Measurement-Based Validation of Z3RO Precoder to Prevent Nonlinear Amplifier Distortion in Massive MIMO Systems

In multiple input multiple output (MIMO) systems, precoding allows the base station to spatially focus and multiplex signals towards each user. However, distortion introduced by power amplifier nonlinearities coherently combines in the same spatial directions when using a conventional precoder such as maximum ratio transmission (MRT). This can strongly limit the user performance and moreover create unauthorized out-of-band (OOB) emissions. In order to overcome this problem, the zero third-order distortion (Z3RO) precoder was recently introduced. This precoder constraints the third-order distortion at the user location to be zero. In this work, the performance of the Z3RO precoder is validated based on real-world channel measurement data. The results illustrate the reduction in distortion power at the UE locations: an average distortion reduction of 6.03 dB in the worst-case single-user scenario and 3.54 dB in the 2-user case at a back-off rate of -3dB.

I. Introduction

In current wireless cellular communication systems, the power amplifier (PA) is the main source of energy consumption [1]. In order to achieve good communication performance, PAs should be highly linear. However, there is a trade-off between power-efficiency and linearity of the PA [2]. Consequently, when using a PA at an energy-efficient operating point, nonlinear distortion will arise. This nonlinear distortion is detrimental for the performance of the wireless system and can furthermore lead to unauthorized OOB emissions. This is especially the case in massive MIMO systems, where beamforming allows the base station to transmit the signal in a certain spatial direction. Recent studies have shown that the nonlinear distortion is not spatially spread out, but follows the dominant beamforming direction [3], [4]. This is witnessed in particular in situations with predominant transmission in one or a few directions (e.g. line-of-sight (LOS) situations with only few users). This beamforming of the nonlinear distortion to the user location can strongly decrease the signal-to-distortion ratio (SDR) at the user location, which inherently limits the user performance. Additionally, this can introduce or strengthen unauthorized OOB emissions at both the user and unintended locations.