Comtech EF Data CDD-880, Installation And Operation Manual

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G–1
Appendix G. DPC (VMS DYNAMIC
POWER CONTROL)
G.1 DPC (Dynamic Power Control) Overview
VMS v3.x.x VIPERSAT Management System User Guide (CEFD P/N MN/22156)
This appendix provides information on Dynamic Power Control in reference to the
return path only – CDM-840 Remote Router to CDD-880 Multi Receiver Router
control.
Satellite networks are subject to uplink/downlink degradations due to rain fade or other
environmental factors. To ensure target link availability, network designers must take into
account the rain region on both sides of a link, and design the terminals and operating point to
include the appropriate link margins.
Satellite network operators typically lease Power Equivalent Bandwidth (PEB) from satellite
owners. In clear sky operation, the terminals on both sides of the link transmit with RF power on
the ground as a means to employ the PEB of the satellite that is assigned to that link. The
presence of rain or other environmental attenuation lowers the RF power received at the
satellite relative to clear sky conditions.
In principle, terminals with sufficient RF power could increase their transmit power during rain
loss conditions, to compensate for the loss and maintain the PEB on the satellite. However, rain
fade may occur at the uplink or downlink location, which presents an ambiguity for each
transmitting modem. If you control power solely on receiving quality and not power-limited at
the transmitter, you may exceed the satellite PEB – thereby oversubscribing service level
agreements or, worse, resulting in adjacent carrier interference.
Such oversubscription is common in traditional uplink power control technology. Modems, in
general, can automatically compensate for rain attenuation, but with power constraints and set
limits not to exceed site margins. Running uplink power at margin, however, tends to flat-line
power availability. If the modems are fed sufficient information to determine which side of the
link is being impacted by rain fade, the power may exceed margin limits without compromising
PEB, whereby the terminal knows the contribution of attenuation on the downlink and the
remainder of uplink attenuation is between the transmitting terminal and the satellite. The
additional information is obtained by monitoring the hub’s outbound transmission and taking