Comtech EF Data CDM-600/600L - Manual (Page 121)

6–1
Chapter 6. FORWARD ERROR
CORRECTION OPTIONS
6.1 Introduction
As standard, the CDM-600/600L Modem is equipped with four Forward Error Correction (FEC)
encoders/decoders – Viterbi, Sequential, concatenated Reed-Solomon, and Trellis (TCM,
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 interoperability.
Turbo Coding represents a very significant development in the area of FEC and, optionally, the
CDM-600/600L 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, OQPSK, 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.
Now, Low-Density Parity Check (LDPC) coding is being offered in addition to the TPC options.
In some instances this provides even better performance than TPC. A third option card, again
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 any conditions.
6.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost universal
standard for satellite communications. The CDM-600/600L 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 constraint length, fixed at 7, for all code rates. (The constraint