Densifying MIMO: Channel Modeling, Physical Constraints, and Performance Evaluation for Holographic Communications

TL;DR

This work addresses the practical limits of holographic MIMO by integrating mutual coupling and polarization into an electromagnetic channel model. It derives physical bounds on antenna efficiency, demonstrates that capacity gains saturate as dense packing reduces efficiency, and validates the model through indoor measurements, showing antenna efficiency as a key limiter. The combination of theoretical bounds, numerical simulations, and real-world data provides a concrete design guideline: densification yields diminishing returns beyond roughly half-wavelength element spacing, and polarization leakage can critically affect multi-user performance. Overall, the paper advances a physically grounded framework for dense-array holographic communications with practical implications for antenna design and system implementation.

Abstract

As the backbone of the fifth-generation (5G) cellular network, massive multiple-input multiple-output (MIMO) encounters a significant challenge in practical applications: how to deploy a large number of antenna elements within limited spaces. Recently, holographic communication has emerged as a potential solution to this issue. It employs dense antenna arrays and provides a tractable model. Nevertheless, some challenges must be addressed to actualize this innovative concept. One is the mutual coupling among antenna elements within an array. When the element spacing is small, near-field coupling becomes the dominant factor that strongly restricts the array performance. Another is the polarization of electromagnetic waves. As an intrinsic property, it was not fully considered in the previous channel modeling of holographic communication. The third is the lack of real-world experiments to show the potential and possible defects of a holographic communication system. In this paper, we propose an electromagnetic channel model based on the characteristics of electromagnetic waves. This model encompasses the impact of mutual coupling in the transceiver sides and the depolarization in the propagation environment. Furthermore, by approximating an infinite array, the performance restrictions of large-scale dense antenna arrays are also studied theoretically to exploit the potential of the proposed channel. In addition, numerical simulations and a channel measurement experiment are conducted. The findings reveal that within limited spaces, the coupling effect, particularly for element spacing smaller than half of the wavelength, is the primary factor leading to the inflection point for the performance of holographic communications.

Figures (20)

• Figure 1: Holographic communication system with homogeneous scattering environment.
• Figure 2: The wavenumber vector of the receive array and its sampling block $\Omega_{\mathrm{R}}(l_x,l_y)$ in the $k_xk_y$ plane.
• Figure 3: An $N$-element uniform linear array excited by a voltage source.
• Figure 4: The upper bound of radiation efficiency with different antenna sizes.
• Figure 5: An infinite planar array with rectangular grids.