Rotatable antenna (RA) is a promising technology that can exploit new spatial degrees-of-freedom (DoFs) by flexibly adjusting the three-dimensional (3D) boresight direction of antennas. In this letter, we investigate an RA-enhanced cell-free system for downlink transmission, where multiple RA-equipped access points (APs) cooperatively serve multiple single-antenna users over the same time-frequency resource. Specifically, we aim to maximize the sum rate of all users by jointly optimizing the AP-user associations and the RA boresight directions. Accordingly, we propose a two-stage strategy to solve the AP-user association problem, and then employ fractional programming (FP) and successive convex approximation (SCA) techniques to optimize the RA boresight directions. Numerical results demonstrate that the proposed RA-enhanced cell-free system significantly outperforms various benchmark schemes.

翻译：可旋转天线（RA）是一种能够通过灵活调整天线三维（3D）视轴方向来开发新空间自由度（DoFs）的潜力技术。本文研究了一种用于下行链路传输的RA增强无蜂窝系统，其中多个配备RA的接入点（APs）在相同时频资源上协作服务多个单天线用户。具体而言，我们通过联合优化AP-用户关联与RA视轴方向，以最大化所有用户的和速率。为此，我们提出一种两阶段策略求解AP-用户关联问题，进而采用分数规划（FP）和逐次凸近似（SCA）技术优化RA视轴方向。数值结果表明，所提RA增强无蜂窝系统显著优于多种基准方案。