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Chapter 7. forward error correction options, 1 overview, 2 viterbi – Comtech EF Data CDM-625 User Manual

Page 273

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7–1

Chapter 7. FORWARD ERROR

CORRECTION OPTIONS

7.1 Overview

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

®

. (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.

7.2 Viterbi

The combination of convolutional coding and Viterbi decoding has become an almost universal

standard for satellite communications. The unit 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