Adaptation of a digital predistortion technique based on intersymbol interpolation

This paper investigates the performance of several adaptation strategies of a digital predistortion technique which efficiently compensates for power amplifier nonlinearities in bandwidth-efficient digital communication systems. The study includes the gradient algorithm in Cartesian coordinates, the gradient algorithm in polar coordinates, the predistorter optimization using averaging, as well as the computation of the adaptation terms at the receiver side and their feeding back to the transmitter using a return channel. Our results indicate that the best performance is achieved when the algorithm is implemented in polar coordinates and the adaptation terms are computed at the transmitter using an auxiliary local demodulator. Numerical results are given for a radio system employing the 32-QAM signal format.