Comtech EF Data CDM-750 - Manual (Page 248)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Appendix C MN-CDM750
C–12
C.4.6 ModCod Selection
Figure C‐5 shows considerable overlap of ModCod combinations. The overlap depicted in this
figure implies omitting some of the ModCods; therefore, without dropping some of the
ModCods, there is a non‐monotonic change of Bits/Hz with changing Es/No. As this would result
in an unstable system, re‐sorting the DVB‐S2 table resolves this.
Also, a practical system needs some hysteresis to provide orderly transition to adjacent
ModCods and avoid dithering. The minimum distance between adjacent ModCods must be at
least the amount of hysteresis – the figure used for hysteresis is fixed in the CDM‐750 at 0.5 dB.
The actual Es/No values used are based on the guaranteed Es/No values + Target Es/No Margin
+ modulation type impairment, not the DVB‐S2 ideal Es/No figures.
Taking into account the ModCod spacing, hysteresis, Target Es/No Margin, modulation type
impairment and monotonic behavior suggests a practical way to select the Es/No thresholds for
switching ModCods, and a way to prune ModCods from the list. During parameter configuration,
the modem selects the usable ModCods.
Figure C-5. Spectral Efficiency vs. Ideal Es/No @ QEF