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

Advanced satellite modem (18 kbps – 25 mbps)

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CDM-625 Advanced Satellite Modem

Forward Error Correction Options

7.7.3 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 7-7

shows, for the CDM-625, the processing delays for the major FEC types, including the three TPC

Turbo Product 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 n o pr onounced t hreshold ef fect – fails

• Exceptional bandwidth efficiency.

• Coding gain independent of data rate (in this implementation).

• Low decoding delay for TPC.

• Easy field upgrade in CDM-625.

FEC Mode (64 kbps data rate)

Rate 1/2 + Reed Solomon

Rate 1/2 + Reed Solomon

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

End-to-end delay (ms)