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In satellite communications, the uplink and downlink paths, to and from the satellite are affected from time to time by propagation effects which include rain attenuation and scintillation. Variations also occur due to antenna mis-pointing, water or snow on the antennas etc.
It is desirable to keep these variations to a minimum. Avoiding reduced levels improves the quality of the service, minimises bit errors and reduces outage times. Avoiding excessively high levels is even more important, as abnormally high levels can cause serious problems of interference into the services of others on the same satellite.
I know of several methods of up-path power control:
1. A radiometer is used at the transmitting earth station to measure the sky noise temperature in the direction of the satellite. By monitoring the noise emitted by the warm water droplets in the rain the slant path attenuation may be estimated and the transmit power adjusted accordingly. The radiometer needs to look at a frequency band where there can be no possibility of interference either from satellites, aircraft or terrestrial sources.
2. Monitoring the downlink beacon signal from the wanted satellite. This is a popular method. The beacon signal is reliable and a good beacon receiver can remain locked to the beacon in spite of severe rain fading. The beacon will be a frequency in the downlink band so a calculation of an estimate of the equivalent fade depth at the transmit frequency is required.
3. In iDirect VSAT networks, the hub TDMA receive modems (called "line cards" by iDirect) detect the receive burst signal quality from each remote site. The hub station then tells the remote site to increase or decrease its transmit power so as to maintain a nominal adequate quality to the hub. Ideally, the remote sites are commissioned in clear sky conditions and the power at commissioning is 6 dB below the maximum power output (P-1dB) of the BUC the remote site.
Up-link power control should be applied with caution, particularly with large carriers as there is serious risk of overloading the satellite if the power is increased accidentally when the sky is clear. This will arise when there is a fault in the receive fade detection system.
Uplink power control is not an accurate system or magic panacea as fading varies with time and it is not possible to keep up with the changes when they are occurring quickly. This is particularly the case when scintillation is occurring. Scintillation due to low elevation tropospheric effects and scintillation due to the ionosphere in tropical regions sometimes involves changes of several dB per second.
Uplink power control may also be referred to as Up path power control (UPPC), Up link power control (ULPC) or Automatic uplink power control (AUPC). 'Open loop' refers to methods that use a radiometer or beacon measurement and then apply a calculated adjustment to the uplink. 'Closed loop' refers to methods such as the iDirect approach, where the far end receive quality is fed back the transmit site and progressive adjustments to accurately adjust the levels is theoretically possible. Beware of the path length latency time delay however; if the propagation loss is varying rapidly it won't work well.
Page started 11th June 2013, amended 1st Aug 2015 ECJ