B.8 versafec (short-block ldpc), B.8 versafec, Short-block ldpc) – Comtech EF Data CDM-625A User Manual
Page 549
CDM-625A Advanced Satellite Modem
MN-CDM625A
Appendix B
Revision 3
B–13
B.8
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 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:
1) To provide an expanded choice of combinations of modulation and coding that maintain the
same coding gain performance as Comtech EF Data’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 a patent-
pending 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 M. V
ERSA
FEC
/
V
ERSA
FEC-2
ACM
(ADAPTIVE
CODING
AND
MODULATION)
OPTION.
Table B-9. The VersaFEC ModCod set
ModCod Modulation Code
Rate
Spectral
efficiency,
bps/Hz
Block size,
kbits
Typical
Eb/No, for
BER = 5 x
10
-8
(dB)
Latency at
64 kbps (ms)
Min. Data
Rate,
CCM mode
(kbps)
Max. Data
Rate,
CCM mode
(Mbps)
00
BPSK
0.488
0.49
2
2.4
26
18
5.7
01
QPSK
0.533
1.07
4.1
2.2
53
20
10
02
QPSK
0.631
1.26
4.1
2.7
59
23
10
03
QPSK
0.706
1.41
4.1
3.4
62
26
10
04
QPSK
0.803
1.61
4.1
3.8
66
28
12
05
8-QAM
0.642
1.93
6.1
4.6
89
35
12
06
8-QAM
0.711
2.13
6.1
5.2
93
39
12
07
8-QAM
0.780
2.34
6.1
5.6
97
43
12
08
16-QAM
0.731
2.93
8.2
6.3
125
53
12
09
16-QAM
0.780
3.12
8.2
7.0
129
57
14
10
16-QAM
0.829
3.32
8.2
7.5
131
60
14
11
16-QAM
0.853
3.41
8.2
8.0
132
62
16