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

2026-04-29

AI Summary

This work addresses the common oversight in existing reconfigurable antenna systems that neglect mutual coupling, thereby missing its potential to enhance channel capacity. For the first time, mutual coupling is treated not as an impairment but as a designable mechanism for performance improvement. The paper develops a unified joint optimization framework for both narrowband and wideband MIMO systems, actively exploiting mutual coupling–induced superdirectivity through coordinated optimization of antenna positions and beamforming. To this end, it introduces a circuit-theory–based mutual coupling matrix design, a unified antenna placement strategy suitable for wideband scenarios, and integrates block coordinate ascent, a Sylvester equation–based trust-region algorithm, and subcarrier joint optimization techniques. Extensive simulations under diverse channel conditions demonstrate that the proposed approach significantly improves system capacity and sum rate, confirming the effectiveness and superiority of deliberately harnessing 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.

Problem

• movable antenna
• mutual coupling
• MIMO systems
• capacity gain
• antenna positioning

Innovation

• mutual coupling
• movable antenna
• MIMO optimization
• trust region method
• superdirectivity