Comtech EF Data CDM-625A - Page 551

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix B Revision 4
B–15
B.8.1 VersaFEC Extensions
B.8.1.1 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 (Rate 0.576 8-QAM and Rate 0.644 16-QAM) 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-10. 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
Max. Data
Rate,
CCM mode
8-QAM 0.576 1.73 4.5 dB 87 32 kbps 11 Mbps
16-QAM 0.644 2.58 6.5 dB 121 47 kbps 11 Mbps
B.8.1.2 Ultra-Low-Latency (ULL) Codes
Four additional ultra-short block LDPC codes address the needs of users with applications
requiring even lower latency than the ‘standard’ VersaFEC code set. These codes (which are
limited to BPSK and QPSK only) use significantly shorter code blocks than ‘standard’ VersaFEC,
and as a result, the coding gain is slightly reduced. However, in terms of latency, the
performance is exceptional – far lower than any other FEC method ever offered, with the
exception of Viterbi.
Table B-11. ULL 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
Max. Data
Rate,
CCM mode
BPSK 0.493 0.49 3.3 dB 18 18 kbps 5.7 Mbps
QPSK 0.493 0.99 3.3 dB 19 18 kbps 6 Mbps
QPSK 0.654 1.30 3.8 dB 21 24 kbps 9 Mbps
QPSK
0.734
1.47
4.3 dB
23
27 kbps
9 Mbps