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（UAV）因其布设灵活性和扩展通信网络功能的潜力，在通信研究领域备受关注。然而，现有研究在详细且经测量验证的空对地（A2G）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采集的无人机MaMIMO通信平台，然后利用该测试平台测量了旋翼无人机与64阵元MaMIMO基站（BS）之间的上行信道状态信息（CSI）。在表征分析中，我们重点研究多维信道平稳性——这是通信系统中的一项基础度量指标。随后，我们展示了测量结果，并基于空间、时域和频域的功率时延谱（PDPs）对信道统计特性进行了分析。我们提出将平稳角度（SA）作为时域中平稳距离（SD）的补充度量指标，同时针对频域平稳性分析了相干带宽和均方根时延扩展。最后，通过阵元间空间相关性分析揭示了阵列的空间平稳性特征。该空-时-频信道平稳性表征将为MaMIMO-UAV通信的物理层设计提供重要支撑。