Multi-User MIMO with Rotatable Antennas and IRS: Joint Antenna Boresight and IRS Orientation Design

2026-05-22

AI Summary

This work addresses the severe multiplicative path loss caused by cascaded channels in intelligent reflecting surface (IRS)-assisted multi-user MIMO systems by proposing a joint optimization framework that coordinates the rotation of both the base station’s directional antenna and the IRS panel. The framework jointly designs receive beamforming, IRS phase shifts, the mainlobe orientation of the base station antenna, and the IRS orientation to maximize the system sum rate, revealing that the dual-rotation gain is separable in the far-field regime but coupled in the near-field regime. Algorithmically, closed-form beamforming updates, a fractional programming–assisted Riemannian conjugate gradient method for phase-shift optimization, and a projected gradient method for rotation-angle adjustment are employed. Simulations demonstrate that the proposed scheme significantly outperforms fixed or single-rotation baselines, offering crucial theoretical insights for near-field cooperative rotation design.

Abstract

In this paper, we investigate an intelligent reflecting surface (IRS)-assisted multi-user system, where the base station (BS) employs rotatable antennas (RAs) and the IRS can adjust the panel orientation.To alleviate the severe multiplicative path loss of the cascaded channel, the IRS is deployed near the BS, while the user-BS and user-IRS links remain in the far field. We formulate a sum-rate maximization problem by jointly optimizing the receive beamforming, IRS phase shifts, BS antenna boresights, and IRS panel orientation. To tackle the resulting highly coupled and non-convex problem, we first study a single-user case to reveal the structure of the dual-rotation gain, which is shown to be multiplicatively separable in the far field but coupled in the near field. For the general multi-user case, we develop an alternating optimization algorithm, where the receive beamforming is updated in closed form, the IRS phase shifts are optimized by an FP-assisted Riemannian conjugate gradient method, and the BS antenna boresights and IRS panel orientation are updated via projected gradient methods. Simulation results demonstrate the significant sum-rate gains achieved by the proposed coordinated rotation design over fixed-orientation and single-rotation benchmark schemes, and provide useful insights into near-field dual-rotation design.

Problem

• Multi-User MIMO
• Intelligent Reflecting Surface
• Rotatable Antennas
• Beamforming
• Sum-Rate Maximization

Innovation

• Rotatable Antennas
• Intelligent Reflecting Surface (IRS)
• Joint Orientation Optimization
• Dual-Rotation Gain
• Alternating Optimization