On the Effects of PA Nonlinearities for Hybrid Beamforming Based Wideband Massive MIMO Systems

In this paper, we present a general framework for the investigation of the effects of power amplifier (PA) non-linearities on the hybrid beamforming based massive multiple-input multiple-output (MIMO) systems. In literature, much of the attention is devoted to phase shifters as the analog beamformer stage; however, in this work, a more comprehensive scenario, based on fully connected analog beamformer architecture, is considered. Firstly, out-of-band (OOB) radiation pattern is evaluated to investigate the spatial characteristics of the undesired radiation. It is observed that OOB radiation due to nonlinear distortion is concentrated on the beamforming directions. In addition, a framework is proposed to design digital predistorters (DPD) and a conventional DPD architecture is adopted to the proposed design framework. It is shown that conventional DPD can solve the OOB problem for a single user environment; however, its performance degrades in the multi-user case. Even though, there are many studies on the effects of nonlinear PA distortions, a few of them consider the achievable information rate (AIR). In this work, a framework is adopted to study the capacity of the considered system under PA nonlinearities.

I. Introduction

Millimeter-wave (mmWave) frequency band (28–72 GHz) has attracted significant attention as a suitable candidate for 5G systems in order to meet high data throughput requirements [1]. However, systems operating in mmWave frequencies suffer from severe amount of propagation loss, which makes beamforming at base station (BS) and user equipment (UE) vitally important. Massive MIMO systems, which is equipped with massive amount of antennas, are promising solution since BS becomes able to concentrate the energy in the desired direction. However, using a large number of antennas limits the implementation of beamforming architecture. In conventional fully digital beamforming, there should be a single dedicated RF chain for each antenna element, which increases the cost and the complexity of the system tremendously. Hybrid analog-digital beamforming is developed so that desired spatial processing is achieved with less number of RF chains then antenna elements. In literature, various design methods for hybrid beamformers are proposed [2]- [3]. In this work, a two stage beamforming technique, called Joint Spatial Division and Multiplexing (JSDM) is adopted [3]. JSDM framework enables the make use of user grouping so that users having same channel covariance matrices can be processed jointly.