Non-fixed flexible antenna architectures, such as fluid antenna system (FAS), movable antenna (MA), and pinching antenna, have garnered significant interest in recent years. In this paper, we propose a new rotatable antenna (RA) model to improve the performance of wireless communication systems. Different from conventional fixed antennas, the proposed RA system can flexibly and independently alter the boresight direction of each antenna via mechanical or electronic means to exploit new spatial degrees-of-freedom (DoFs). Specifically, we investigate an RA-enabled uplink communication system, where the receive beamforming and the boresight directions of all RAs at the base station (BS) are jointly optimized to maximize the minimum signal-to-interference-plus-noise ratio (SINR) among all the users. In the special single-user and free-space propagation setup, the optimal boresight directions of RAs are derived in closed form with the maximum-ratio combining (MRC) beamformer applied at the BS. In the general multi-user and multipath channel setup, we first propose an alternating optimization (AO) algorithm to alternately optimize the receive beamforming and the boresight directions of RAs in an iterative manner. Then, a two-stage algorithm that solves the formulated problem without the need for iteration is proposed to further reduce computational complexity. Moreover, we extend the channel model to incorporate polarization effects and frequency-selective fading while catering to antenna boresight rotation. Simulation results are provided to validate our analytical results and demonstrate that the proposed RA system can significantly improve the communication performance as compared to other benchmark schemes.

翻译：非固定柔性天线架构，如流体天线系统（FAS）、可移动天线（MA）和夹捏天线，近年来引起了广泛关注。本文提出了一种新的可旋转天线（RA）模型，以提升无线通信系统的性能。与传统固定天线不同，所提出的RA系统能够通过机械或电子方式灵活且独立地改变每根天线的视轴方向，从而利用新的空间自由度（DoFs）。具体而言，我们研究了一种RA使能的上行通信系统，其中基站的接收波束赋形和所有RA的视轴方向被联合优化，以最大化所有用户中最小的信干噪比（SINR）。在特殊的单用户自由空间传播场景中，当基站采用最大比合并（MRC）波束赋形器时，推导了RA最优视轴方向的闭式解。在一般的多用户多径信道场景中，我们首先提出了一种交替优化（AO）算法，以迭代方式交替优化接收波束赋形和RA的视轴方向。随后，为进一步降低计算复杂度，提出了一种无需迭代的两阶段算法来求解所构建的问题。此外，我们将信道模型扩展到包含极化效应和频率选择性衰落，并适应天线视轴旋转。仿真结果验证了我们的分析结论，并表明与其它基准方案相比，所提出的RA系统能够显著提升通信性能。