An adaptive predistortion system

Abstract

A slowly adapting predistorter is presented. The approach is to minimize the transmitter output power in spectral regions occupied only by intermodulation (IM) products. For single channel data signals, a suitable region is in the adjacent channel. This technique relies on the principle that the power amplifier's characteristics vary slowly with time. In general, the main performance drift in power amplifiers occurs because of transistor degradation, temperature changes, channel switching, and power supply variations; these characteristics do not require fast adaptation. By monitoring the out-of-band power one can obtain an estimate for the distortion introduced by the power amplifier and use this quantity to optimize the predistorter. The approach utilized to obtain a measurement of the out-of-band power is to perform a complex convolution of the RF input signal to the predistorter with the power amplifier's output signal. >