Mocomtech CDM-QX - Manual (Page 167)

7–1
Chapter 7. FORWARD ERROR
CORRECTION OPTIONS
7.1 Introduction
As standard, the CDM-Qx/QxL Modem is equipped with three Forward Error Correction (FEC)
encoders/decoders – Viterbi, concatenated Reed-Solomon, and Trellis (TCM, available with the
8-PSK FAST option). The constraint lengths and encoding polynomials are not only Open
Network (IESS-315) 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 Product Coding represents a very significant development in the area of FEC and,
optionally, the CDM-Qx/QxL may be fitted with Turbo Product Codec in the form of a plug-in
daughter card (SIMM module) that is field upgradeable. The codec provides data rate capability up
to 20 Mbps, and code rates of:
• Rate 5/16 (BPSK)
• Rate 21/44 (BPSK, QPSK)
• Rates 3/4 and 7/8 (QPSK, 8-PSK, and 16-QAM)
• Rate 17/18 (QPSK and 8-PSK)
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, such 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-Qx/QxL 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 interoperability 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.)