Unmanned aerial vehicle (UAV) is regarded as a key enabling platform for low-altitude economy, due to its advantages such as 3D maneuverability, flexible deployment, and LoS air-to-air/ground communication links. In particular, the intrinsic high mobility renders UAV especially suitable for operating as a movable antenna (MA) from the sky. In this paper, by exploiting the flexible mobility of UAV swarm and antenna position adjustment of MA, we propose a novel UAV swarm enabled two-level MA system, where UAVs not only individually deploy a local MA array, but also form a larger-scale MA system with their individual MA arrays via swarm coordination. We formulate a general optimization problem to maximize the minimum achievable rate over all ground user equipments (UEs), by jointly optimizing the 3D UAV swarm placement positions, their individual MAs' positions, and receive beamforming for different UEs. To gain useful insights, we first consider the special case where each UAV has only one antenna, under different scenarios of one single UE, two UEs, and arbitrary number of UEs. In particular, for the two-UE case, we derive the optimal UAV swarm placement positions in closed-form that achieves IUI-free communication when the uniform plane wave (UPW) model holds, where the UAV swarm forms a uniform sparse array (USA) satisfying minimum safe distance constraint. While for the general case with arbitrary number of UEs, we propose an efficient alternating optimization algorithm to solve the formulated non-convex optimization problem. Then, we extend the results to the case where each UAV is equipped with multiple antennas. Numerical results verify that the proposed low-altitude UAV swarm enabled MA system significantly outperforms various benchmark schemes, thanks to the exploitation of two-level mobility to create more favorable channel conditions for multi-UE communications.

翻译：无人机因其三维机动性、灵活部署能力以及视距空对空/地对空通信链路等优势，被视为低空经济的关键使能平台。特别是其固有的高移动性使得无人机尤其适合作为空中可移动天线运行。本文通过利用无人机集群的灵活机动性和可移动天线的位置调节能力，提出了一种新型的无人机集群赋能两级可移动天线系统。在该系统中，无人机不仅各自部署一个本地可移动天线阵列，还通过集群协同形成一个更大规模的可移动天线系统。我们构建了一个通用优化问题，旨在通过联合优化无人机集群的三维部署位置、各无人机可移动天线的位置以及针对不同地面用户设备的接收波束成形，最大化所有地面用户设备的最小可达速率。为获得有益见解，我们首先考虑每个无人机仅配备单根天线的特殊情况，并分别研究了单一用户、两个用户及任意数量用户的不同场景。特别地，针对双用户场景，在均匀平面波模型成立的条件下，我们推导出了实现无用户间干扰通信的最优无人机集群部署位置的闭式解，此时无人机集群形成一个满足最小安全距离约束的均匀稀疏阵列。而对于具有任意数量用户的一般情况，我们提出了一种高效的交替优化算法来求解所构建的非凸优化问题。随后，我们将结果扩展到每个无人机配备多根天线的情况。数值结果表明，得益于利用两级移动性为多用户通信创造更有利的信道条件，所提出的低空无人机集群赋能可移动天线系统性能显著优于多种基准方案。