Breaking the Degrees-of-Freedom Limit of Holographic MIMO Communications: A 3-D Antenna Array Topology

TL;DR

This work identifies the DOF limitation in holographic MIMO systems employing 2-D planar arrays and introduces a 3-D antenna topology that exploits the vertical dimension to access additional EM DOF. It develops two modeling frameworks, the 3-D Clarke model and the Kronecker model, to quantify DOF, diversity, and capacity gains, and validates the concept with a practical EBG-enabled dipole array. Theoretical analysis and full-wave simulations, complemented by experimental measurements, demonstrate that the 3-D topology can surpass the conventional DOF and capacity limits, particularly in rich-scattering and large-angular-spread environments, with modest gains in more LOS-like scenarios. The results across Rayleigh and 3GPP channels suggest that 3-D holographic MIMO arrays are a promising path to substantially enhance MIMO performance in future wireless systems.

Abstract

The performance of holographic multiple-input multiple-output (MIMO) communications, employing two-dimensional (2-D) planar antenna arrays, is typically compromised by finite degrees-of-freedom (DOF) stemming from limited array size. The DOF constraint becomes significant when the element spacing approaches approximately half a wavelength, thereby restricting the overall performance of MIMO systems. To break this inherent limitation, we propose a novel three-dimensional (3-D) antenna array that strategically explores the untapped vertical dimension. We investigate the performance of MIMO systems utilizing 3-D arrays across different multi-path scenarios, encompassing Rayleigh channels with varying angular spreads and the 3rd generation partnership project (3GPP) channels. We subsequently showcase the advantages of these 3-D arrays over their 2-D counterparts with the same aperture sizes. As a proof of concept, a practical dipole-based 3-D array, facilitated by an electromagnetic band-gap (EBG) reflecting surface, is conceived, constructed, and evaluated. The experimental results align closely with full-wave simulations, and channel simulations substantiate that the DOF and capacity constraints of traditional holographic MIMO systems can be surpassed by adopting such a 3-D array configuration.

Figures (19)

• Figure 1: Diagrams of the traditional 2-D antenna array and the proposed 3-D antenna array, where a one-row array is set up for demonstration. (a) Traditional 2-D antenna array over a PEC surface, where the heights of antennas are the same. (b) Proposed 3-D antenna array over an EBG surface and a height difference is introduced between the nearby antennas.
• Figure 2: Intuitive explanations of the benefits from a 3-D antenna array. (a) Under the incidence of a large-angle plane wave with the direction of $\mathbf{k}$, the 3-D array would have a larger projection area compared to the 2-D array. (b) In the angular spectrum analysis of the DOF limit, the blue circle with the radius of $k_0$ represents the available angular spectrum resource, $\Delta k_x$ and $\Delta k_y$ denote the resolutions along the $x$ and $y$ directions. The 3-D array would have better resolutions compared to the 2-D array at large angles.
• Figure 3: Illustration of the angular spread, where the plane waves are uniformly distributed in an angular range characterized by $\theta$.
• Figure 4: Diversity measure based on the 3-D Clarke model, and the array length is fixed as 5$\lambda_0$. (a) Diversities of the 3-D arrays with different $h$ and antenna numbers. (b) Diversities of the 3-D arrays with different $h$ and angular spreads, where the antenna number is fixed as 25.
• Figure 5: Diversity measure based on the Kronecker model, and the array length is fixed as 5$\lambda_0$. (a) Diversities of the 3-D arrays when $h=0$ and $h=0.5$$\lambda_0$. (b) Diversities of the 3-D arrays with different $h$ and angular spreads, where the antenna number is fixed as 25.