# Study on Drone Handover Methods Suitable for Multipath Interference Due to Obstacles
## 1 Introduction  
The fifth-generation (SG) [1] network was developed as a wireless network technology, and services using 5G provide attractive functions, such as communication speed, communication capacity, and user capacity. Additionally, design methods for the optimal installation of base stations and evaluation and verification methods for the multipath fading effects of interference and reflections caused by buildings and shields between base stations and users, propagation attenuation, and improvement methods have been established. Beyond 5G and 6G, next-generation wireless network technologies have attracted attention. Beyond 5G and 6G include the requirement of extending the communication area to the sea, sky, and space, which is different from a terrestrial network [2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. Communication networks that satisfy these requirements are called non-terrestrial networks (NTN) [12; 13; 14; 15; 16; 17; 18]. As shown in Figure 1, satellites and relay stations, such as high-altitudeplatform stations (HAPS), are placed in the sky, and drones and robots near the ground are expected to have various applications, such as information collection. In addition, unlike conventional network designs that approximate a flat surface on the ground, three-dimensional network construction that includes three-dimensional movement in the air offers high potential for creating new services due to the flexibility of the design. In the future, a shift toward NTNs linked to conventional ground networks is expected. Relay communication using drones has been proposed in conventional relayed networks when it is difficult to set up a ground station to expand the communication area or eliminate blind zones [19; 20]; for example, research on relay transmission using drones and 5G technology already exists, and demonstrative experiments have been conducted [21]. For example, demonstration experiments have been reported for transmitting 4K high-resolution video shots from the sky using 5G, as shown in Figure 2. Moreover, transmission technology for coverage holes where wireless signals cannot reach directly are already in the process of being put to practical use. However, although simple relaying by drones can be achieved, weight limitations make it impossible to carry heavy payloads, such as devices that transmit high-power radio waves or directional antennas, which makes it difficult to achieve a wide communication area. Moreover, because drones are mobile, no method has been proposed to exploit their ability to connect with various ground-based stations while moving. Furthermore, unlike terrestrial communications, it is necessary to consider three-dimensional propagation paths with height differences for drones to communicate with the ground and mobile stations. For example, multipath interference, diffraction, and shielding caused by reflections from structures vary greatly with the height of the structure and flying drone. Therefore, this study focused on airborne networks that use drones as radio sta  
Figure 1: NTN image model configuration. Figure 2: Relay transmission using drones. shown in Figure 3, the network is expected to be used for capturing pictures, collecting oceanographic data, relaying between terminals and base stations with no line of sight, and providing a communication environment for users in blind areas. To realize such a network, it is desirable to assume that the drone moves while flying, and to switch connections so that it can continuously communicate while searching for the most appropriate base station on the ground during its movement. The method of switching the destination is called handover, and we propose a handover method suitable for drones. For the handover evaluation, we developed a cross-layer simulator that combines multiple analysis methods of the communication layer [22] to obtain the communication speed or throughput at the service level by combining the propagation characteristics in a three-dimensional (3D) space that considers structures and network protocols.