AN ADAPTIVE PREDISTORTER FOR A POWER-AMPLIFIER BASED ON ADJACENT CHANNEL EMISSIONS

Linearization of power amplifiers has become an important issue, with the advent of linear modulation methods in mobile communications. This paper presents a slowly adapting predistorter. 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. For multichannel inputs, it is in an inter-channel or "sacrificial channel" location. In this way, only a spot power measurement is required. 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, power supply variations; these characteristics do not require fast adaptation. By monitoring the out-of-band power we can obtain an estimate for the distortion introduced by the power amplifier. Adaptation is accomplished by iterative adjustment of the predistorter parameters to minimize the IM power. For a polynomial predistorter (e.g., cubic or quintic), we can analytically demonstrate that the IM power is a quadratic function of the coefficients. A variety of algorithms therefore apply. This paper presents an analog static predistortion linearizer circuit, although the technique can be implemented in digital signal processing or other variations of analog predistorters. The analog predistorter presented in this paper uses the envelope of the baseband signal to generate the nonlinear functions used in predistorting the input signal. The improvement obtained with an amplitude modulated (AM) input signal was 15 dB in the third- and 5 dB in the fifth-order intermodulation products. The IM improvement could be maintained with the use of a robust direct search algorithm.