Downlinks for DBS: Design and Engineering Considerations

The design of the downlink in a DBS system requires optimization of a set of complex interrelated objectives. This paper describes the objectives, demonstrates the interrelationships, and discusses the process of selecting system design parameters to best balance the objectives of the DBS business entity. The Satellite Television Corporation's (STC's) system design is presented as a case study. Performance requirements are stated in terms of consumer perceived picture quality and rain-fade protection. The requirements are generated by a subjective testing program. The system objectives are formulated consistent with performance requirements, competitive products, and economic reality. To meet the objectives, four major design parameters can be adjusted: satellite power, satellite antenna beam shape, home equipment performance, and signal format characteristics. This paper deals with the first three of these parameters. The manipulation of these parameters to meet system objectives is illustrated by the STC case study. It is demonstrated, for this sample system, that the choice of design parameters varies across the targeted service area, depending on demographics, competition, consumer expectations, weather, and other factors. However, in general, the STC design provides antenna sizes of 0.6-0.75 m for 92 percent of TV households, extremely high clear-sky picture ratings, and a sufficient rain-fade margin to provide acceptable levels of service outage.