Influence of design on performance of a satellite modem transmitting coded PSK signals

Abstract

The paper studies a digital modem that transmits a convolutionally encoded eight-phaseshift- keyed (CE8PSK) signal over a satellite link. Several different earth stations are assumed to have simultaneous access to a given transponder in the satellite, in an arrangement of frequency division multiple access. Thus, adjacent channel interference (ACI) can be introduced into the CE8PSK signal by the corresponding CE8PSK signals that occupy the immediately adjacent frequency bands. The high power amplifier (HPA) at the earth station transmitter, within the satellite link, may introduce nonlinear distortion into the CE8PSK signal, but the satellite transponder is assumed to be linear. Predistortion may or may not be used. Results are presented of an extensive series of computer-simulation tests. These measure the effects of bandlimiting, nonlinear distortion with and without predistortion, ACI and the demodulation process at the receiver, on the tolerance of the modem to additive white Gaussian noise. Four different bandwidths of the transmitted signal are considered, together with three different levels of output back-off in the HPA at the transmitter, and three different arrangements of the demodulator. The predistorter operates on the baseband signal at the input to the modulator and assumes a prior knowledge of the HPA characteristics. An equivalent baseband model of the transmission system is used to determine the performance of the modem at each of a number of different combinations of the various conditions studied, and the results are used to determine the preferred modem designs. It has been shown that a transmission rate of 16/9 bit/s per hertz of bandwidth can be achieved, with a loss in tolerance to additive white Gaussian noise of less than 1/2 dB relative to the ideal undistorted CE8PSK system.