Comtech EF Data CDM-625A, Installation And Operation Manual

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B–1
Appendix B. FEC (FORWARD
ERROR CORRECTION) OPTIONS
B.1 FEC Overview
As standard, the CDM-625A Advanced Satellite Modem is equipped with four Forward Error
Correction encoders/decoders – Viterbi, Sequential, concatenated Reed-Solomon and Trellis
(which is available with the 8-PSK FAST option). The constraint lengths and encoding
polynomials are not only Open Network compatible, but are also Closed Network compatible
with the vast majority of existing modems from other manufacturers. Comtech EF Data has
performed compatibility testing to ensure inter-operability.
Turbo Product Coding ( TPC ) and Low-Density Parity Check ( LDPC ) coding represent a significant
development in the area of FEC. An option card, field upgradeable, combines LDPC and TPC
together on one module.
While the performance of LDPC is exceptional in terms of coding gain, its higher latency is
considered disadvantageous in some applications. For this reason, Comtech EF Data and
Comtech AHA Enterprise Products Group have jointly developed a family of short-block LDPC
codes, which we have called
VersaFEC
®
. (The name VersaFEC is a trademark registered to
Comtech AHA.) It is ideal for lower data rates that demand the shortest possible latency, and is
available as a field upgradeable option card.
VersaFEC represents the best Forward Error Correction technology currently available, and is
offered with a sufficient range of code rates and modulation types that link performance can be
optimized under almost any conditions.