Efficient Utilization of Satellite Transponders via Time-Division Multibeam Scanning

The trend toward higher frequency communication satellites employing multiple spot beams affords significant capacity advantages relative to lower frequency, wide-coverage area systems, since the allocated spectral band can be reused in the various spot beams. 1 - 2 When used in conjunction with digital modulation techniques and time-division multiple-access, the various coverage regions are readily interconnected via an onboard satellite switch operating in the time-division multiplex mode. In addition to the frequency reuse capability, the down-link transmitter power requirements are generally reduced because the antenna gain is higher than for a wide coverage area system. Despite these advantages, however, multiple spot beam satellites have several distinct drawbacks. These are generally associated with conflicting requirements concerning reliability, 3 coverage and blackout areas, 4 efficient transponder utilization, and nonuniform traffic density demands. 2901 In the following sections we explore these conflicting requirements and review some partial solutions proposed to date. Then we present a space segment configuration for a satellite communication system which provides high reliability with a minimum of redundancy, access from any location within a wide service area, and up to 100-percent efficiency in transponder capacity utilization matched to an arbitrary nonuniform density of traffic demand over the entire service region. This system employs N identical transponders which are shared on a time-division basis among M > N antenna ports spanning the entire service region.