Comtech EF Data Vipersat CDM-570L, User Manual

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Appendix
C -
Dynamic Power Control
C-1
A
P P E N D I X
D
YNAMIC
P
OWER
C
ONTROL
Introduction
Dynamic Power Control (DPC) provides a mechanism whereby VIPERSAT
satellite links have their transmit power levels adjusted in order to optimize the
receive signal quality (as measured by the demodulator Eb/No). This optimiza-
tion process acts to either increase or decrease transmitted signal levels in order
to:
• Achieve a minimum level of received Eb/No consistent with providing an
error free link.
• Reduce transmit power where sufficient link margin exists in order to
optimize station uplink and satellite transponder power usage.
DPC can be selectively enabled and disabled on a link-by-link basis and works
on both point-to-point as well as point-to-multipoint links. DPC relies on the
passing of an IP message called Path Loss Data Multicast (PLDM) between the
receive site (where the receive quality is being measured) and the transmit site
(where the power level is to be adjusted). PLDM's are IP unicast messages, and
only the transmit site whose frequency and Network ID number matches that
contained in the PLDM message will act on the message.
The Vipersat CDM (modulator) units have an algorithm that can, if activated by
the VMS, automatically adjust the modulator transmit power to maintain a
constant nominal receive signal quality (Eb/No) at the corresponding receive
station(s). This provides a mechanism to compensate for varying signal levels
caused, for example, by the use of different antenna sizes in a mesh network, or
if Ku band is being used as a means to compensate for rain fade conditions. The
power control algorithm is a closed loop servo-mechanism with the received