperformthisscalingwithoutapplyingthe-1factor.WithTCP,thisrequirestheDSPto
performthe-1multiplicationastheTCPexpectsthisscaling.InTCP2,the-1multiplicationcanbe
performedautomaticallybytheTCP2basedontheINPUTSIGNbitfieldoftheTCPIC3configuration
register.ThisreducesDSPoverheadanddoesnotcostextraTCP2cycles.
TheexactmathematicalequationforforwardandbackwardrecursioninaMAPdecoderisbasedonalog
ofasumoftwoexponentialterms,In(e
A
+e
B
).Duetothecomplexityofhardwareimplementation,this
expressionisoftenapproximatedwiththeco-calledmax*term,whichiscomputedasmax*(A,B)=max
(A,B)+In(l+e|-
A-B
|));i.e.,themaximumoftheexponentsandacorrectiontermwhichisafunctionofthe
differenceoftheexponents.Thecorrectiontermisoftenimplementedasatablelook-up.Suchdecoderis
calledMax*Log-MAP.Ifthecorrectiontermisdropped,theimplementationbecomesmax-log-MAP.
WhileTCPusesaMax*-log-MAPimplementation,TCP2offersbothMax*Log-MAPandmax-log-MAP
implementations.Thiscanbeselectedonaframe-by-framebasis.Thesecondimplementationdoesnot
requiretheinputLLRs(log-likelihoodratios)tobescaledbyafactorinverselyproportionaltonoise
variance,andisthereforemorerobustinsituationswhereSNRcannotbeaccuratelyestimated.
There-encodeblockisdirectlyconnectedtotheCRCblock.Duringthesub-blockexecution,upto256
setsofdatawillbestoredinadoublebufferedmemory.Twobitseachwillbestoredforx0,p0,andp1.
Onebitisthesignbitandtheotherbitissetifthesymbolisequaltoazero.These6bitswillbeusedfor
re-encoding.Theseventhbitwillbetheharddecisionbit.Thisbitisthesignofthefollowingsummation:
(x+a+w).
Thedecisionbitscanbere-encodedwithaconvolutionalencoder.Theoutputoftheencoderis3bit
streams:systematicbitand2paritybits.Thesebitscanbecomparedwiththesignsofthestored
systematicandparitysymbols.Ifthebitsmatch,thennoerrorhasoccurred.Ifthebitsdonotmatch,then
anerrorhasoccurred.Ifthestoredsymbolisazero,thennoinformationcanbedeterminedfromthis
data.Azerosymbolrepresentseitheradepuncturedsymbolorasymbolthatisequaldistancefromthe
idealmodulated+1or-1.Asthesignsarecompared,arunningcountofthetotalnumberofsign
differencesiscalculated.Thesecountscanbeusedasanestimateofthechannelquality.
Thecnt_re_map0outputregisterisasumofthenumberofsigndifferencesforMAP0.Thecnt_re_map1
outputregisterisasumofthenumberofsigndifferencesforMAP1.Table41showsthevalidsymbols
thatcanbeusedduringthecalculationoferrorsforeachMAP.
Table41.ValidRe-EncodeSymbolsUsedforComparison
MAPValidSystematic(x)ValidParity(p0)ValidParity(p1)
0YesYesYes
1NoYesYes
TheTCP2requiressettingupthefollowingcontextperuserchannel:
•Standalone(SA)mode
–3to5EDMA3parameters(seeTable42)
–Theinputconfigurationsparameters
•Shared-processing(SP)mode
–3to4EDMA3parameters(seeTable43)
–Theinputconfigurationsparameters
64TMS320C6457Turbo-DecoderCoprocessor2SPRUGK1–March2009
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