Comtech EF Data CDD-562AL, Installation And Operation Manual

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CDD-562AL/564A/L Multi-Channel Demodulator Revision 0
Forward Error Correction MN-CDD562AL564AL
A–3
A.3 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.
There are 12 ModCods in the VersaFEC set (see Table A-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.
Table A-1. 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 16.0 1.462
01 QPSK 0.533 1.07 4.1k 2.2 dB 53 16.0 3.200
02 QPSK 0.631 1.26 4.1k 2.7 dB 59 16.0 3.785
03 QPSK 0.706 1.41 4.1k 3.4 dB 62 16.0 4.233
04 QPSK 0.803 1.61 4.1k 3.8 dB 66 16.0 4.818
05 8-QAM 0.642 1.93 6.1k 4.6 dB 89 16.0 5.782
06 8-QAM 0.711 2.13 6.1k 5.2 dB 93 16.0 6.401
07 8-QAM 0.780 2.34 6.1k 5.6 dB 97 16.0 7.021
08 16-QAM 0.731 2.93 8.2k 6.3 dB 125 16.0 8.776
09 16-QAM 0.780 3.12 8.2k 7.0 dB 129 16.0 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