## 5 Conclusions  
In this paper, we present the results of a demonstration focused on data transfer between UAVs using 60 GHz band wireless communication, with the aim of realizing non-terrestrial networks for Beyond 5G applications. Although mmWave or terahertz frequencies enable ultra high speed communication, their characteristics exhibit a spot-like  
Figure 11: Throughput, amount of transferred data, and RSSI of the experimental inter-UAV 60 GHz data transfer in platooning flight for 30 s.  
nature, constraining the spatial and temporal opportunities for communication due to their distinctive directivity and significant propagation losses. This paper delves into the challenges entailed in effectively utilizing the short-duration, high-frequency communication windows unique to these bands. We analytically quantify the ultra-narrow properties arising from escalating carrier frequencies. In contrast to conventional microwave bands, the spot size is shrunk to a few meters for the 60 GHz band, and to several centimeters at 300 GHz in the terahertz range. These features differ from traditional wireless systems, introducing novel barriers when employing mmWave or terahertz bands in UAVs. Firstly, we evaluated the data transfer characteristics between two hovering UAVs at varying distances. Shorter distances yielded smaller spot sizes, leading to the throughput being highly sensitive to positional fluctuations and angular misalignment. This causes difficulties in keeping a peak maximal throughput. Spot sizes enlarged as distances increased due to beam spreading, diminishing the impact of throughput fluctuation. When we tried to achieve high-speed communication between close UAVs, the impact of misalignment became significant. We also investigated the characteristics of data transfer using instantaneous proximity opportunities, assuming scenarios where UAVs were passing each other, or pass-by flight. The data transmission of 126 MB was performed over a 516 ms duration during their passing with a relative speed of roughly 5 m/s. By using wireless communication devices compliant with IEEE 802.15.3e, we were able to use these sub-second limited zone efficiently, resulting in 99% of the total time to data transmission. Other communication standards, which require several seconds for link establishment or association, have difficulties in utilizing such proximity opportunities. The pass-by flight, even when UAVs move in different destinations, exemplifies the potential of leveraging ultra-narrow communication zone to facilitate substantial data exchange between UAVs. Furthermore, for scenarios demanding extensive data exchange, we showed that cooperative route navigation and communication zone tracking can extend the communication timeframe. In one example, by maintaining UAV formation for 30 s, we achieved a consistent transmission of over 15 GB of data. The strategic utilization of these two types of flight methods considering mission requisites emerges as of significance. These results anticipate that UAVs with different destinations will be able to plan appropriate flight routes to establish over-the-air networks for data exchange and sharing. Even aircraft that do not have hovering capabilities can utilize ultra high frequency communication during passing each other, and this technology is expected to be deployed in high-capacity data relay networks where aircraft collaborate. We expect the findings of our study to be a promising foundation for aerial wireless networks, wherein UAVs equipped with ultra high frequency devices, can engage in cooperative flight to leverage proximity opportunities. Such technology finds potential applications in various scenarios, such as the rapid collection of high-volume monitoring data in domains such as agriculture, transportation, and infrastructure without the need for landing or hovering, helping to establish high-capacity data distribution networks through drone-based logistics. Conceptualization, R.I. and Y.S. ; methodology, R.I., K.K. and Y.S. ; software, R.I., K.K. and S.Y.