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7–1
Chapter 7. FORWARD ERROR
CORRECTION OPTIONS
7.1 Introduction
As standard, the CDM-600 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-600 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 povides
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.
7.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The CDM-600 complies with the Intelsat