Holographic MIMO Communications: What is the benefit of closely spaced antennas?

TL;DR

This paper investigates the benefits of closely spaced antennas in holographic MIMO systems, revealing that spectral efficiency gains are limited to moderate array sizes and diminish for larger arrays.

Contribution

It provides a physically consistent analysis of closely spaced antennas in holographic MIMO, highlighting the limited spectral efficiency improvements for large arrays.

Findings

Spectral efficiency depends on signal directions and array matching.

Closely spaced antennas improve efficiency mainly in moderate-sized arrays.

Larger dense arrays show marginal efficiency gains over half-wavelength arrays.

Abstract

Holographic MIMO refers to a (possibly large) array with a large number of individually controlled and densely deployed antennas. The objective of this paper is to provide further insight into the use of closely spaced antennas in the uplink and downlink of a multi-user Holographic MIMO system. To this end, we utilize multiport communication theory, which ensures physically consistent uplink and downlink models. We first consider a simple uplink scenario with two side-by-side half-wavelength dipoles, two users, and single-path line-of-sight propagation, and show both analytically and numerically that the array gain and average spectral efficiency strongly depend on the directions from which the signals are received and on the array matching network used. The numerical results are then used to extend the analysis to more practical scenarios involving larger arrays of dipoles (arranged in a uniform linear array) and a larger number of users. The case where the antennas are densely packed in a space-constrained factor form is also considered. It is found that the spectral efficiency increases with decreasing antenna spacing only for arrays of moderate size, e.g. in the order of a few wavelengths. In comparison, larger arrays with closely spaced antennas show only marginal improvements in spectral efficiency compared to half-wavelength arrays.