An adaptive predistortion system

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