Comtech EF Data CDM-570 - Manual (Page 167)

8–1
Chapter 8. FORWARD ERROR
CORRECTION OPTIONS
8.1 Introduction
As standard, the CDM-570/570L Modem is equipped with an industry-standard Viterbi Forward
Error Correction (FEC) encoder/decoder. The constraint lengths and encoding polynomials are
compatible with the vast majority of existing modems from other manufacturers.
Comtech EF Data has performed compatibility testing to ensure interoperability. In addition,
there are two plug-in daughter cards (SIMM modules), both field upgradeable, for adding other
FEC functionality. The first of these is a Concatenated Reed-Solomon Codec, which is combined
with Viterbi coding, to significantly improve BER versus E
b
/N
o
performance. It is required for
running 8-PSK/TCM, and for the 16-QAM Viterbi modes.
The second optional plug-in card is the Turbo Product Codec. Turbo Coding represents a very
significant development in the area of FEC, and Comtech EF Data’s Turbo Product Codec offers
Rate 5/16 and Rate 21/44 for BPSK, Rate 21/44 QPSK, Rate 3/4 and Rate 7/8 for QPSK,
OQPSK, 8-QAM, 8-PSK and 16-QAM, and Rate 0.95 for QPSK, 8-QAM and 8-PSK. Turbo
Product Coding provides the best Forward Error Correction technology currently available, and is
now offered with a sufficiently broad range of code rates and modulation types that link
performance can be optimized under any conditions.
8.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost universal
standard for satellite communications. The CDM-570/570L 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), you can trade off coding gain for
bandwidth expansion. Rate 1/2 coding gives the best improvement in error rate, but doubles the