Near-Field Communications for Extremely Large-Scale MIMO: A Beamspace Perspective

TL;DR

This paper explores near-field beamspace for XL-MIMO systems, addressing the limitations of far-field models by mapping antenna space to near-field beamspace using fractional Fourier transform, and analyzing its components and advantages.

Contribution

It introduces a novel near-field beamspace mapping method for XL-MIMO, analyzing its components and demonstrating its benefits over traditional far-field approaches.

Findings

Near-field beamspace mapping improves XL-MIMO performance.

Analysis of high mainlobe, low mainlobe, and sidelobe components.

Advantages over existing far-field beamspace methods.

Abstract

Extremely large-scale multiple-input multiple-output (XL-MIMO) is regarded as one of the key techniques to enhance the performance of future wireless communications. Different from regular MIMO, the XL-MIMO shifts part of the communication region from the far field to the near field, where the spherical-wave channel model cannot be accurately approximated by the commonly-adopted planar-wave channel model. As a result, the well-explored far-field beamspace is unsuitable for near-field communications, thereby requiring the exploration of specialized near-field beamspace. In this article, we investigate the near-field communications for XL-MIMO from the perspective of beamspace. Given the spherical wavefront characteristics of the near-field channels, we first map the antenna space to the near-field beamspace with the fractional Fourier transform. Then, we divide the near-field beamspace into three parts, including high mainlobe, low mainlobe, and sidelobe, and provide a comprehensive analysis of these components. Based on the analysis, we demonstrate the advantages of the near-field beamspace over the existing methods. Finally, we point out several applications of the near-field beamspace and highlight some potential directions for future study in the near-field beamspace.