Comtech EF Data CDM-750, Installation And Operation Manual

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H–1
Appendix H. FEC (FORWARD
ERROR CORRECTION)
H.1 Introduction
The CDM ‐ 750 Advanced High ‐ Speed Trunking Modem operates with error correction based on
the DVB ‐ S2 standard for QPSK, 8PSK, 16APSK and 32APSK with concatenated L ow D ensity P arity
C ode (LDPC) and B ose ‐ C haudhuri ‐ H ocquenghem (BCH).
H.2 DVB-S2: LDPC and BCH
The DVB ‐ S2 specification defines a generation of performance that boosts throughput by about
30% over DVB ‐ S while using the same amount of bandwidth. The result is coding and
modulation that surpasses the capability of concatenated Viterbi and Reed Solomon coding.
LDPC and BCH is also a concatenated error correction technique; the LDPC coding scheme features
significant, Near ‐ Shannon Bound Performance.
In some cases, LDPC error correction starts flaring toward an error floor as the carrier ‐ to ‐ noise ratio
increases. To compensate, BCH error correction follows LDPC and eliminates the flare for any
practical range of error rates.
LDPC also functions differently than Viterbi decoding by using iterative 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. Through soft decision output from the LDPC decoder and a
high ‐ speed processor operating at a rate much higher than the data rate, the iterative process is
run 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 correction operates by
passing data through the convolutional error correction process using a single error correction
pass.
The error correcting capability of LDPC is enhanced by use of large block sizes. Although large
block sizes can increase latency in low bitrate applications (typically less than 2Mbps), this is not
a drawback in one ‐ way broadcast applications. Links with LDPC normally operate at multi ‐