Directivity-Aware Degrees of Freedom Analysis for Extremely Large-Scale MIMO

TL;DR

This paper analyzes how antenna directivity affects the effective degrees of freedom in extremely large-scale MIMO systems, providing insights into capacity and element spacing impacts using a wavenumber-domain approach.

Contribution

It introduces a directivity-aware analysis of EDoF in XL-MIMO systems, revealing the uneven distribution of channel coupling coefficients and their influence on system capacity.

Findings

EDoF distribution is uneven due to antenna directivity.

Element spacing significantly impacts EDoF and capacity.

Numerical methods for coupling coefficients are provided.

Abstract

Extremely large-scale multiple-input multiple-output (XL-MIMO) communications, enabled by numerous antenna elements integrated into large antenna surfaces, can provide increased effective degree of freedom (EDoF) to achieve high diversity gain. However, it remains an open problem that how the EDoF is influenced by the directional radiation pattern of antenna elements. In this work, empowered by the wavenumber-domain channel representation, we analyze the EDoF in a general case where the directivity of antennas, determined by the antenna structure and element spacing, is considered. Specifically, we first reveal the uneven distribution of directivity-aware wavenumber-domain coupling coefficients, i.e., channel gain towards different directions, in the isotropic Rayleigh fading channel. EDoF is then calculated based on such distribution of coupling coefficients. A numerical method is also provided to obtain coupling coefficients via electromagnetic full-wave simulations. Due to the influence of antenna directivity, how EDoF and ergodic channel capacity vary with the element spacing are explored via simulations for different antenna types.