Rain Margin Improvement Using Resource Sharing in 12-GHz Satellite Downlinks

Abstract

In this paper we consider the effectiveness of sharing a small pool of reserved time slots of a Time-Division-Multiple-Access ( T D M A ) frame among a large number of ground stations to overcome rain fading. With this approach, a system dynamically assigns time slots from the reserved pool to ground stations experiencing fade depths above the built-in margin. Powerful error correcting codes can be introduced to occupy the extra time slots, providing 10 dB or more of extra fade margin. Because a large number of ground stations are competing for the limited reserved pool, blockage can occur if the number of simultaneous fades exceeds the maximum number that can be accommodated. Some factors that influence the effectiveness of resource sharing are the mutual fade statistics at the various sites, the traffic distribution within the network, the number of earth stations, the size of the reserved pool, and the rain outage objective. Since the mutual fade and traffic statistics are unavailable, we develop models that can be used to find a conservative bound on the required size of the reserved pool. The rain model accounts for diurnal, seasonal, and geographical correlation among attenuation events. Results for a maximum resource-sharing gain of 10 dB show that reserving six percent of the time slots ensures a realized fade gain in excess of 9 dB for a down-link outage objective of 0.005 percent if there are more than 50 ground stations in the network, each with two percent or less of the traffic. I. I N T R O D U C T I O NIn an earlier paper,' a shared-resource concept was described for increasing the rain fade margin of a digital satellite system by as much as 10 dB above the design fade margin. With this approach, unused time slots of the Time-Division-Multiple-Access ( T D M A ) frame are made available to ground stations experiencing rain fading above the R A I N M A R G I N I M P R O V E M E N T 1 6 7