I. Introduction
The demand of the unmanned aerial vehicle (UAV) service market, such as air taxi and drone cargo delivery, is expected to grow explosively, and UAV communication is envisioned as an essential factor in the upcoming
We here mean the time unit as a frame in a certain communication system such as 5G/LTE cellular standards, or a packet in IEEE 802.11 standard.
aerial network [1]. UAVs/drones have attractive elements due to flexible deployment, movement, and high line-of-sight (LOS) probability. Thus they can be highly utilized as base stations, users, and relays in communication systems. For UAVs to play a role in the communication network, they need to provide a stable link through directional beamforming based on the large-sized antenna array to build connectivity with a UAV at a relatively long distance. Therefore, the transceiver must steer the directional beam along the movement path of the UAV, and this is called beam tracking [2], [3]. Especially, the transceiver based on a massive antenna array can implement a narrow beam with high directivity mitigating severe path loss in the millimeter-wave band [4], [5]. A narrow beam can increase SNR performance through a high beamforming gain. However, the communication link secured by the narrow beam is vulnerable to movement and perturbation of the UAV [6]. In addition, if maximizing the transmission rate is not the primary goal, it may be more practical to establish a stable link that does not cause packet loss with a small overhead while satisfying the minimum transmission rate.