Satellite Communications Payload and System

Chapter 11

Communications Link

11.1 Introduction

This chapter describes the remaining elements of the end-to-end satellite communications system not discussed in earlier chapters, namely the communication links. The payload has been extensively described in Part I of this book. The modulating transmitter and the demodulating receiver, minus the antennas, have been described in Chapter 10. In this chapter, we look at what happens to the signal when it goes between a ground transmitter and the satellite (the uplink) and between the satellite and a ground receiver (the downlink). Some of the effects happen on the link, and some happen only as the transmitted signal is received by the antenna. The payload is assumed to be bent-pipe but it should be rather easy to see how things would be for a processing payload.

The rest of this chapter is organized as follows:

• Section 11.2 End-to-end C/N₀
• Section 11.3 Signal power on link: what determines it and what makes it vary, including atmospherics
• Section 11.4 Noise level on link: what determines it, including antenna noise, and what makes it vary
• Section 11.5 Interference on link: how it can arise and what mitigates it
• Section 11.6 Link budget, combining these facets.

11.2 End-to-End C/N₀

At this point neglecting interference, the quality of the (impaired) signal on the end-to-end communications system is represented to first order by the C/N₀ out of the ground receiver's RF/IF front end. C is signal power (sometimes denoted by “