Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix K K–16 MN-CDM-425
K.7 VersaFEC-2 ACM and CCM
Comtech EF Data’s second generation VersaFEC-2 comprises 74 new ModCods, with a new
family of constellations.
•
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.
•
Es/No span from -1.3 dB (ModCod 0) to 16.3 dB (ModCod 37), giving an average of
approx. 0.45 dB per ModCod, and a 17.6 dB total span.
•
The overhead associated with AUPC is eliminated. (A framed mode such as EDMAC is
normally required.)
•
Combined AUPC and ACM operation is possible - VersaFEC-2 permits up to 10 dB of
AUPC power increase, followed by 17.6 dB of ACM range, for a total of 27.6 dB of fade
protection.