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（UAV）凭借其三维机动性、灵活部署以及视距空对空/地通信链路等优势，被视为支撑低空经济的关键使能平台。特别是，其固有的高机动性使得无人机特别适合作为来自空中的可移动天线（MA）。本文通过利用无人机群的灵活机动性和MA的天线位置调整能力，提出了一种新颖的无人机群使能的两级MA系统：在此系统中，无人机不仅各自部署局部MA阵列，还通过群集协作将各自的MA阵列组合成更大规模的MA系统。我们构建了一个通用优化问题，通过联合优化无人机群的3D部署位置、其各自MA的位置以及面向不同地面用户设备（UE）的接收波束赋形，来最大化所有UE中的最小可达速率。为获得有意义的见解，我们首先考虑了每架无人机仅配置单根天线的特殊情况，并分别针对单个UE、两个UE以及任意数量UE的场景进行了分析。特别地，在双UE场景下，当均匀平面波（UPW）模型成立时，我们推导出了实现无用户间干扰（IUI）通信的无人机群最优部署位置的闭式表达式，此时无人机群形成为满足最小安全距离约束的均匀稀疏阵列（USA）。而对于任意数量UE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，我们将结果推广至每架无人机配备多根天线的情形。数值结果验证了所提出的低空无人机群使能MA系统显著优于各种基准方案，这得益于利用两级移动性为多UE通信创造了更有利的信道条件。