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The modem fully implements the IESS-310 specification at data rates up to 51.84 Mbps. In
accordance with the specification, the R-S outer code cannot be disabled.
Exceptionally bandwidth efficient compared to QPSK
Interoperable with legacy Intelsat networks
D.4
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. 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.
Appendix D
Table D-2. 8-PSK/TCM Coding Summary
FOR
D–3
SLM-5650B Satellite Modem
AGAINST
Needs concatenated RS outer codec to give acceptable
coding gain performance
Demodulator acquisition threshold much higher than for
QPSK
8-PSK is more sensitive to phase noise and group delay
distortion than QPSK
Revision 2
MN-SLM-5650B
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