I. Introduction
During the past decade, there has been a dramatic increase in the use of unmanned aerial vehicles (UAVs), i.e., the remote surveillance, filming, disaster relief, goods transport, and communication relaying, due to their affordable prices, multiple flight controls, high maneuverability, ease operability and increasing payload weight [1]. UAVs also have been considered as the aerial base station in the fifth generation (5G) communication systems [2], [3]. Since multi-input multi- output (MIMO) technologies can improve the capacity and efficiency of communication systems significantly, the UAV communication systems with MIMO technologies have gradu- ally become a research hotspot. However, it is vital to establish accurate and reliable UAV-MIMO channel models for the system design, optimization, and performance evaluation.