Simulation and analysis of an adaptive predistorter utilizing a complex spectral convolution

An adaptive predistorter for linearizing a power amplifier in a mobile transmitter is studied and the analytical, simulation, and measured results are presented. Unlike most other predistorters found in the literature, this predistorter does not have the problems of loop delay or phase shifting in its feedback path. The feedback is used only periodically to update the predistorter parameters so that it adapts to changes in the amplifier characteristics due to device aging, temperature changes, channel switching, power supply variations, and component drift. An adaptation method for predistorters of the polynomial type is described. Its complexity is significantly lower than that of the previously described methods and its convergence speed, though low, is more than sufficient to track amplifier drift. There are three main contributions. The first is analytical verification that the measurement of out-of-band power is sufficient to drive the adaptation. The second is a complex convolution method for measuring the out-of-band power that requires no additional local oscillator or phase reference. The third is a demonstration of the system performance utilizing a 16-QAM signal in a 25-kHz bandwidth, centered at 850 MHz.