## 4 Experimental Results
### Pass-By Flight  
Figure 10 presents the 60 GHz data transfer results during the mutual encounter of UAVs. The RSSI distribution exhibits a mountain-shaped pattern, with weaker RSSI values observed at the edges of the communication zone, reflecting a similar pattern in throughput.  
Data transmission between UAVs is confined within a delimited communication window. In the case shown in Figure 10a, where the drones maintained a relative velocity of 5 m/s, the duration of data transmission throughout the passing event was 516 ms, enabling the transfer of a cumulative 126 MB of data. Using a wireless device compliant with IEEE 802.15.3e in our experiments, which establishes the link in less than 1 millisecond, enabled over 99% use of the communication zone for data transfer. If alternative standards, which take a longer time for association processes, had been employed, it would not have completed the association phase within the available timeframe and no actual data would have been transferred. The effective transmission rate remained consistent at 1.86 Gbps for both uplink and downlink across the entire segment, with a peak transmission rate reaching approximately 3 Gbps. Furthermore, the investigation reveals that lower UAV passing speeds lead to prolonged link maintenance intervals and augmented data transmission volumes, as shown in Figure 10b. This indicates that data volume can be adaptively regulated by adjusting flight speeds without the need for flight interruptions or modifications in destination.  
In addition to the pass-by flight, which involves both UAVs in motion, data transmission can also be accomplished by employing one stationary UAV as a base station while another UAV passes in proximity. This configuration is demonstrated in Figure 10c,d, and its trends are analogous to the pass-by scenario involving both UAVs in motion. When the relative speeds of the UAVs match, the transmission characteristics exhibit nearly equivalent behaviors in both cases, as observed in the Figure 10c,d, respec  
Figure 10: Throughput, amount of transferred data and RSSI of the experimental inter-UAV 60 GHz data transfer in pass-by flight on the situation that both of two UAVs were in flight at (**a**) the relative velocity \(v_{r}=5.0\) m/s (2.5 m/s and 2.5 m/s), (**b**) \(v_{r}=2.0\) m/s (1.0 m/s and 1.0 m/s), and that one was in hovering and the other was in flight at (**c**) \(v_{r}=5.0\) m/s, and (**d**) \(v_{r}=2.0\) m/s.  
Figure 10a,b. These observations indicate that 60 GHz band data transfer for pass-by flight can be effectively characterized based on the relative speed between the two UAVs.  
This approach may also offer a suppression of throughput instability arising from disturbances, as we observed during hovering. While three-dimensional beam tracking is generally required to mitigate this effect, the zone utilization during the pass-by scenario is analogous to beam scanning by getting across the communication zones, hence only two dimensions are required for beam tracking; there is no need for scanning against the axis of travel. In the pass-by flights, the data transfer capacity will be more limited compared to communication during hovering due to the constrained communication time. Nevertheless, when the anticipated data transfer volume by pass-by communication is projected to surpass the desirable content size to send, this method holds the potential to realize high-volume data relay with high-speed transfer between UAVs, while retaining the maneuverability of the drones, as described later.