Table B-7. Tpc/Ldpc Processing Delay Comparison; Table B-8. Tpc/Ldpc Summary - Comtech EF Data CDM-625A Installation And Operation Manual

CDM-625A Advanced Satellite Modem
Appendix B
B.7.3 End-to-End 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 B-7
shows, for the CDM-625A, the processing delays for the major FEC types, including the three
TPC modes.
Viterbi
Sequential
Turbo Product Coding
LDPC Coding
*A larger block is used for the Rate 7/8 code, which increases decoding delay.
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 seen that the concatenated Reed-Solomon
cases increase the delay significantly (due mainly to interleaving/de-interleaving), while the TPC
cases yield delays which are less than or equal to Sequential.
• Exceptionally good B ER per formance – significant improvement
compared with every other FEC method in use today.
• Most m odes hav e no pr onounced t hreshold ef fect – fails
gracefully.
• Exceptional bandwidth efficiency.
• Coding gain independent of data rate (in this implementation).
• Low decoding delay for TPC.
• Easy field upgrade in CDM-625A.

Table B-7. TPC/LDPC Processing Delay Comparison

FEC Mode (64 kbps data rate)
Rate 1/2
Rate 1/2 + Reed Solomon
Rate 1/2
Rate 1/2 + Reed Solomon
Rate 3/4
Rate 21/44
Rate 5/16
Rate 7/8
Rate 0.95
Rate 1/2
Rate 2/3, O/QPSK
Rate 2/3, 8-PSK, 8-QAM
Rate 3/4, O/QPSK
Rate 3/4, 8-PSK, 8-QAM, 16-QAM

Table B-8. TPC/LDPC Summary

FOR
B–10
Revision 0
MN-CDM625A
End-to-end delay (ms)
9
266
74
522
47
41
69
245 *
69
198
234
350
248
395
AGAINST
Nothing!