Mocomtech CDM-570 - Manual (Page 192)

CDM-570/570L Satellite Modem with Optional IP Module Revision 4
Forward Error Correction Options MN/CDM570L.IOM
7.2 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) 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 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).
Note that in BPSK mode, the CDM-570/570L only permits a coding rate of 1/2. Because
the method of convolutional coding used with Viterbi, the encoder does not preserve the
original data intact, and is called non-systematic.
Table 7-1. Viterbi Decoding Summary
FOR AGAINST
Good BER performance - very useful coding gain. Higher coding gain possible
with other methods
Almost universally used, with de facto standards for
constraint length and coding polynomials
Shortest decoding delay (~100 bits) of any FEC
scheme - good for coded voice, VOIP, etc.
Short constraint length produces small error bursts -
good for coded voice.
No pronounced threshold effect - fails gracefully.
Coding gain independent of data rate.
7.3 Reed-Solomon Outer Codec (Hardware Option)
IMPORTANT
It cannot be emphasized strongly enough that the purpose of the concatenated Reed-
Solomon is to dramatically improve the BER performance of a link under given
noise conditions. It should NOT be considered as a method to reduce the link EIRP
to the point where rain-fade margin, particularly at Ku-band, is no longer required.
The concatenation of an outer Reed-Solomon Codec with Viterbi decoder first became
popular when Intelsat introduced it in the early 1990s. It permits significant
7–2