Comtech EF Data CDM-550T - Manual (Page 105)

Chapter 12. AUTOMATIC UPLINK
POWER CONTROL
12.1 Introduction
Automatic Uplink Power Control (AUPC) is a feature whereby a local modem is permitted to
adjust its own output power level in order to attempt to maintain the Eb/No at the remote modem.
WARNING
The user MUST obtain permission from the Satellite Operator to use this feature.
Improper use of this feature could result in a transmitting terminal seriously
exceeding its allocated flux density on the Operator’s satellite. This could
produce interference to other carriers, and could cause transponder saturation
problems.
To accomplish this, the framed (EDMAC) mode of operation must be used. The remote modem
constantly sends back information about the demodulator Eb/No using reserved bytes in the
overhead structure. The local modem then compares this value of Eb/No with a pre-defined target
value. If the Remote Eb/No is below the target, the local modem will increase its output power,
and hence, a closed-loop feedback system is created over the satellite link. A particularly
attractive benefit of this feature is that whenever framed operation is selected, the remote
demodulator’s Eb/No can be viewed from the front panel display of the local modem. Note also
that AUPC can be used simultaneously with EDMAC.
There are several important parameters associated with this mode of operation, and the user needs
to understand how the AUPC feature works, and the implications of setting these parameters.
12.2 Setting AUPC Parameters
1) The user, under the menu (CONFIG, FRAME) first ensures that Framed mode is
selected. EDMAC may be disabled, or the unit may be defined as an EDMAC Master or
Slave. The important consideration is that Framing should be enabled.
2) The user should verify that the remote modem also has Framing enabled.
3) The user, under the menu (CONFIG, TX, PWR) sets the nominal output power of the
modem. This is done by selecting the MANUAL mode, then editing the TX output power
level displayed.
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