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.

翻译：本文研究了一种智能反射面（IRS）辅助的多用户系统，其中基站（BS）采用可旋转天线（RAs），且IRS能够调整面板朝向。为缓解级联信道严重的乘积路径损耗，我们将IRS部署在基站附近，而用户-基站链路与用户-IRS链路均保持远场条件。通过联合优化接收波束赋形、IRS相移、基站天线轴向与IRS面板朝向，我们构建了一个和速率最大化问题。针对该高度耦合的非凸问题，我们首先研究单用户场景以揭示双旋转增益的结构——该增益在远场中呈现乘法可分离性，但在近场中表现为耦合。对于一般多用户场景，我们提出了一种交替优化算法：接收波束赋形采用闭式更新，IRS相移通过FP辅助的黎曼共轭梯度法优化，基站天线轴向与IRS面板朝向则通过投影梯度法更新。仿真结果表明，所提出的协同旋转设计相较于固定朝向与单旋转基准方案实现了显著的和速率增益，并为近场双旋转设计提供了重要启示。