Comtech EF Data CDM-750 - Manual (Page 265)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Appendix D MN-CDM750
D–7
CDM‐750 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 D‐1 illustrates how QPSK, when used with CnC, approaches the bandwidth efficiency of
16APSK.
Table D-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.