Performance of a 16-ARY DEQAM modem employing a baseband or RF predistorter over a regenerative satellite link

The paper presents the results of a series of computer-simulation tests that determine the effects of bandlimiting (nonlinear distortion with and without predistortion) on the tolerance to additive white Gaussian noise of a digital modem. The modem transmits a 16-ary differentially encoded quadrature amplitude modulation (16-ary DEQAM) signal over a regenerative satellite link, where the high power amplifier (HPA) on board the satellite transmitter may introduce AM-AM and AM-PM conversion effects into the 16-ary DEQAM signal. Four different bandwidths of the transmitted signal are tested, together with different levels of output back-off in the HPA at the transmitter. The arrangements achieving the most efficient use of the power are selected. There sults show that a surprisingly good improvement in performance can be achieved by using predistortion of the 16-ary DEQAM signal to reduce the nonlinear distortion of the high power amplifier.