Comtech EF Data SDM-300A - Manual (Page 240)

SDM-300A Satellite Modem Revision 6
Forward Error Correction (Options) MN/SDM300A.IOM
9–4
9.4 Reed-Solomon Outer Codec
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
requirement to produce a given BER. Factors such as rain-fade margin, particularly
at Ku-band, are extremely important, and reducing link EIRP can seriously degrade
the availability of such a link.
The concatenation of an outer Reed-Solomon (R-S) Codec with Viterbi decoder first
became popular when Intelsat introduced it 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, Reed-Solomon
codes are limited in how well they can correct errors that occur in bursts. This,
unfortunately, is the nature of the uncorrected errors from both Viterbi and Sequential
decoders, which produce clusters of errors that are multiples of half the constraint length.
(This is particularly severe in the case of Sequential, where the constraint lengths are
considerably longer than Viterbi).
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
or Sequential 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 SDM-300A, different R-S code rates are used, according to the mode of
operation: