Comtech EF Data CDM-840, Инструкция по установке и эксплуатации, Страница: 87 / 110

CDM-840 Remote Router Revision 0
Forward Error Correction MN-CDM840
6–3
2) To provide combinations of modulation and coding ( ModCods ) that are not only suitable for
Constant Coding and Modulation ( CCM ) applications, but are also the basis for a patent-
pending Adaptive Coding and Modulation ( ACM ) system (ACM will be available in future
releases of the CDD-880).
VersaFEC represents the best FEC technology currently available, and is offered with a sufficient
range of code rates and modulation types that link performance can be optimized under almost
any conditions. The performance of the VersaFEC codes compared with the Shannon bound is shown
in
Figure 6-1 .
Note that SNR is used in place of Eb/No – a convention for comparing ACM ModCods. SNR is
defined as Eb/No + 10
log
(Spectral Efficiency).
Figure 6-1. The VersaFEC Codes versus Shannon Capacity
There are 12 ModCods in the VersaFEC set ( Table 6-1 ). The modulation types (BPSK,
QPSK, 8-QAM and 16-QAM) and the code rates have been chosen to give a continuous
progression of performance in terms of both Eb/No and spectral efficiency – an essential
aspect of a well-engineered ACM system.