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Chapter 6. FORWARD ERROR
6.1
Introduction
As standard, the Modem is equipped with three Forward Error Correction encoders/decoders:
Viterbi, 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 (IESS-315), 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 modem may be fitted with a Turbo Product Codec. They are plug-in daughter cards
(SIMM modules) that are field upgradeable. The codec provides data rate capability up to
20 Mbps, and code rates of:
•
•
•
•
Turbo Product Coding provides one of the best Forward Error Correction technologies
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.
6.2
Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The Modem complies with the Intelsat
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
CORRECTION OPTIONS
Rate 5/16 (BPSK
Rate 21/44 (BPSK, QPSK)
Rate 3/4, and 7/8 (QPSK, 8-PSK, and 16-QAM)
Rate 17/18 (QPSK and 8-PSK)
6–1
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