Table D-1. The Versafec Modcod Set - Comtec CDM-840 Installation And Operation Manual

CDM-840 Remote Router
Appendix D
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.
Modulation Code Rate
BPSK
QPSK
QPSK
QPSK
QPSK
8-QAM
8-QAM
8-QAM
16-QAM
16-QAM
16-QAM
16-QAM
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

Table D-1. The VersaFEC ModCod set

Spectral
Block
efficiency,
size, kbits
bps/Hz
0.488 0.49 2
0.533 1.07 4.1
0.631 1.26 4.1
0.706 1.41 4.1
0.803 1.61 4.1
0.642 1.93 6.1
0.711 2.13 6.1
0.780 2.34 6.1
0.731 2.93 8.2
0.780 3.12 8.2
0.829 3.32 8.2
0.853 3.41 8.2
(Spectral Efficiency).
log
Typical
Latency at
Eb/No, for
64 kbps, in
BER = 5 x 10
-8
milliseconds
(dB)
2.4 26
2.2 53
2.7 59
3.4 62
3.8 66
4.6 89
5.2 93
5.6 97
6.3 125
7.0 129
7.5 131
8.0 132
D–3
MN-CDM840
Revision 3
Min. Max.
Data Rate, Data Rate,
CCM mode CCM mode
(kbps) (Mbps)
16.00 2.19
17.07 4.80
20.19 5.67
22.577 6.34
25.69 7.22
30.83 8.67
34.14 9.60
37.44 10.53
46.80 13.26
49.92 14.04
53.04 14.91
54.60 15.35