CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix M Revision 4
M–12
M.7 VersaFEC-2 ACM
Comtech EF Data’s second generation VersaFEC-2 comprises 74 new ModCods, with a new
family of constellations. This new functionality requires the plug-in VersaFEC-2 Option Card,
Firmware Version 1.3.1 or higher, and activation of the appropriate FAST codes.
• VersaFEC-2 has two modes:
o Long Block – Maximum coding gain with higher latency.
38 ModCods – BPSK to 32-ARY modulation, 9600 symbol block size.
Spectral efficiency up to 4.4 bps/Hz for Long Block VersaFEC-2.
o Short Block – Slightly lower coding gain with much lower latency.
36 ModCods - BPSK to 32-ARY modulation, 1600 symbol block size.
Short Block performance is, overall, considerably better than 1st generation
VersaFEC, with similar latency performance.
Short Block deliberately limits throughput to 2 Mbps in CCM (Constant Coding and
Modulation) and 1 Msps in ACM (Adaptive Coding and Modulation). Above these
limits, latency is no longer an issue, and Long Block is preferred.
Spectral efficiency up to 4.15 bps/Hz for Short Block VersaFEC-2.
• CCM and point-to-point ACM are supported in both modes.
• In CCM, links can be run with VersaFEC-2 in one direction, and any other supported FEC type
in the other (Example: Rx = VersaFEC-2, Tx = TPC).
• In ACM, the range of rate bit rate adaptation is approximately 9:1.
• In ACM, both sides need to be VersaFEC-2, but Tx and Rx can be Short or Long – no
restrictions (but NOT compatible in any way with 1st generation VersaFEC).
• If VersaFEC-2 is used on both sides of the link then, even in CCM operation, the remote
modem’s Eb/No can be seen on the front panel display/web pages, etc, of the local modem.
• VersaFEC-2 has increased throughput compared to VersaFEC – the full symbol rate of 12.5
Msps, and full bit rate of 25 Mbps in CCM and ACM (Long Block) are supported.
• VersaFEC-2 provides five modulation types – BPSK, QPSK, 8-ARY, 16-ARY, and 32-ARY.
• VersaFEC-2 adds new circular or quasi-circular constellations for higher-order modulations
(above QPSK) to replace the square VersaFEC constellations.
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