This paper investigates a multiuser millimeter-wave (mmWave) uplink system in which each user is equipped with a multi-antenna fluid antenna system (FAS) while the base station (BS) has multiple fixed-position antennas. Our primary objective is to maximize the system capacity by optimizing the transmit covariance matrices and the antenna position vectors of the users jointly. To gain deeper insights, we commence by deriving upper bounds and approximations for the maximum capacity. Then we delve into the capacity maximization problem. Beginning with the simple scenario of a single user equipped with a single-antenna FAS, we reveal that a closed-form optimal solution exists when there are only two propagation paths between the user and the BS. In the case where multiple propagation paths are present, a near-optimal solution can be obtained through a one-dimensional search method. Expanding our focus to multiuser cases, where users are equipped with either single- or multi-antenna FAS, we show that the original capacity maximization problems can be reformulated into distinct rank-one programmings. Then, we propose alternating optimization algorithms to deal with the transformed problems. Simulation results indicate that FAS can improve the capacity of the multiple access (MAC) system greatly, and the proposed algorithms outperform all the benchmarks.

翻译：本文研究了一种多用户毫米波上行链路系统，其中每个用户配备多天线流体天线系统(FAS)，而基站(BS)则具有多个固定位置天线。我们的主要目标是通过联合优化用户的发射协方差矩阵和天线位置向量来最大化系统容量。为深入理解，我们首先推导出最大容量的上界和近似表达式，随后深入探讨容量最大化问题。从单用户配备单天线FAS的简单场景入手，我们发现当用户与BS之间仅存在两条传播路径时，存在闭式最优解；而在存在多条传播路径的情况下，可通过一维搜索方法获得近优解。进一步将研究扩展到多用户场景（用户配备单天线或多天线FAS），我们证明原始容量最大化问题可被重构为不同的秩一规划问题。随后，我们提出交替优化算法以处理转化后的优化问题。仿真结果表明，FAS可显著提升多址接入(MAC)系统容量，且所提算法优于所有基准方案。