Spatial Characteristics of Distortion Radiated from Antenna Arrays with Transceiver Nonlinearities

TL;DR

This paper analyzes the spatial radiation pattern of nonlinear distortion from massive MIMO antenna arrays, revealing how beamforming influences distortion directionality and isotropy, aiding in system design and interference management.

Contribution

It derives a theoretical model for the spatial characteristics of nonlinear distortion in massive MIMO systems using Hermite representation.

Findings

Distortion beams in the same direction as dominant input beams.

When no dominant beam exists, distortion becomes nearly isotropic.

The theory helps predict out-of-band radiation and optimize user scheduling.

Abstract

The distortion from massive MIMO (multiple-input--multiple-output) base stations with nonlinear amplifiers is studied and its radiation pattern is derived. The distortion is analyzed both in-band and out-of-band. By using an orthogonal Hermite representation of the amplified signal, the spatial cross-correlation matrix of the nonlinear distortion is obtained. It shows that, if the input signal to the amplifiers has a dominant beam, the distortion is beamformed in the same way as that beam. When there are multiple beams without any one being dominant, it is shown that the distortion is practically isotropic. The derived theory is useful to predict how the nonlinear distortion will behave, to analyze the out-of-band radiation, to do reciprocity calibration, and to schedule users in the frequency plane to minimize the effect of in-band distortion.