Influence And Compensation Of Non-idealities In Digital Predistortion Of Power Amplifiers

With the remarkable evolution of wireless communication techniques, modern wireless communication base stations utilize different standards to meet the increasing demand of users. So, it is more desirable to improve transmitter efficiency as much as possible within limited bandwidth. Radio Frequency (RF) power amplifiers (PAs), which contribute greatly to energy consumption of base station transmitters, are usually driven to nonlinear region in order to achieve higher efficiency. However, the distortions introduced by nonlinear behavior of PAs, like spectral regrowth and in-band distortions, must be compensated. There have been a lot of investigations into the linearization techniques of PAs over the past decades. Digital predistortion (DPD) emerged as one of the most promising approaches.RF transmitters are composed of linear and nonlinear circuits and they are dynamic nonlinear systems. The detailed investigations and specific discussions about the estimation and compensation of nonlinearities and distortions in modern transmitters are conducted. The main contributions and innovations are summarized as follows:First, the focus is on the source of nonlinear distortions in RF transmitters, which include distortions both from power amplifier and from I/Q imbalance. Then the overview of DPD theory and model architecture is introduced.In Chapter3, the influence of time misalignment on the PA modeling and performance of DPD is analyzed. Then different model’s sensitivity to time misalignment is compared and the simulation results are given. The generalized memory polynomial is found to have increased the degree of freedom in modeling and to have good tolerance to the inaccuracy of time alignment.Then the discussion about time misalignment extends to the situation when concurrent dual-band amplifiers are utilized. The characteristics of time-delay estimation method of concurrent dual-band amplifiers are analyzed. The time-delay-tolerant model with interpolation is simplified by pruning and comparisons in model complexity and performance are given.Chapter5proposes a new method to prune the joint2D digital predistortion (2D-DPD) model for concurrent dual-band transmitter where in-phase and quadrature (I/Q) imbalance and local oscillator (LO) leakage often exist. The phenomenon of over compensation of the DPD model is found if too many terms in the DPD model are adopted. Thus redundant polynomial terms are removed in order to solve this problem,and meanwhile, to reduce the model complexity. The experiment compares linearization performances and model complexities of the proposed model with other state-of-the-art models. The new method proved to have the advantage of mitigated computational load and increased numerical stability, and of good performance on linearization and spectral regrowth suppression.