Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix K K–10 MN-CDM-425
Figure K-2. VersaFEC Codes vs. Constrained Capacity
Note that SNR is used in place of Eb/No – a convention for comparing ACM ModCods. SNR is
defined as Eb/No + 10
log
(Spectral Efficiency).
VersaFEC ACM requires the appropriate FAST code for the maximum operating
symbol rate.
K.6.1
VersaFEC ACM Latency
In an ACM system that has a number of ModCods, each having a different latency, what defines
the overall system latency? The answer is simple – the latency of the worst-case ModCod. (This
may not seem obvious to some, and it is beyond the scope of this appendix to provide a rigorous
defense of this statement. It
is , however, a correct statement despite certain believers in non-
causal systems and encoders that possess the magical quality of negative latency.)
Examining the data in Table K-2, latency for each ModCod is shown for the example of
VersaFEC ACM at a fixed 100 ksymbols/second rate. Of particular note is that even though the
ModCods span a 7:1 variation in throughput, the latency is only varying between 25 and 34
milliseconds. A careful analysis will show that this is a consequence of using a constant number
of symbols per block. In the example shown the worst-case latency for this ACM scheme is 34
milliseconds.