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SLM-7650 Satellite Modem
Theory of Operation
The available CEVD rates offered are:
•
Rate 1/2 is offered for BPSK
•
Rates 1/2, 3/4, and 7/8 for QPSK and OQPSK
•
Rates 3/4 and 7/8 are offered for 16-QAM.
Good BER performance - very useful coding gain.
Almost universally used, with de facto standards for
constraint length and coding polynomials
Shortest decoding delay (~100 bits) of any FEC scheme -
good for coded voice, VOIP, etc
Short constraint length produce small error bursts - good
for coded voice.
No pronounced threshold effect - fails gracefully.
Coding gain independent of data rate.
4.3.3.2
Reed-Solomon
Reed-Solomon is an added coding method to convolutional encoding and Viterbi
decoding. This is called Concatenated Code when more than one error correction
methods is used at the same time.
The concatenation of an outer Reed-Solomon Codec with Viterbi decoder first became
popular when it was introduced by Intelsat in the early 1990's. 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), 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 the Viterbi decoder, 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 R-S 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 modulator, different RS code rates are used, according to the mode of
operation. Refer to the modulator specification in Appendix C for details on the code
rates and interleave depth for each operating mode.
A characteristic of concatenated RS coding is the very pronounced threshold effect. For
any given modem design, there will be a threshold value of Eb/No below which the
demodulator cannot stay synchronized. This may be due to the carrier-recovery circuits,
or the synchronization threshold of the primary FEC device, or both. In the SLM-7650,
and Rate 1/2 operation, this threshold is around 4 dB Eb/No. Below this value, operation
is not possible, but above this value, the error performance of the concatenated RS system
produces exceptionally low error rates for a very small increase in Eb/No.
Table 4-1. Viterbi Decoding Summary
FOR
4–5
MN/SLM7650.IOM
AGAINST
Higher coding gain possible with
other methods
Revision 4
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