Degrees of Freedom of Holographic MIMO in Multi-user Near-field Channels

TL;DR

This paper introduces a new analytical method to evaluate the spatial degrees of freedom of holographic MIMO systems in multi-user near-field channels, accounting for spatial blocking effects that reduce capacity.

Contribution

It develops a novel approach to calculate the DoF in near-field HMIMO systems and extends it to multi-user scenarios considering spatial blocking effects.

Findings

The DoF in multi-user near-field channels is less than the sum of single-user DoF due to spatial blocking.

The proposed method accurately predicts a 21.2% average reduction in DoF caused by spatial blocking.

Simulation results validate the effectiveness of the analytical framework.

Abstract

Holographic multiple-input multiple-output (HMIMO) is an emerging technology for 6G communications, in which numerous antenna units are integrated in a limited space. As the HMIMO array aperture expands, the near-field region of the array is dramatically enlarged, resulting in more users being located in the near-field region. This creates new opportunities for wireless communications. In this context, the evaluation of the spatial degrees of freedom (DoF) of HMIMO multi-user systems in near-field channels is an open problem, as the analytical methods utilized for evaluating the DoF in far-field channels cannot be directly applied. In this paper, we propose a novel method to calculate the DoF of HMIMO in multi-user near-field channels. We first derive the DoF for a single user in the near field, and then extend the analysis to multi-user scenarios. In this latter scenario, we focus on the impact of spatial blocking between HMIMO users. The derived analytical framework reveals that the DoF of HMIMO in multi-user near-field channels is not in general given by the sum of the DoF of the HMIMO single-user setting because of spatial blocking. Simulation results demonstrate the effectiveness of the proposed method. In the considered case study, the number of DoF reduces by $21.2\%$ on average due to spatial blocking.