Versafec (Short-Block Ldpc; Table B-3. The Versafec Modcod Set - Comtech EF Data CDM-570A Installation And Operation Manual

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.6. ADAPTIVE CODING
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.
ModCod Modulation
00 BPSK
01 QPSK
02 QPSK
03 QPSK
04 QPSK
05 8-QAM
06 8-QAM
07 8-QAM
08 16-QAM
09 16-QAM
10 16-QAM
11 16-QAM
FEC (Forward Error Correction) Options
CDM-570A/570AL Satellite Modem with Optional Packet Processor

Table B-3. The VersaFEC ModCod Set

Spectral
Code Block
efficiency,
Rate size, bits
bps/Hz
0.488 0.49 2k
0.533 1.07 4.1k
0.631 1.26 4.1k
0.706 1.41 4.1k
0.803 1.61 4.1k
0.642 1.93 6.1k
0.711 2.13 6.1k
0.780 2.34 6.1k
0.731 2.93 8.2k
0.780 3.12 8.2k
0.829 3.32 8.2k
0.853 3.41 8.2k
B–7
MODULATION OPTION
AND
Typical Latency at
Eb/No, for 64 kbps, in
BER = 5 x 10 milliseconds
-8
2.4 dB 26
2.2 dB 53
2.7 dB 59
3.4 dB 62
3.8 dB 66
4.6 dB 89
5.2 dB 93
5.6 dB 97
6.3 dB 125
7.0 dB 129
7.5 dB 131
8.0 dB 132
Revision 5
Min. Data Max. Data
Rate (kbps) Rate (Mbps)
2.4 1.462
5.2 3.200
6.1 3.785
6.8 4.233
7.8 4.818
9.3 5.782
10.3 6.401
11.3 7.021
14.1 8.776
15.0 9.361
16.0 9.946
16.4 10.239
MN-CDM570A