Comtech EF Data OM20 - Manual (Page 216)

OM20 Universal Outdoor Modem Appendix G
MN-OM20– Revision 5 H–2
drop in the remote E
b
/N
o
, it slowly begins to raise the output power, and will continue to adjust if
the remote E
b
/N
o
continues to drop. As the rain increases in intensity, the remote E
b
/N
o
decreases
but Local modem continues to increase its power level to compensate.
When the rain diminishes, Local modem will see the remote E
b
/N
o
begin to increase. Local
modem will lower its power level. The operation is therefore a feedback control loop with the
added complication of a significant time delay.
H.1.2 EF AUPC
In EF AUPC mode, the Target E
b
/N
o
indicates the local unit wants the remote unit to maintain a
power level sufficient to provide the local E
b
/N
o
value.
EF AUPC can be set to operate on either or both directions of a link but always require a bi-
directional channel. Enabling AUPC on one side of the link will activate AUPC on the distant
end of the link. It is necessary that both the Modulator and Demodulator be set to the appropriate
framing for AUPC options to be editable and for the AUPC function to operate properly.
Examples of the basic EF AUPC Operations are described as follows:
Assume that the two modems, one at each end of the link, are set to AUPC operation. Only one
direction is discussed, but the same functions could be occurring in both directions
simultaneously.
The local modem is transmitting to modem at a remote locale under normal conditions. The
remote modem has a receive E
b
/N
o
of 7.5 dB. The local modem has been set with a Target E
b
/N
o
of 7.5 dB, and has a current power output of –30 dBm.
It begins to rain at the local site, and the E
b
/N
o
drops to –7.0 then –6.8 dB. The local modem is
constantly sending update messages of its E
b
/N
o
to the remote modem. When the remote modem
sees the drop in the E
b
/N
o
, it slowly begins to raise it’s output power, and will continue to do so
until the Target E
b
/N
o
is restored at the local site.
When the rain diminishes, the local modem’s E
b
/N
o
will begin to increase. The remote modem
will now lower its power level to restore the target value. The operation is therefore a feedback
control loop with the added complication of a significant time delay.
H.1.3 Near Side AUPC
Near Side AUPC is a loop back system that adjusts the broadcast uplink signal when local
conditions change. This is done by having the Near Side AUPC attempt to adjust the outbound
power to compensate for local weather.
The local receiver must be tuned and locked to the transmitter and then the internal E
b
/N
o.
, is
used for feedback. This creates a Tx-Satellite-Rx control loop.
Near Side AUPC is primarily used for broadcast applications since the modem cannot expect to
receive data from a distant location. Near Side AUPC can be utilized with any satellite framing
or Network mode.