Comtech EF Data CDM-625 - Manual (Page 402)

CDM-625 Advanced Satellite Modem Revision 15
Adaptive Coding and Modulation (ACM) MN-CDM625
17–6
• In an ACM mode, no overhead channel was defined by DVB-S2 for the purpose of
reporting SNR metrics to the originating end. It has been left to individual equipment
manufacturers to decide their own method. This illustrates that all ACM systems, DVB-S2 or
not, are proprietary. In addition, it implies that additional bandwidth needs to be consumed
for the SNR reporting, and this is not accounted for in the code rate.
17.6 VersaFEC ACM
VersaFEC (a registered trademark of Comtech AHA), in concert with a novel ACM approach,
addresses all of the shortcomings of DVB-S2 outlined above. There are patents pending for both
VersaFEC and the ACM scheme.
VersaFEC covers a family of 12 short-block LDPC ModCods, specifically designed for low latency
and ACM applications. However, the VersaFEC codes are equally well suited to Constant Coding
and Modulation (CCM) applications.
The requirements for an ACM system that approaches the minimum possible latency are:
• The shortest possible LDPC codes that give performance at or very close to DVB-S2, in order
to minimize latency, and which do not use interleaving.
• Encoder design that further reduces latency to the minimum possible.
• A constant number of symbols per block, to reduce the demodulator and decoder
complexity, and significantly, also reduces latency in the ACM case.
• The elimination of the need for pilot symbols for carrier tracking at low SNR by substitution
of other modulation techniques. This further reduces the complexity of the demodulator.
• Reduction in the number of ModCods that further reduces complexity.
• The inclusion, at the physical layer, of an overhead channel to permit the reporting of SNR
metrics back to the originating end. Note that this does not have to be enabled or disabled –
it is part of the fundamental frame structure of VersaFEC ACM, and has been take into
account in the code rate.
The family of VersaFEC short-block LDPC codes is presented in Table 17-1. The modulation types
include BPSK, QPSK, 8-QAM, and 16-QAM. It will be seen from the table that in order to
maintain a constant number of symbols per block, the block size in bits (data + parity) must
necessarily change, depending on both the modulation type (which affects the number of bits
per symbol) and the code rate. For VersaFEC, the block size varies between 2k and 8.2k bits. At
worst, therefore, the VersaFEC codes are 50% shorter than the ‘short’ DVB-S2 codes.