Unmanned aerial vehicles (UAVs) have gained popularity in the communications research community because of their versatility in placement and potential to extend the functions of communication networks. However, there remains still a gap in existing works regarding detailed and measurement-verified air-to-ground (A2G) Massive Multi-Input Multi-Output (MaMIMO) channel characteristics which play an important role in realistic deployment. In this paper, we first design a UAV MaMIMO communication platform for channel acquisition. We then use the testbed to measure uplink Channel State Information (CSI) between a rotary-wing drone and a 64-element MaMIMO base station (BS). For characterization, we focus on multidimensional channel stationarity which is a fundamental metric in communication systems. Afterward, we present measurement results and analyze the channel statistics based on power delay profiles (PDPs) considering space, time, and frequency domains. We propose the stationary angle (SA) as a supplementary metric of stationary distance (SD) in the time domain. We analyze the coherence bandwidth and RMS delay spread for frequency stationarity. Finally, spatial correlations between elements are analyzed to indicate the spatial stationarity of the array. The space-time-frequency channel stationary characterization will benefit the physical layer design of MaMIMO-UAV communications.

翻译：无人机因其部署灵活性和扩展通信网络功能的潜力，在通信研究领域日益受到关注。然而，现有研究在经测量验证的空对地大规模多输入多输出（MaMIMO）信道特性方面仍存在空白，而这些特性对实际部署至关重要。本文首先设计了一套用于信道获取的无人机MaMIMO通信平台，然后利用该测试平台测量了旋翼无人机与64单元MaMIMO基站之间的上行信道状态信息。在信道表征方面，我们聚焦于通信系统中的基础度量——多维信道平稳性。随后，我们基于功率时延谱给出了测量结果，并从空间、时间和频率维度分析了信道统计特性。我们提出将平稳角度作为时域中平稳距离的补充度量，并通过相干带宽和均方根时延扩展分析频率平稳性。最后，通过分析阵列单元间的空间相关性来表征阵列的空间平稳性。这种空-时-频三维信道平稳性表征将有助于MaMIMO无人机通信的物理层设计。