Fluid antenna system (FAS)/movable antenna (MA) has emerged as a promising technology to fully exploit the spatial degrees of freedom (DoFs). In this paper, we propose a new rotatable antenna (RA) model, as a simplified implementation of six-dimensional movable antenna (6DMA), to improve the performance of wireless communication systems. Different from conventional fixed-position antenna (FPA), the proposed RA system can independently and flexibly change the three-dimensional (3D) orientation of each antenna by adjusting its declination angles to achieve desired channel realizations. Specifically, we study an RA-enabled uplink communication system, where the receive beamforming and the declination angles of all RAs 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 declination angles are derived in closed form with the maximum-ratio combining (MRC) beamformer applied at the base station (BS). In the general multi-user and multi-path setup, we propose an alternating optimization (AO) algorithm to alternately optimize the receive beamforming and the declination angles in an iterative manner. Simulation results are provided to demonstrate that the proposed RA-enabled system can significantly outperform other benchmark schemes.

翻译：流体天线系统（FAS）/可移动天线（MA）已成为一种充分利用空间自由度（DoFs）的前沿技术。本文提出了一种新型旋转天线（RA）模型，作为六维可移动天线（6DMA）的一种简化实现方案，旨在提升无线通信系统的性能。与传统固定位置天线（FPA）不同，所提出的RA系统能够通过独立灵活地调整各天线的俯仰角，改变其三维（3D）朝向，从而实现期望的信道状态。具体而言，我们研究了一个基于RA的上行链路通信系统，其中联合优化了接收波束成形与所有RA的俯仰角，以最大化所有用户中的最小信号与干扰加噪声比（SINR）。在特殊的单用户自由空间传播场景下，推导出了闭式最优俯仰角表达式，此时基站（BS）采用最大比合并（MRC）波束成形器。在一般的多用户多径传播场景下，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成形与俯仰角。仿真结果表明，所提出的基于RA的系统性能显著优于其他基准方案。