Adaptive digital/RF predistortion using a nonuniform LUT indexing function with built-in dependence on the amplifier nonlinearity

This paper reports on an integrated adaptive digital/RF predistorter using a nonuniform spaced lookup table (LUT) and in-phase/quadrature (I/Q) RF vector multiplier (VM). The LUT contents are directly deduced from the baseband input and output signals of the power amplifier (PA). In addition, a new nonlinear indexing function of the predistortion LUT with built-in dependence on the PA nonlinearity is proposed. This function is made to be robust to the input signal statistics. A comparison of this new indexation method with conventional approaches, namely, power and logarithmic power indexation functions, is carried out. The superiority of the proposed scheme is demonstrated in particular for class-AB amplifiers where the gain of the PA varies over the whole input range of the drive signal. The measured output spectrum of a linearized 90-W peak lateral double-diffused metal-oxide-semiconductor PA reveals a significant reduction of the power emission at the adjacent channels of approximately 15 dB under IS95, single-carrier, and multicarrier wide-band code-division multiple-access signals. The experimental evaluation is carried out using an RF/digital predistorter prototype that mainly includes an envelope detector, a linear I/Q RF VM, field-programmable gate array and digital signal processor, and fast analog/digital and digital/analog converters.