Comtech EF Data CDM-425, Installation And Operation Manual

Page: 563 / 744

CDM-425 Advanced Satellite Modem
Revision 0
Appendix B B–1 MN-CDM-425
Appendix B. FORWARD ERROR
CORRECTION (FEC) OPTIONS
B.1 VersaFEC (Short-block LDPC)
Appendix K. V
ERSA
FEC / V
ERSA
FEC-2 ACM (ADAPTIVE CODING
AND
MODULATION) OPTION
VersaFEC is a system of short-block LDPC codes that have been designed as a low-latency
alternative to both Comtech EF Data’s existing LDPC codes, and to the system set forth in the
DVB-S2 specification (EN 302307) ratified by ETSI.
Since the introduction of Comtech EF Data’s LDPC, considerable research has been carried out
to find ways to reduce the block size of LDPC (and hence its latency) while preserving the coding
gain performance very close to the Shannon bound.
The set of VersaFEC codes was developed with two distinct purposes:
•
To provide an expanded choice of combinations of modulation and coding that maintain
the same coding gain performance as Comtech EF Data’s legacy LDPC, while
significantly reducing latency. The existing LDPC approach (and DVB-S2 short-block)
uses blocks that are 16 kbits in length. VersaFEC, on the other hand, uses block sizes
that vary between 2k and 8.2 kbits, and at the same time reduces latency still further
through a novel design approach.
•
To provide combinations of modulation and coding (ModCods) that are suitable not only for
Constant Coding and Modulation (CCM) applications, but are also the basis for an Adaptive
Coding and Modulation (ACM) system. 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. There are 12 ModCods in the VersaFEC set. For a detailed
discussion on ACM, see
Appendix K. V
ERSA
FEC / V
ERSA
FEC-2 ACM (ADAPTIVE
CODING
AND
MODULATION) OPTION.