Distortion Countermeasures

Abstract

As already mentioned in previous chapters, the role of a satellite is to receive a signal from an earth station or another satellite (uplink) and, acting as a simple payload, to transmit it to another earth station or satellite (downlink) [72]. The satellite channel introduces linear distortion to the transmitted signal due to linear filtering, shadowing and multipath fading. Moreover, the need to maximally exploit on-board resources often imposes driving a High Power Amplifier (HPA) at or near its saturation point, resulting in nonlinear distortion of the signal, and rendering the overall link nonlinear. For this reason, modulated signals with a constant envelope have been widely used in practice until now [263]. However, contemporary and future mobile satellite communication systems require high bandwidth efficiency. Thus, bandwidth efficient modulation schemes, with large signal constellations, must be also considered and evaluated [264].