A Statistical Analysis of Transmit Power Control to Compensate Up- and Down-Link Fading in an FDMA Satellite Communications System

The application of 14/12 GHz and higher allocated frequency bands in commercial satellite communications systems has been retarded by the problem of slow deep fading due principally to rain attenuation on the earth-station-to-satellite and satellite-satellite-to-earthstation radio links. The large carrier-to-noise ratio margins required to ensure good link performance for a large percentage of time may be excessive, and a more sophisticated method of dealing with fading is required. The methods available fall broadly into two categories, one employing diversity and the other adaptive compensation techniques. This paper examines a link compensation method, namely up-link power control at the transmitting earth station, that effectively reduces the effects of both up- and down-link fading in a frequency-division multiple-access (FDMA) satellite system having a large number of accesses. The transmitted level of each carrier accessing the satellite transponder is dynamically adjusted to compensate the combination of up- and down-link fading experienced by the carrier. While this leads to wide variations in individual carrier levels into the satellite, the assumed independence of fades on the down-link to each of the receiving earth stations means that variations in multicarrier TWT operating point tend to be small (providing no one earth station receives a highly disproportionate share of the traffic). By essentially pooling among many links the effects of deep fades simultaneously present on only a few links, this technique will reduce variations on individual carriers, thereby reducing required fade margins. This paper derives expressions for the earth-station-to-earth-station link fade probability density function (pdf) with and without transmit power control. The theoretical effects of noisy estimates of the up- and down link fades and quantization of the transmitted powers are considered. Simple asymptotic expressions applicable when the number of accesses is large are also derived. Computed results demonstrate that transmit power control can substantially reduce the fade margins required in homogeneous single channel per carrier (SCPC) systems operating in the 14/12 GHz band. Some detection and control problems associated with- a practical implementation of the scheme are introduced.