Comtech EF Data CDM-600/600L - Manual (Page 153)

CDM-600/600L Open Network Satellite Modem Revision 3
Automatic Uplink Power Control (AUPC) MN/CDM600L.IOM
7–3
7.2.4 Demod Unlock
DEMOD UNLOCK defines the action the modem will take if the remote demodulator loses lock.
The two choices are:
• NOMINAL (reduce the Tx Output Power to the nominal value – this is the default
setting).
• MAXIMUM (increase the Tx Output Power to the maximum value permitted by the
parameter MAX RANGE).
(Note: If the local demod loses lock, the modem will automatically move its output power to the
nominal value.)
7.3 Compensation Rate
As with any closed-loop control system, the loop parameters must be chosen to ensure stability at
all times. Several features have been incorporated to ensure that the AUPC system does
overshoot, or oscillate.
• First, the rate at which corrections to the output power can be made is fixed at once every
4 seconds. This takes into account the round trip delay over the satellite link, the time
taken for a power change to be reflected in the remote demodulator’s value of E
b
/N
0
, and
other processing delays in the modems.
• Second, if the comparison of actual and target E
b
/N
0
yields a result that requires a change
in output power, the first correction made will be 80% of the calculated step. This avoids
the possibility of overshoot. Subsequent corrections are made until the difference is less
than 0.5 dB. At this point, the output power is only changed in increments of 0.1 dB, to
avoid ‘hunting’ around the correct set point.
7.4 Monitoring
The remote demodulator’s value of E
b
/N
0
can be monitored at all times, either from the front panel
(MONITOR: AUPC) or via the remote control interface. The resolution of the reading is 0.2 dB.
For all values greater than or equal to 16 dB, the value 16.0 dB will be displayed. As long as
framing is enabled, the value will still be available, regardless of the AUPC mode, or framing mix.
Also displayed is the current value of Tx power increase. If EDMAC framing is enabled, but
AUPC is disabled, this will indicate 0.0 dB. This value is also available via the remote control
interface.
Comtech EF Data strongly cautions against the use of large values of
permitted power level increase under AUPC control. Users should
consider using the absolute minimum range necessary to improve rain-
fade margin.
CAUTION