Comtech EF Data CDM-550T, Manual

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Forward Error Correction Options CDM-550T Satellite Modem
7–6 Rev. 1.3
7.5.1 End-to-End Processing Delay
In many cases, FEC methods which provide increased coding gain do so at the expense of
increased processing delay. However, with TPC, this increase in delay is very modest.
The table below shows, for the CDM-550T, the processing delays for the major FEC
types, including the three TPC modes:
FEC Mode (64 kbps data rate) End-to-end delay, ms
Viterbi, Rate 1/2 12
Sequential, Rate 1/2 74
Viterbi Rate 1/2 + Reed Solomon 266
Sequential Rate 1/2 + Reed Solomon 522
Turbo Product Coding, Rate 3/4, QPSK/OQPSK
79
Turbo Product Coding, Rate 21/44, BPSK 64
Turbo Product Coding, Rate 5/16, BPSK 48
(Note that in all cases, the delay is inversely proportional to data rate, so for 128 kbps, the
delay values would be half of those shown above.)
It can be clearly seen that the concatentaed Reed-Solomon cases increase the delay very
significantly (due mainly to interleaving/de-interleaving), while the TPC cases yield
delays which are less than or equal to Sequential decoding.
FOR AGAINST
Exceptionally good BER performance - significant
improvement compared with every other FEC method in use
today Nothing!
No pronounced threshold effect - fails gracefully
Exceptional bandwidth efficiency for Rate 3/4 QPSK - 40%
less than Viterbi and concatenated Reed-Solomon
Coding gain independent of data rate (in this implementation)
Low decoding delay
Easy field upgrade in
CDM-550T