Satellite Communications Payload and System

Chapter 12

Probabilistic Treatment of Multibeam Downlinks

12.1 Introduction

This chapter presents a link-level analysis of a multibeam payload that shows lower EIRPs to be necessary than a traditional, overly conservative analysis would dictate. The new analysis treats variations via probability theory. (Here, the word “variation” is used instead of “uncertainty” as in Chapter 8, even though the definition of variation is the same as the definition of uncertainty, because “variation” is the term used in a payload specification.) Even relatively more EIRP can be saved if the downlinks are subject to rain loss and the traditional EIRP requirements are replaced with a link-availability requirement. Recall that the downlink is available if the link “closes,” that is, if the signal arriving at the ground-station receiver meets a minimum SNR requirement.

In a traditional analysis of this kind, worst-case value is piled upon worst-case value and held against C (=EIRP) and used to build up I_self (=self-interference—see Section 11.5). This characterizes an extreme situation that occurs with miniscule probability. A probabilistic characterization more fully and accurately portrays the on-orbit payload performance. If the links are subject to rain loss, traditionally a rain margin is separated out from a payload-variation margin and applied one-to-one to increase the specified EIRPs. In reality, though, most of the time the payload's performance is fairly close to nominal, so for the rare ...