Product specifications

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):