Comtech EF Data CDM-550T - Manual (Page 75)

Chapter 7. FORWARD ERROR
CORRECTION OPTIONS
7.1 Introduction
As standard, the CDM-550T Modem is equipped with two Forward Error Correction (FEC)
Decoders, Viterbi, and Sequential. The constraint lengths and encoding polynomials are
compatible with the vast majority of existing modems from other manufacturers, and Comtech EF
Data has performed compatibility testing to ensure inter-operability.
As an option, the CDM-550T may be fitted with two additional FEC devices. The first is a
concatenated Reed-Solomon Codec. This is always used in conjunction with, rather than instead
of, the main FEC method (Viterbi or Sequential). It comprises a small daughter card, and may be
fitted at the factory at the time of order, or may be added by the user at a later date. The second
option is Comtech EF Data’s Turbo Product Codec (TPC), which is an entirely stand-alone
method of FEC. This too is a small daughter card.
7.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost universal
standard for satellite communications. The CDM-550T 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 which 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). This is not true of the Sequential decoding
method, as explained in the next section.
7–1