Adaptive Digital Predistortion for Wideband High Crest Factor Applications Based on the WACP Optimization Objective: A Conceptual Overview

Abstract

This paper proposes a method of digital predistortion suitable for wideband high crest factor applications such as those encountered in DAB, DVB-T and WCDMA transmitters. The proposed method is advantageous for four main reasons. Firstly, it utilizes a reliable frequency domain measure of transmitter output nonlinearity, specifically the Weighted Adjacent Channel Power (WACP), as the objective for predistortion filter parameter estimation. This is in direct contrast to traditional approaches which utilize a time domain measure obtained via a full feedback path and potentially corrupted by gain and phase compensation error as well as ADC distortion. Secondly, the method models predistortion filter parameter estimation as a generic nonlinear mathematical optimization problem. This model assumes a nonconvex objective function and therefore utilizes both global and local optimization algorithms to achieve true global convergence. This is once again in direct contrast to traditional approaches which model predistortion filter parameter estimation as a linear regression problem. Such a model incorrectly assumes a convex error surface and therefore restricts itself to inadequate local optimization algorithms which unfortunately cannot guarantee true global convergence. Thirdly, the method's predistortion filter is a pruned Volterra Series with memory which utilizes a hybrid pruning strategy in order to keep high order kernels to a practically manageable size, suitable for optimization parameter estimation. Predistortion filter memory ultimately makes the method highly suited to wideband applications. Finally, predistortion filter parameter estimation does not require known test signals to be injected into the transmitter and therefore the technique is on-air adaptive. This means any transmitter using this method of digital predistortion will be both on-air and optimally linearized for its entire operational life. Preliminary results obtained from actual hardware are presented.

Direct learning predistortion algorithm based on PSO in frequency domain

“…If the magnitude of the outband spectrum is used as the direct optimization objective, the optimizer tends to optimize the area far away from the transmission band [3]. Therefore, in order to guide the optimizer to optimize outband spectrum leakage evenly, a weighting function will be added to outband spectrum area.…”

“…In the studies of Volterra se ries simplification, Zhu et al found that the coefficients of different delay product cross-tenns would reduce rapidly along with delay expansion, and thus presented a dynamic deviation reduction (Dynamic Deviation Reduction, DDR) model [6]. In [3], the power of the deviation terms are limited to 2, which we call simplified dynamic deviation reduction model here. The symbol interpolation rate of modern communication signal is very high, and the corre lation between the different samples is very strong.…”

“…And be cause of the memory effect, broadband predistorters have a lot of parameters, which results in a more difficult iden tification. Recently, Laki et al proposed a complicated frequency domain predistortion algorithm [3] [4], which required different kinds of algorithms in several alternat ing global and local optimizations. In this paper, we use a 8 basic and an improved PSO algorithm as the optimization algorithm.…”

An Average Power Tracking Method for Wideband Highly Nonlinear Power Amplifiers

“…In the past decade, based on the basic architecture of digital predistortion subsystem (DPDS), various DPD models are presented, such as polynomial model [3], [4], Poly-Harmonic model [5] and adaptive model [6], [7]. In these studies, the average power gain between predistorter input and the feedback signal was revised using all original data directly in the preprocessing step and PA output will drop more or less when the predistorter is applied [8].…”

Study and design of CMOS RF power circuits and modulation capabilities for communication applications

“…Knowing the behavior of the power amplifier may shift due to aging effects, heating and memory effects, this technique finds use in base stations[53]. It is also possible to make predistortion adaptive based on feedback loops or look-up tables[54] [55].…”