Spatial Distribution of Distortion due to Nonlinear Power Amplification in Distributed Massive MIMO

TL;DR

This paper analyzes how nonlinear power amplifier distortion distributes spatially in distributed massive MIMO systems, revealing that it is more uniformly spread and localized in a beamspot, which can improve system efficiency.

Contribution

It extends the analysis of distortion distribution from central to distributed massive MIMO in LOS scenarios, highlighting differences and potential benefits.

Findings

Distortion is more uniformly distributed in distributed MIMO.

Coherent combining occurs in a beamspot rather than arbitrary directions.

Distributed MIMO allows operation closer to PA saturation, improving efficiency.

Abstract

Due to the nonlinearity of power amplifiers (PAs), the transmit signal is distorted. Previous works have studied the spatial distribution of this distortion for a central massive MIMO array. In this work, we extend the analysis for distributed massive MIMO in a line-of-sight scenario. We show that the distortion is not always uniformly distributed in space. In the single-user case, it coherently combines at the user location. In the few users case, the signals will add up at the user locations plus several others locations. As the number of users increases, it becomes close to uniformly distributed. As a comparison with a central massive MIMO system, having the same total number of antennas, the distortion in distributed massive MIMO is considerably more uniformly distributed in space. Moreover, the potential coherent combining is contained in a zone rather than in generic directions, i.e., in a beamspot. Furthermore, a small-scale fading effect is observed at unintended locations due to the non-coherent combining of the signals. As a result, one can expect that going towards distributed systems allows working closer to saturation, increasing the PA operating efficiency and/or using low-cost PAs.