Comtech EF Data CDM-760 - Manual (Page 384)

Appendix J Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
J–6
Traditional Full Duplex Link Duplex Link with DoubleTalk Carrier-in-Carrier
Figure J-4. Duplex Link Optimization
The CDM-760 CnC module operates on the near-zero signal before the demodulator, and is
waveform agnostic. This means that no prior knowledge of the underlying modulation, FEC, or
any other waveform specific parameter is required in order to perform the signal cancellation
operation. The only caveat to this is that the waveform must be sufficiently random.
CDM-760 CnC operates from 1 to 63 Msps. Interference will compromise CnC links just as they do
non-CnC links. Acquisition of the demodulator is improved by minimizing the delay search range;
for example, if the delay is known to be around 240ms, set the minimum search delay to 230ms
and the maximum search delay to 250ms.
As advances in modem technologies – including advanced modulation and FEC techniques –
approach their theoretical limits of power and bandwidth efficiencies, CnC allows satellite users to
achieve spectral efficiencies (bps/Hz) that cannot be achieved with modulation and FEC alone.
Table J-1 illustrates how QPSK, when used with CnC, approaches the bandwidth efficiency of
16APSK.
Table J-1. Spectral Efficiency using DoubleTalk Carrier-in-Carrier
As shown here, CnC allows equivalent spectral efficiency using a lower order modulation and/or
FEC Code Rate; CAPEX is therefore reduced by allowing the use of a smaller BUC/HPA and/or
antenna. And, as CnC can be used to save transponder bandwidth and/or transponder power, it
can be successfully deployed in bandwidth-limited as well as power-limited scenarios.