Comtech EF Data CDM-570A - Manual (Page 436)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix B Revision 2
B–8
As a comparison, looking at LDPC Rate 2/3 8-QAM and VersaFEC Rate 0.642 8-QAM essentially
identical spectral efficiency and Eb/No performance. However, at 64 kbps, the latency has been
reduced from 350 milliseconds to 89 milliseconds – a factor of 4 reduction.
The performance of the VersaFEC codes compared with the Shannon bound is shown in Figure B-1.
This puts the performance of VersaFEC at or near the DVB-S2 performance with 16 kbit blocks.
Figure B-1. VersaFEC Codes versus Shannon Capacity
Note that SNR is defined as Eb/No + 10
log
(Spectral Efficiency).
B.6.1 VersaFEC Extensions
B.6.1.1 Extended CCM Codes
Two additional rates are available with the VersaFEC family of codes. Note that these are CCM
(Constant Coding and Modulation) only – they are NOT included in the ACM set of ModCods.
These two codes (see Table B-4) have been added to fill in perceived gaps in the progression of
spectral efficiency versus SNR. They do not, in fact, provide any discernible improvement in
Eb/No performance (compared with their nearest neighbors in the initial VersaFEC family), but
exhibit a modest SNR improvement.
Table B-4. Extended CCM Codes
Modulation
Code
Rate
Spectral efficiency,
bps/Hz
Typical
Eb/No, for
BER = 5 x 10 -8
Latency at
64 kbps, in
milliseconds
Min. Data Rate,
CCM Mode (kbps)
Max. Data Rate, CCM
Mode (Mbps)
8-QAM 0.576 1.73 4.5 dB 87 8.3 5.179
16-QAM 0.644 2.58 6.5 dB 121 12.4 7.726