Analysis of Applicability of the Dipole Mutual Coupling Model for Configuring Holographic MIMO Surfaces

Holographic MIMO (H-MIMO) surfaces are planar structures composed of multiple active or passive unit cells capable of changing their configuration to form a wave with specified parameters. In H‑MIMO surfaces, the spacing between unit cells is less than half the wavelength, which, on the one hand, enables the formation of highly directive beams, but on the other hand, leads to the need to take into account the mutual coupling between cells. This study presents a comparative analysis of the accuracy of a dipole-based analytical model for computing the mutual coupling matrix in H-MIMO surfaces. The discrepancy between the mutual coupling matrices derived through the CST Microwave Studio numerical simulation and those calculated using an analytical model that relies on pairwise interaction calculation for dipole-like unit cells is investigated. The numerical results obtained show that, as the spacing between H-MIMO surface elements decreases below half the wavelength, the fidelity of the analytical model degrades substantially due to the pronounced mutual coupling. It is shown that employing this approximate analytical matrix for the H‑MIMO surface configuration leads to a significant channel gain degradation as compared with the configuration based on the CST-simulated coupling matrix.