Rethinking Mutual Coupling in Movable Antenna MIMO Systems: Modeling and Optimization

TL;DR

This paper explores how mutual coupling in movable antenna MIMO systems can be modeled and optimized to enhance capacity, revealing that MC can be a beneficial factor rather than just an electromagnetic limitation.

Contribution

It introduces a novel optimization framework leveraging circuit-theoretic models to exploit mutual coupling effects for capacity gains in MA-MIMO systems.

Findings

Mutual coupling can be harnessed for capacity enhancement in MA-MIMO systems.

The proposed algorithms effectively optimize antenna positions for narrowband and wideband scenarios.

Simulation results show significant performance improvements by exploiting mutual coupling.

Abstract

Movable antennas (MAs) have attracted growing interest for their ability to improve channel conditions via adaptive antenna movement. Nevertheless, such movement inevitably introduces mutual coupling (MC), whose impact has been largely overlooked in existing MA literature. In this paper, we show that MC is not merely an unavoidable electromagnetic effect, but also a new source of capacity gains in MA-enabled multiple-input multiple-output (MIMO) systems. To leverage MC effects, we develop an optimization framework for both narrowband and wideband systems based on a rigorous circuit-theoretic model. For narrowband systems, capacity maximization is formulated as a non-convex optimization problem, which is solved via a block coordinate ascent (BCA) framework. Because optimizing MA positions is challenging due to analytically intractable MC matrices, we develop a trust region method (TRM)-based algorithm that utilizes Sylvester equations to compute the derivatives of the inverse square roots of the MC matrices. We further consider wideband systems and formulate a sum-rate maximization problem. To find a unified set of MA positions that balances varying subcarrier conditions, the BCA framework and the TRM-based MA position optimization algorithm are extended to wideband systems. Simulation results demonstrate that exploiting MC effects in MA-MIMO systems yields significant performance gains in both narrowband and wideband systems under various channel conditions. These gains highlight the benefits of MC-induced superdirectivity and designable MC matrices.