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CDD-562AL/564A/L Multi-Channel Demodulator
Forward Error Correction
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.
Code
ModCod Modulation
Rate
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
Table A-1. The VersaFEC ModCod Set
Spectral
Block
efficiency,
size,
bps/Hz
bits
0.488
0.49
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
Typical
Latency at
Eb/No, for
64 kbps, in
-8
BER = 5 x 10
milliseconds
2k
2.4 dB
2.2 dB
2.7 dB
3.4 dB
3.8 dB
4.6 dB
5.2 dB
5.6 dB
6.3 dB
7.0 dB
7.5 dB
8.0 dB
A–3
Revision 0
MN-CDD562AL564AL
Max. Data
Min. Data
Rate
Rate (kbps)
(Mbps)
26
16.0
1.462
53
16.0
3.200
59
16.0
3.785
62
16.0
4.233
66
16.0
4.818
89
16.0
5.782
93
16.0
6.401
97
16.0
7.021
125
16.0
8.776
129
16.0
9.361
131
16.0
9.946
132
16.4
10.239
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