Distortion Adaptation / Modulation Type Impairment; Table 7-4. Modulation Type Impairment - Comtech EF Data CDM-750 Installation And Operation Manual

CDM-750 Advanced High-Speed Trunking Modem
Adaptive Coding and Modulation (ACM) Option
7.4.2

Distortion Adaptation / Modulation Type Impairment

Distortion is a very serious impairment, particularly for ModCods using 16APSK and 32APSK.
These higher order modulations rely on amplitude modulation (AM) and phase modulation (PM)
compared to 'phase only' modulations such as QPSK/8PSK. The amplitude modulation gives rise
to AM/AM and AM/PM distortion when operating near the satellite amplifier compression limit.
To compensate, the user may enter additional impairments by modulation type; this allows
correct parameter adjustment in cases of additional degradation when a transponder is operated
close to saturation.
This distortion also penalizes higher-order ModCods (16APSK and 32APSK) much more than
QPSK/8PSK. The default for all degradation values is 0.0 dB – when operating in the linear
portion of the ground amplifier and satellite amplifier segment, degredation should be minimal
and can be left at 0.0 dB for most applications.
The additional impairment allows programming of an offset for each DVB-S2 constellation type,
which will ideally match the estimated satellite link distortion. These impairments apply to the
forward (transmit) link. The factors are added to CDM-750 specified Es/No for a given
constellation. Table 7-4 describes the impairment entry:
Additional Impairment
QPSK Degradation
8PSK Degradation
16APSK Degradation
32APSK Degradation
Note: Impairment Setting – QPSK ≤ 8PSK ≤ 16APSK ≤ 32APSK
Figure 7-4. Single Hop Latency

Table 7-4. Modulation Type Impairment

Variable Name
QPSK_DEG
8PSK_DEG
16APSK_DEG
32APSK_DEG
7–8
Revision 1
MN-CDM750
dB (Factory Default Shown)
Adjustable 0.0 to 9.9 dB
0.0
0.0
0.0
0.0