Effects of baseband predistortion at the transmitter on the performance of a DEQPSK satellite modem

Abstract

AbstractThe paper presents the results of an extensive series of computer‐simulation tests to determine the effects of bandlimiting, nonlinear distortion with and without predistortion, and adjacent channel interference (ACI), on the tolerance to additive white Gaussian noise of a digital satellite modem. The modem transmits a differentially encoded quaternary phase‐shift keyed (DEQPSK) signal over a satellite link, where the high‐power amplifier (HPA) at the earth‐station transmitter may introduce AM—AM and AM—PM conversion effects into the DEQPSK signal, but the satellite transponder is linear. Four different bandwidths of the transmitted signal are tested, together with three different levels of output back‐off in the HPA at the transmitter. The predistorter operates on the baseband signal at the input to the modulator in the transmitter and assumes a prior knowledge of the HPA characteristics. ACI can be introduced into the transmitted DEQPSK signal by the corresponding DEQPSK signals, which occupy the two immediately adjacent frequency bands. The latter signals are assumed to have the same transmission rate and bandwidths as the first signal.Various combinations of the different distortion and interference effects are studied, using an equivalent baseband model of the transmission system, and the arrangements selected are those achieving the most efficient use of bandwidth together with the best tolerance to additive white Gaussian noise. A surprisingly good performance is achieved.