CDM-625 Advanced Satellite Modem
Revision 1
Forward Error Correction Options
MN-0000036 (Ref MN/CDM625.IOM)
Table 7-6. Turbo Product Coding Processing Delay Comparison
FEC Mode (64 kbps data rate)
End-to-end delay (ms)
Viterbi
Rate 1/2
9
Rate 1/2 + Reed Solomon
266
Turbo Product Coding
Rate 3/4, O/QPSK
47
Rate 21/44, BPSK
41
Rate 5/16, BPSK
69
Rate 7/8, O/QPSK
245 *
Rate 0.95, O/QPSK
69
LDPC Coding
Rate 1/2
198
Rate 2/3, O/QPSK
234
Rate 2/3, 8-PSK, 8-QAM
350
Rate 3/4, O/QPSK
248
Rate 3/4, 8-PSK, 8-QAM, 16-QAM
395
*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 decoding (which is not offered as an
option in the CDM-625).
Table 7-7. TPC and LDPC Summary
FOR
AGAINST
Exceptionally good BER performance - significant improvement
compared with every other FEC method in use today
Nothing!
Most modes have no pronounced threshold effect - fails gracefully
Exceptional bandwidth efficiency
Coding gain independent of data rate (in this implementation)
Low decoding delay for TPC
Easy field upgrade in CDM-625
7.7
Uncoded Operation (No FEC)
There are occasions where a user may wish to operate a satellite link with no forward error
correction of any kind. For this reason, the CDM-625 offers this uncoded mode for three
modulation types - BPSK, QPSK, and OQPSK. However, the user should be aware of some of
the implications of using this approach.
PSK demodulators have two inherent undesirable features. The first, known as ‘phase ambiguity’,
is due to the fact the demodulator does not have any absolute phase reference and, in the process
7–9
