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4.4StoppingTestUnit
4.4.1SNRThresholdTermination
4.4.2CRCTermination
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Standalone(SA)Mode
TheCRC-basedstoppingcriterioncanbeusedbysettingtheCRCpolynomiallength(CRCLEN)andthe
numberofCRCiterationsrequiredtopassCRCITERPASS.Aftereachiteration,harddecisionsare
computedandaCRCisperformed.TheCRCpolynomialisaprogrammable32-bitnumber.Toavoid
situationswhereaCRCtestpassesforaverynoisyframeofdata,theharddecisionsneedtopassthe
CRCtestforanumberofconsecutiveiterations,whichisuser-definedviatheCRCITERPASSbitfield.
Turbodecodersareiterativedecoders.EachiterationconsistsoftwoMAPdecodesexceptthelast
iterationthatexecutesonlythefirstMAPdecode.Theturbodecodercaniterateupto32iterations.The
decoderwillcontinuetoiterateuntiloneofthefollowingconditionsoccur:meetparameterconditions,
CRCpassed,orSNRthresholdpassed.
Thestoppingcriteriaalgorithmgeneratesthefirsttwomomentsoftheextrinsics,generatesanSNRratio,
andcomparestheratiowithathreshold.Ifthecalculatedratioexceedsthethreshold,thenthedecoder
hasfoundanoptimumsolution.Thedecodercanthenstopexecutinganyfurtheriterations.The
calculatedSNRratioisgeneratedaftereachMAPprocess.Thethresholdisauserinputandcanrange
from0to100.Largerthresholdsgivebetterresultsbutrequiremoreiterations.Smallerthresholdsrequire
feweriterationsandgivecangivepoorerresults.Settingthethresholdto0disablesthestoppingcriteria
algorithm.
Thestoppingcriteriacontainstwoparts.Thefirstpartexecutesoneachextrinsicvalue.Thesumofthe
extrinsicsandthesumoftheextrinsicssquaredarecalculated.Thesecondpartisexecutedonceatthe
endofeachMAPblock.Thefirstmomentissquaredandmultipliedbythesumof1plustheinverseofthe
threshold.Thesecondmomentismultipliedbythenumberofsymbolsperframe.Thetworesultsare
compared.Iftheresultispositive,thenthestoppingcriteriahasbeenmet.
TheturbodecoderwillgenerateablockofextrinsicsaftereachMAPdecode.TheSNRstoppingcriteria
blockcalculatesthemeanandthevarianceforthisblock.Itwilldividethetwoandcomparetheresultwith
thesnr_threshold.Iftheresultisgreaterthenthesnr_thresholdfortwoconsecutiveMAPdecodes,then
thedecoderwillstopexecuting.TheDSPsetsthesnr_thresholdparameter.TheSNRstoppingcriteria
canbeturnedoffwithavalueof0.Enabledvaluesforsnr_thresholdrangefrom1to100.Avalueof100
givesthebestBERperformanceatacostofthemostiterationsexecuted,andavalueof1givesthe
worstBERperformanceatacostofthefewestiterations.Recommendedsettingforthisparameteris100.
AframeofdataissentthroughaCRCblockwhichappendscrc_lengthnumberofbitstotheframe.This
frameisencodedbytheturboencoder.ThepolynomialfortheCRCcheckisdefinedwiththecrc_poly
parameter.Theturbodecoderwillgenerateharddecisionbitsaftereachnon-interleavedMAPdecode.
ThesebitsareprocessedbytheCRCblockwithinthedecoder.Ifthelastcrc_lengthbitsmatchtheCRC
pattern,thentheCRCcheckhaspassed.TheturbodecoderwillstopexecutingafterCRCITERPASS
numberofconsecutiveCRCpassesasprogrammedinTCPIC4.
ThecoefficientsandthesizeoftheCRCpolynomialareprogrammable.Thesizeofthepolynomialis
definedwiththeparametercrc_lengthandcanbesetfrom0to32bits.Avalueof0disablestheCRC
check,valuesbetween1and32enabletheCRCcheck.TheCRCpolynomialisdefinedwiththecrc_poly
parameter.TheCRCunitwillnotbeenableduntilthedecoderiterationcountisequalorgreaterthanthe
min_iterparameter.Theturbodecoderwillgenerateharddecisionsaftereachnon-interleavedMAP
decode.ThesebitsareprocessedbytheCRCblockwithinthedecoder.Ifthelastsetofframebitsmatch
theCRCpattern,thentheCRCcheckhaspassed.Theturbodecodermustpassanumberofconsecutive
iterationstoterminatebeforemax_iter.Thenumberofconsecutiveiterationspassedisdefinedwiththe
crc_iter_passparameter.Thecrc_iter_passparametercanbesetfrom0to31,azeroisequalto1
iteration.Thedec_passoutputparameterwillbesettoa1ifthedecoderterminatedduetoapassing
CRC.
Duringthesub-blockexecution,upto256setsofdatawillbestoredinadoublebufferedRAMwhosesize
is265x7x2.Twobitseachwillbestoredforx0,p0,andp1.Onebitisthesignbitandtheotherbitissetif
thesymbolisequaltoazero.These6bitswillbeusedforre-encoding.Theseventhbitwillbethehard
decisionbit.Thisbitisthesignofthefollowingsummation:(x+a+w).
SPRUGK1–March2009TMS320C6457Turbo-DecoderCoprocessor217
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