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

TL;DR

This article investigates the near-field communications for XL-MIMO from the perspective of beamspace, and points out sever-al applications of the near-field beamspace, and highlights potential directions for future study in the near-field beamspace.

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.

Figures (4)

• Figure 1: Illustration of characteristics of the near and far fields.
• Figure 2: Comparisons of far-field and near-field beamspaces.
• Figure 3: Characteristics of the high mainlobe in the near-field beamspace.
• Figure 4: Spatial region and intensity distribution of the low mainlobe in the near-field beamspace.