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Chapter 7. FORWARD ERROR
7.1
Introduction
As standard, the CDM-625 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 very
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 have jointly developed a completely new family of short-block LDPC codes, which
we have called VersaFEC
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.
7.2
Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost universal
standard for satellite communications. The CDM-625 complies with the Intelsat IESS 308/309
standards for Viterbi decoding with a constraint length of seven. This is a de facto standard,
even in a closed network environment, which means almost-guaranteed interoperability with
other manufacturer's equipment. It provides very useful levels of coding gain, and its short
decoding delay and error-burst characteristics make it particularly suitable for low data rate
coded voice applications. It has a short constraint length, fixed at 7, for all code rates. (The
CORRECTION OPTIONS
®
. (The name VersaFEC is a trademark registered to Comtech AHA.) It is
7–1
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