CDM-625 Advanced Satellite Modem Revision 13
Forward Error Correction Options MN-CDM625
7–8
It is clear that, in order to provide the best possible performance over the range of code rates
from 1/2 to 0.95, both an LDPC and a TPC codec need to be offered.
In order to meet this requirement, Comtech EF Data has developed a combination LDPC/TPC
Codec module that can be added to the CDM-625 Modem. Table 7-5 outlines the operating
modes provided by this module.
Table 7-5. Available TPC/ LDPC Modes
Mode Code Rate/Modulation
Lower Data Rate Limit
(kbps)
Upper Data Rate Limit
(kbps)
Rate 21/44 BPSK (aka 1/2)
Rate 21/44 QPSK, OQPSK (aka 1/2)
Rate 3/4 8-PSK , 8-QAM
Rate 0.95 QPSK, OQPSK (aka 17/18)
Rate 0.95 8-PSK , 8-QAM (aka 17/18)
52.0 25000.0
Rate 1/2 BPSK
This LDPC/TPC codec module may be installed in any existing CDM-625 as a simple field
upgrade, or it can be pre-installed in new modems ordered from the factory. It also requires the
appropriate FAST codes for enabling operation beyond the base data rate limit of 5 Mbps.
Contact a Comtech EF Data Sales representative during normal business hours for pricing and
delivery information.
Table 7-6 compares all TPC and LDPC modes available in Comtech EF Data’s CDM-625, 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).
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