Joint Transmit and Receive Antenna Orientation Design for Secure MIMO Communications

2026-05-13

AI Summary

This work addresses the limitations of conventional physical-layer security, where fixed antenna orientations hinder effective eavesdropping suppression. For the first time, it treats the orientations of rotatable transmit and receive antennas as active optimization variables, jointly designing beamforming, artificial noise covariance matrices, and antenna directions to dynamically reshape the MIMO channel structure and enhance secrecy rate. Leveraging a Riemannian manifold optimization framework combined with alternating optimization, the proposed approach integrates semi-closed-form beamforming, minimum mean square error reconstruction, and the Riemannian Frank–Wolfe algorithm to achieve rapid convergence. The resulting scheme significantly outperforms fixed-orientation baselines in terms of secrecy performance and naturally extends to multiuser secure transmission scenarios.

Abstract

Physical layer security (PLS) is a promising paradigm for safeguarding 6G wireless networks by exploiting the inherent characteristics of wireless channels. However, the efficiency of conventional PLS is often limited by fixed orientation antennas. This paper investigates a rotatable antenna (RA)-aided secure multiple-input multiple-output (MIMO) communication system, where both the transmitter and the receiver are equipped with RAs in the presence of an eavesdropper. By dynamically optimizing the orientations of RAs, we can proactively reshape the effective MIMO channels to enhance legitimate transmission while simultaneously suppressing information leakage to the eavesdropper. We formulate a secrecy rate maximization problem by jointly optimizing the transmit beamforming, artificial noise (AN) covariance matrix, and the transmit/receive RA orientations, subject to the transmit power budget and antenna orientation constraints. To tackle the resulting highly coupled and non-convex problem, we first study a simplified single-input single-output (SISO) case to reveal the structure of the optimal RA orientation. For the general MIMO case, we develop an alternating optimization algorithm by reformulating the original problem through the minimum mean-square error framework. In particular, the transmit beamforming and AN covariance matrix are derived in semi-closed forms, while the RA orientations are updated via the Riemannian Frank-Wolfe method. The proposed design is further extended to the multi-receiver secure transmission scenario. Simulation results show that the proposed scheme converges rapidly and achieves significant secrecy rate gains over the conventional fixed-orientation scheme.

Problem

• Physical layer security
• Rotatable antenna
• MIMO communications
• Secrecy rate
• Eavesdropping

Innovation

• rotatable antenna
• physical layer security
• MIMO
• alternating optimization
• Riemannian Frank-Wolfe