Comtech EF Data CDM-570A - Manual (Page 435)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix B Revision 2
B–7
B.6 VersaFEC (Short-block LDPC)
VersaFEC is a system of short-block LDPC codes that have been designed as a low-latency
alternative to both Comtech’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’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:
1) To provide an expanded choice of combinations of modulation and coding that maintain the
same coding gain performance as Comtech’s existing 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.
2) 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 Comtech
Comtech EF Data and Comtech AHA Enterprise Products Group’s patent-pending Adaptive
Coding and Modulation (ACM) system. The VersaFEC ACM feature requires activation of the
appropriate FAST codes.
Appendix P. ADAPTIVE CODING AND MODULATION OPTION
There are 12 ModCods in the VersaFEC set (see Table B-3).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.
Table B-3. The VersaFEC ModCod Set
ModCod Modulation Code
Rate
Spectral
efficiency,
bps/Hz
Block
size, bits
Typical
Eb/No, for
BER = 5 x 10 -8
Latency at
64 kbps, in
milliseconds
Min. Data
Rate (kbps)
Max. Data
Rate (Mbps)
00 BPSK 0.488 0.49 2k 2.4 dB 26 2.4 1.462
01 QPSK 0.533 1.07 4.1k 2.2 dB 53 5.2 3.200
02 QPSK 0.631 1.26 4.1k 2.7 dB 59 6.1 3.785
03 QPSK 0.706 1.41 4.1k 3.4 dB 62 6.8 4.233
04 QPSK 0.803 1.61 4.1k 3.8 dB 66 7.8 4.818
05 8-QAM 0.642 1.93 6.1k 4.6 dB 89 9.3 5.782
06 8-QAM 0.711 2.13 6.1k 5.2 dB 93 10.3 6.401
07 8-QAM 0.780 2.34 6.1k 5.6 dB 97 11.3 7.021
08 16-QAM 0.731 2.93 8.2k 6.3 dB 125 14.1 8.776
09 16-QAM 0.780 3.12 8.2k 7.0 dB 129 15.0 9.361
10 16-QAM 0.829 3.32 8.2k 7.5 dB 131 16.0 9.946
11 16-QAM 0.853 3.41 8.2k 8.0 dB 132 16.4 10.239