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Chapter 7. FORWARD ERROR
7.1
Introduction
As standard, the CDM-625 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, 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. This provides 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 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 constraint length, fixed at 7, for all code rates. (The constraint length is
defined as the number of output symbols from the encoder that are affected by a single input bit.)
By choosing various coding rates (Rate 1/2, 3/4 or 7/8) the user can trade off coding gain for
bandwidth expansion. Rate 1/2 coding gives the best improvement in error rate, but doubles the
transmitted data rate, and hence doubles the occupied bandwidth of the signal. Rate 7/8 coding, at
the other extreme, provides the most modest improvement in performance, but only expands the
transmitted bandwidth by 14 %. A major advantage of the Viterbi decoding method is that the
performance is independent of data rate, and does not display a pronounced threshold effect (i.e.,
does not fail rapidly below a certain value of Eb/No). Note that, in BPSK mode, the CDM-625
only permits a coding rate of 1/2. Because the method of convolutional coding used with Viterbi,
the encoder does not preserve the original data intact, and is called non-systematic.
CORRECTION OPTIONS
7–1
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