# Multi-IRS-Assisted mmWave UAV-BS Network for Coverage Extension
## 5 Simulation Results and Discussion
From Figure 8, we can see that the SNR tended to be very high in the western part of the area near the UAV, that is, the part surrounded by (300, 300), (600, 400), (400, 1000), (200, 1000), and low in almost all other areas. The reason for this is a large park and a few tall buildings in the western part of the area, thus making it easier to create the LoS from the UAVs. On the other hand, the central area, for x axis values at approximately 600 to 800 and for y axis values at 500 to 1000, was lined with high-rise buildings, and the southeast and southwest areas surrounded by three points (600, 100), (1200, 600), (1200, 100) or (100, 100), (400, 100) (100, 800) were densely populated with small buildings, which may have prevented a clear LoS from the UAV and caused significant radio wave attenuation due to obstacles. Figure 9 shows the received SNR of each UE when using the IRS, with the same UE distribution as in Figure 6. Compared to Figure 8, the area with a higher SNR is spread over the southern and eastern parts of the park. The UEs located near the area's northeastern edge (1200, 900) also provided a high SNR. This is thought to be because multiple IRSs around (300, 200), (400, 400), (500, 600), (100, 800) and so on were used to reflect the radio  
Figure 6: Position of UEs, buildings, IRSs, and UAV. waves according to Figure 6 and establish a virtual LoS in the area that was shielded in Figure 8.  
Figure 8: UE received SNR when not using IRS. Figure 7: UE distribution and pairing with IRS on July 2024 weekday. (**a**) 6:00; (**b**) 12:00; (**c**) 18:00; (**d**) 0:00. Figure 10 shows the received SNR of each UE in the situation shown in Figure 9, when it further utilized its neighboring BSs. Even if an UE could not establish an LoS with any IRS, it can establish a communication path by establishing an LoS with its neighboring BSs. As a result, it can be confirmed that the SNR was slightly improved around \((x,y)=(200,600)\) in the southwest part of the area compared to Figure 9.  
Figure 11 shows the cumulative density function (CDF) of the change in UE received SNR as visually confirmed in Figures 8-10. The UE distribution was varied 10 times, and the results were tabulated. The horizontal axis represents the magnitude of the UE received SNR, and the vertical axis represents the CDF. The blue line indicates the case where the UAV position was randomly selected, the orange line indicates the case where the UAV position was optimized but an IRS was not used, the yellow line indicates the case where the UAV position was optimized and an IRS was used, and the purple line indicates the case where the UAV position was optimized and both an IRS and nearby BS were used. From Figure 11, the graphs changed significantly around SNR = 15 dB, which is considered to be the borderline between the NLoS and LoS in this simulation. Compared to the case where the access UAVs were placed randomly (blue line), the percentage of UEs that were NLoS decreased from 73.41 % to 70.40 % when deployed at the optimal location (orange line). In other words, 3 % of the UEs were changed to the LoS, and the communication quality was improved. In addition, 16 % of the UEs improved their communication quality by reflecting the radio waves through the IRS. Finally, for UEs that could not create a virtual LoS even with the IRS, we confirmed that providing radio waves from adjacent BSs improved the communication quality for 0.3 % of the UEs. Although this value may seem small at first glance, it is expected to have a large impact depending on the simulation environment, since the adjacent BS was set to a specific location. Figure 12 shows how many IRSs were used and how many UEs were improved for coverage extension at different times of 6:00, 12:00, 18:00, and 0:00. The bar chart shows how many IRSs were used at each time, thereby referring to the value on the left vertical axis. The line graph shows the percentage of UEs whose SNR was improved by the IRS relative to the total UEs in the area, and this refers to the value on the right vertical axis.