The Ultimate Capacity of Frequency-Reuse Communication Satellites

Communication satellites employing large antennas capable of providing multiple, nonoverlapping, high-gain spot beams have attracted 1089 considerable attention in recent years because the allocated spectral bands can be reused in the various spot beams.1-17 For a fixed level of available RF power, such satellites exhibit a much higher throughput capability relative to area coverage satellites because the higher antenna gain reduces the RF power required per spot beam to maintain the same effective radiated power (and therefore the same communication performance) on the earth; the excess power can then be applied to the remaining spot beams, thereby increasing the capacity of the satellite. For widely separated spot beams, each such beam can be considered as providing an independent, full-bandwidth channel between the satellite and the earth. A satellite offering N independent spot beams thereby provides a factor of N higher capacity than does an area coverage satellite. In fact, the total RF power required by the multiple spot beam satellite to provide this increased capacity is often less than that needed by the lower capacity area coverage system because the number of spot beams provided is usually smaller than the ratio of the spot beam to area coverage antenna gain. In this paper, we first briefly review several spot-beam, frequencyreuse concepts proposed to date. These include the familiar satelliteswitched, time-division multiple access (SS-TDMA) approach with fixed spot beams,1-9 integration of the basic SS-TDMA structure with a rapidly scannable spot beam to provide service over those regions not covered by a fixed spot beam,10"15 and multiple scanning beam approaches which efficiently accommodate nonuniform service demand across some wide service region.14,17 This background material appears in Section II and is optional reading for those already familiar with these concepts.