Comtech EF Data CDD-880, Installation And Operation Manual

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CDD-880 Multi Receiver Router MN-CDD880
Appendix G Revision 2
G–8
When you
disable
DPC, you modify the current modulator power, adding the margin at the
current rate that remains as part of all other bandwidth changes. This means that the terminal is
running at fixed power margin. The corresponding demodulator is notified of the state change,
stopping the DPC’s fast LQRM reporting on power greater than the target power. This prevents
the CDD-880 from sending messages at the fast rate for power above target window. ACM, if
enabled, operates normally.
G.3.1 Adaptive Control Loop (ACL) Components
The ACL operates using separate components at both ends of the link. Each component plays a
specific role in maintaining link quality during fade conditions:
• CDD-880 Multi Receiver Router:
The CDD-880 incorporates a mechanism to maintain or
adjust ACM during degraded conditions. This control uses the LQRM that is generated for
each individual demodulator available within the unit, sending signal quality E
S
/N
0
value to
its corresponding CDM-840. Messages are sent on timed intervals: 60 seconds normally, or
0.5 second if measured BER falls below defined thresholds or MODCOD is below maximum
ACM MODCOD. The DPC function reuses this messaging to adjust power in conjunction with
ACM control.
• CDM-840 Remote Router:
The CDM-840 power control has two methods to automatically
modify its transmit power:
1. ECM “Power Hunt” adjusts power depending on burst success or failure, whereby power
is modified from base reference or default setting (–40 dBm) in 3 dB steps, up to a
maximum 9 dB of additional power.
ECM uses an open loop mechanism because burst CDD-880s cannot provide an accurate
E
S
/N
0
per burst. This control is useful, during commissioning to find the initial power
setting to switch to dSCPC, or during normal operation to compensate for conditions
that may exceed the set power.
2. In dSCPC, where the LQRM is used to adjust power during fade conditions and also to
determine power reference during the calibration period.