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CDM-600 Satellite Modem Revision 7
Forward Error Correction Options MN/CDM600.IOM
7–9
This new LDPC/TPC codec module may be installed in any existing CDM-600, as a simple field
upgrade, or already installed in new modems ordered from the factory. It requires Firmware
Version 1.6.0 (or higher) to be installed.
Please contact the Sales Department at Comtech EF Data for pricing and delivery information.
The table that follows compares all TPC and LDPC modes available in Comtech EF Data’s
CDM-600, and shows Eb/No performance and spectral efficiency (occupied bandwidth) for each
case. This information will be of particular interest to satellite operators wishing to
simultaneously balance transponder power and bandwidth. The large number of modes offered
will permit, in the majority of cases, significant power and/or bandwidth savings when compared
with existing schemes such as concatenated Viterbi/Reed-Solomon, or the popular 8-
PSK/Trellis/Reed-Solomon (Intelsat IESS-310)
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 7-6
shows, for the CDM-600, the processing delays for the major FEC types, including the three
TPC modes:
Table 7-6. Turbo Product Coding Processing Delay Comparison
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, O/QPSK 47
Turbo Product Coding, Rate 21/44, BPSK 64
Turbo Product Coding, Rate 5/16, BPSK 48
Turbo Product Coding, Rate 7/8, O/QPSK 245 *
Turbo Product Coding, Rate 0.95, O/QPSK 69
LDPC Coding, Rate 1/2 248
LDPC Coding, Rate 2/3, O/QPSK 296
LDPC Coding, Rate 2/3, 8-PSK, 8-QAM 350
LDPC Coding, Rate 3/4, O/QPSK 321
LDPC Coding, Rate 3/4, 8-PSK, 8-QAM, 16-QAM 395
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
decoding.
*A larger block is used for the Rate 7/8 code, which increases decoding delay.