Comtech EF Data SNM-1001L - Manual (Page 179)

SNM-1001L Satellite Modem Revision 1
Forward Error Correction Options MN/SNM1001L.IOM
7–3
7.3.2 Mod/Demod Processing Delay
In many cases, FEC methods that provide increased coding gain do so at the expense of
increased processing delay. However, with TPC, this increase in delay is very modest.
Table 7-2 shows, the processing delays for the major FEC types, including the three TPC modes:
Table 7-2. Turbo Product Coding Processing Delay Comparison
FEC Mode (64 kbps data rate) Mod/Demod delay, ms
Viterbi, Rate 1/2 12
Sequential, Rate 1/2 74
Viterbi Rate 1/2 with Reed Solomon 266
Sequential Rate 1/2 with Reed Solomon 522
Turbo Product Coding, Rate 3/4, Offset QPSK 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 concatenated 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.
7.3.3 Comparison of all TPC Modes