Mocomtech SLM-5650 - Manual (Page 119)

SLM-5650 Satellite Modem Revision 1
Options MN/SLM5650.IOM
A–3
A.3 Reed-Solomon Outer Codec
The concatenation of an outer Reed-Solomon (R-S) Codec with a Viterbi decoder first
became popular in the early 1990's. It permits significant improvements in error performance
without significant bandwidth expansion.
The coding overhead added by the R-S outer Codec is typically around 10%, which translates
to a 0.4 dB power penalty for a given link. Reed-Solomon codes are block codes (as opposed
to Viterbi and Sequential, which are convolutional), and in order to be processed correctly the
data must be framed and de-framed. Additionally, R-S codes are limited in how well they
can correct errors that occur in bursts.
This, unfortunately, is the nature of the uncorrected errors from Viterbi decoders, which
produce clusters of errors that are multiples of half the constraint length. For this reason, the
data must be interleaved following R-S encoding, and is then de-interleaved prior to
decoding. This ensures that a single burst of errors leaving the Viterbi decoder is spread out
over a number of interleaving frames, so errors entering the R-S decoder do not exceed its
capacity to correct those errors.
In the case of the SLM-5650, different R-S code rates are used, according to the mode of
operation: