Comtech EF Data CDM-625 - Manual (Page 285)

CDM-625 Advanced Satellite Modem Revision 15
Forward Error Correction Options MN-CDM625
7–13
7.8.1 VersaFEC Extensions
VersaFEC Extensions require Firmware Version 1.5.4 (or higher)
7.8.1.1 Extended CCM Codes
In Firmware Version 1.5.4 and later, two additional rates have been added to 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 new 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 7-10. 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
7.8.1.2 Ultra-Low-Latency (ULL) Codes
Four new ultra-short block LDPC codes have been added to address the needs of users with
applications requiring even lower latency than the ‘standard’ VersaFEC code set. These new
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 7-11. 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