B.3 reed-solomon outer codec, B.3.1 closed network modes – Comtech EF Data CDM-570A User Manual
Page 431
CDM-570A/570AL Satellite Modem with Optional Packet Processor
MN-CDM570A
Appendix B
Revision 2
B–3
B.3 Reed-Solomon Outer Codec
The concatenation of an outer Reed-Solomon (RS) Codec with Viterbi decoder first became popular
when it was introduced by Intelsat in the early 1990s. It permits significant improvements in error
performance without significant bandwidth expansion. The coding overhead added by the RS 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, which is convolutional; in order to
be processed correctly, the data must be framed and de-framed. Additionally, RS 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
RS 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 RS decoder do not exceed its capacity to correct those errors.
B.3.1 Closed Network Modes
A 220,200 code is used in transparent closed network modes, and a 200,180 code is used in
framed (EDMAC) modes. (220,200 means that data is put into blocks of 220 bytes, of which 200
bytes are data, and 20 bytes are FEC overhead.)
When Viterbi decoding is used as the primary FEC, an interleaver depth of 4 is used. The
increase in coding gain is at the expense of delay. The interleaving/de-interleaving delay and the
delay through the decoder itself can be as high as 25 kbits. At very low data rates, this equates
to several seconds, making it highly unsuitable for voice applications. Additionally, the de-
interleaver frame synchronization method can add significantly to the time taken for the
demodulator to declare acquisition.