Comtec CDM-840 - Table D-1. the Versafec Modcod Set

CDM-840 Remote Router MN-CDM840
Appendix D Revision 3
D–3
D.2 VersaFEC ACM
VersaFEC covers a family of 18 short-block LDPC ModCods, specifically designed for low latency
and ACM applications. However, the VersaFEC codes are equally well suited to Constant Coding
and Modulation (CCM) applications.
The shortest possible LDPC codes that give performance at or very close to DVB-S2 with
significantly reduced latency in comparison.
The family of VersaFEC short-block LDPC codes is presented in Table D-1. The modulation types
include BPSK, QPSK, 8-QAM, and 16-QAM. This table shows that, in order to maintain a constant
number of symbols per block, the block size in bits (data + parity) must necessarily change,
depending on both the modulation type (which affects the number of bits per symbol) and the
code rate. For VersaFEC, the block size varies between 2k and 8.2k bits. At worst, therefore, the
VersaFEC codes are 50% shorter than the ‘short’ DVB-S2 codes.
Table D-1. The VersaFEC
ModCod set
Modulation Code Rate
Spectral
efficiency,
bps/Hz
Block
size, kbits
Typical
Eb/No, for
BER = 5 x 10
-8
(dB)
Latency at
64 kbps, in
milliseconds
Min.
Data Rate,
CCM mode
(kbps)
Max.
Data Rate,
CCM mode
(Mbps)
BPSK
0.488
0.49
2
2.4
26
16.00
2.19
QPSK
0.533
1.07
4.1
2.2
53
17.07
4.80
QPSK
0.631
1.26
4.1
2.7
59
20.19
5.67
QPSK
0.706
1.41
4.1
3.4
62
22.577
6.34
QPSK
0.803
1.61
4.1
3.8
66
25.69
7.22
8-QAM
0.642
1.93
6.1
4.6
89
30.83
8.67
8-QAM
0.711
2.13
6.1
5.2
93
34.14
9.60
8-QAM
0.780
2.34
6.1
5.6
97
37.44
10.53
16-QAM
0.731
2.93
8.2
6.3
125
46.80
13.26
16-QAM
0.780
3.12
8.2
7.0
129
49.92
14.04
16-QAM
0.829
3.32
8.2
7.5
131
53.04
14.91
16-QAM
0.853
3.41
8.2
8.0
132
54.60
15.35
The VersaFEC
codes compared with the Shannon bound are shown in Figure D-1. It can be seen
that the performance of VersaFEC
at or near the DVB-S2 performance with 16 kbit blocks.
Note that SNR is used in place of Eb/No, a convention for comparing ACM ModCods. SNR is
defined as Eb/No + 10
log
(Spectral Efficiency).