Comtech EF Data CDM-760, Installation And Operation Manual

Page: 364 / 441

Appendix G Revision 4
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
G–15
Figure G-5. DVB-S2 Spectral Efficiency vs. Es/No (Pilot On, 64K Block Size)
Figure G-6. DVB-S2-EB1 Spectral Efficiency vs. Es/No (Pilot On, 64K Block Size)
Also, a practical system needs some hysteresis to avoid dithering and to provide for an orderly
transition to adjacent ModCods. The minimum distance between adjacent ModCods must be at
least the amount of hysteresis – the figure used for hysteresis is fixed in the CDM-760 at 0.5 dB.
The actual Es/No values used are based on the guaranteed Es/No values
plus
Target Es/No
Margin
plus
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.