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Chapter 7. FORWARD ERROR
7.1
Introduction
As standard, the CDM-600L 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 Coding represents a very significant development in the area of FEC, and
optionally, the CDM-600L may be fitted with one of two Turbo Product Codecs They are
plug-in daughter cards (SIMM modules), both field upgradeable. The Low Rate option
provides data rate capability up to 5 Mbps, and code rates limited to Rate 5/16 (BPSK,
Rate 21/44 (BPSK) and Rate 3/4 (QPSK, Offset QPSK, 8-PSK and 16-QAM). The High
Rate option provides data rate capability up to 20 Mbps, in addition to Rate 7/8 and Rate
0.95 capability. Turbo Product Coding provides the best Forward Error Correction
technology currently available, and is now offered with a sufficient range of code rates
and modulation types that link performance can be optimized under any conditions.
7.2
Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The CDM-600L 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 inter-operability 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
CORRECTION OPTIONS
7–1
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