CiM-550 IP Enabled Satellite Modem

Rev. 2

Theory of Operation

CD/CIM550.IOM

6.4.2COMPENSATION 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 Eb/No, and other processing delays in the modems.

Second, if the comparison of actual and target Eb/No 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.

6.4.3MONITORING

The remote demodulator’s value of Eb/No can be monitored at all times, either from the front panel (MONIT, 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, even though AUPC may be disabled.

Also displayed is the current value of TX power increase. If 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.

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