## 1 Introduction  
Highly maneuverable unmanned aerial vehicles (UAVs) are expected to be utilized across a wide range of fields [1, 2]. Recent advances have led to a reduction in the cost and size of UAVs, making them more efficient and safer to deploy in a variety of commercial applications, such as surveillance, agriculture, infrastructure inspection, and transportation [3, 4]. In particular, their use for infrastructure inspection and transportation is anticipated to experience the most significant growth in the future [5]. Infrastructure inspection has a significant impact on the safety, efficiency, and sustainability of society. UAVs can also be used as an alternative to perform more dangerous missions and tasks without putting the operator's life at risk [6, 7, 8]. In the transport sector, there is particular interest in last-mile delivery and the emergency transport of medical supplies. For last-mile delivery, narrow alleys and traffic congestion in urban areas can be a bottleneck, but UAVs can avoid these restrictions and deliver efficiently [9]. They also have the advantage of being able to transport medical supplies quickly and safely to remote areas and disaster zones where ground transport is difficult [10]. These operations often require UAVs to be near buildings. Such environments create multiple signal paths, commonly referred to as multipath environments due to reflected and diffracted waves from surrounding structures, which then cause significant fluctuations in received signal power. Consequently, even when UAVs are flying in a line-of-sight (LOS) environment with no obstructions between the transmitting and receiving antennas, communications may be temporarily interrupted due to the propagation environment or antenna directivity, posing a significant risk to the safe operation of UAVs. Therefore, a thorough understanding of the radio propagation characteristics along a UAV's flight path is crucial to its reliable operation. UAV wireless communications include two types: air-to-air (A2A) communications between UAVs in flight and air-to-ground (A2G) communications between the UAV and a ground pilot or base station (BS). In particular, Wi-Fi 5 (IEEE 802.11 ac) [11] and Wi-Fi 6 (IEEE 802.11 ax) [12], which are often used for A2G communications, can provide convenient and high-speed communication for UAV operations and short-range data transmission. Organizations such as the International Telecommunication Union (ITU), the Unmanned Aircraft Systems International Association (UASIA), and the Federal Aviation Administration (FAA) have initiated several research projects to establish international standards and regulations for UAV communications [13]. These standards and regulations are concerned with UAV communications technology, protocols, and the use of frequency bands. The frequency bands allocated for A2G communications vary between countries and regions, but the 2.4 GHz band is widely used for remote control and data transmission of small UAVs [14]. To evaluate the quality of UAV-based A2G communications, it is essential to perform A2G channel measurements and develop A2G channel models [15; 16; 17]. The third-generation partnership project (3 GPP) technical report (TR) 38.901 [18] and 36.777 [19] has performed detailed studies on channel models for frequencies from 0.5 to 100 GHz and enhanced long-term evolution (LTE) support for aerial vehicles, respectively. It is known that A2G communication is significantly affected by the following factors: UAV altitude [20; 21; 22], movement speed [23; 24], LOS and non-line-of-sight (NLOS) conditions [25; 26; 27], and surrounding objects, such as buildings or trees and geographical features [28; 29]. Especially in LOS environments, the propagation loss of radio waves is relatively small, whereas, in NLOS environments, the shielding effect of buildings and obstacles is strong, causing significant degradation of communication quality [30; 31].