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B–1

Appendix B. FEC (FORWARD

ERROR CORRECTION)

B.1 Introduction

The method of FEC (Forward Error Correction)used by the CDM‐800 Gateway Router  is based

upontheDVB‐S2standardforQPSK,8PSK,16APSKand32APSKwithconcatenatedLowDensity

ParityCode(LDPC)andBose‐Chaudhuri‐Hocquenghem(BCH).

B.2 DVB-S2: LDPC and BCH

TheDVB‐S2specificationhasdefinedanewgenerationofperformancethatbooststhroughput

by about 30% over DVB‐S using the same amount of bandwidth. This new type of coding and

modulationfarexceedsthecapabilityofconcatenatedViterbiandReedSolomoncoding.



LDPC and BCH is also a concatenated error correction technique. LDPC is a ver

y powerful coding

scheme with significant, Near‐ShannonBound Performance.In some cases,asthe carrier‐to‐noise

ratioincreases,theLDPCerrorcorrectionstartsflaringtowardanerrorfloorsoBCHerrorcorrection

followsLDPCandeliminatesthefl areforanypracticalrangeoferrorrates.



LDPC

alsofunctionsdifferentlythanViterbidecodingbyusingi terativedecoding.Inthisprocess

the data initially corrected by the LDPC decoder is re‐encoded and run through the decoder

againtocorrectadditionalerrors.KeytothisisthesoftdecisionoutputfromtheLDPCdecoder

and a high‐

speed processor operating at a rate much higher than the data rate. The LDPC

decoder runs the iterative process as many times as possible before corrected data is finally

outputtomakewayforanewblockofdataenteringthedecoder.LDPCalsousesinterleavingto

spread the errors. In contrast, Viterbi error correct

ion operates by passing data through the

convolutionalerrorcorrectionprocessusingasingleerrorcorrectionpass.

B.2.1 Range of Data Rates

FortherangeofDataRates,refertoSect. 1.4 SUMMARY OF SPECIFICATIONS.

B.2.2 BER, QEF, Eb/No, Es/No Spectral Efficiency, and Occupied Bandwidth

Depending upon the operating mode, the DVB standard uses different modes of specifying

performancewithaunitinIFLoopandAd di tiveWhiteGaussianNoise(AWGN):

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Comtech EF Data CDM-800 Especificaciones Pagina 123