Comtech EF Data SLM-5650A - Manual (Page 397)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–3
D.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 1990s as a means to permit 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 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.