On the Robustness of the Predistortion Function Synthesis for Highly Nonlinear RF Power Amplifiers Linearization

This paper investigates the robustness of the digital predistortion function synthesis process applied to linearize a highly non linear RF power amplifier. For this purpose, a 100-watts peak power amplifier designed for UMTS applications biased in deep class AB is used. Two approaches have been compared: the polynomial curve fitting of the measured AM/AM and AM/PM characteristics and the look-up tables synthesis using the smoothed versions of these same non linear curves. The polynomial based fitting presents badly conditioned matrices that make this approach unpractical for the considered non linearity shape particularly for DSP implementation of the synthesis procedure. The accuracy of both approaches was assessed by measuring the adjacent channel power ratio at the output of the power amplifier under a 2-carriers W-CDMA signal. The look-up tables approach based on the averaging technique showed better correction capabilities than the polynomial based fitting