## 5 Numerical Results
### Effects of System Parameters  
We now proceed to evaluating the effect of system parameters on the UAV blockage probability including the BS height and altitude of UAV, the mean and variance of building height and, finally, the street width and building block's width and length. We start with an assessment of the effects of mmWave height and UAV flying altitude. To this aim, Figure 5 shows UAV LoS blockage probability as a function of these parameters for street width \(l=20\) m, mean building height and standard deviation \(E[H_{B}]=30\) m and \(\sigma[H_{B}]=10\) m, block width and length of \(b_{w}=b_{l}=100\) m. By analyzing the presented results, we see that higher BS heights result in lower UAV LoS blockage probability, see Figure 5a. Particularly, the gain of changing mmWave BS height from 5 m to just 15 m leads to the decrease of UAV LoS blockage probability by approximately 0.15 for 20 mmWave BS deployed in the area. The rationale for these improvements is that higher mmWave BS heights make the visible regions at the UAV flying altitude larger, see Figure 2. Furthermore, this effect is non-linear as the area increases faster when the mmWave BS height increases. We also note that these gains depend heavily on BS deployment density and are minimal highly dense deployments. Analyzing the effect of UAV flying altitude in Figure 5b, qualitatively similar conclusions can be made. More specifically, the higher the altitude the smaller the UAV LoS blockage probability. Specifically, for the density of 20 mmWave BS in the considered area, the gain of changing the altitude from 100 to 200 m is approximately 0.15 and is comparable to that of the change in BS height from 5 to 15 m. We also note that in practice this parameter should be tuned with care. The reason is that higher altitudes may lead to much lower  
Figure 4: Comparison of the developed model and computer simulations. received power, especially for ground-mounted mmWave BS that is usually downtilted to provide better coverage for terrestrial users, e.g., pedestrians. In dense city deployments of mmWave BS, the characteristics of building block height may produce a significant impact on UAV LoS blockage probability. We investigate this hypothesis in Figure 6, where we illustrate the UAV LoS blockage probability as a function of the number of deployed mmWave BS for UAV altitude \(h_{U}=150\) m, BS height \(h_{A}=5\) m, street width \(l=20\) m, block width and length of \(b_{w}=b_{l}=100\) m. Here, in Figure 5(a) we show the effect of different mean values by keeping the standard deviation constant at \(\sigma[H_{B}]=10\), while in Figure 5(b) we vary standard deviation and keep the mean constant at \(E[H_{B}]=30\) m.  
By analyzing the presented data, we may conclude that the mean building height logically produces a significant effect on the UAV LoS blockage probability. The magnitude of this effect is comparable to that of BS height or UAV altitude. Particularly, when considering districts with high building heights, e.g., city centers, one needs to utilize additional ways to improve UAV LoS blockage probability. However, at the same time, the effect of standard deviation is rather limited, leading to differences in the range of 0.05-0.1 for the considered range of the number of deployed mmWave BS. Finally, we consider the effect of street and building block widths on UAV LoS blockage probability illustrated in Figure 7 for UAV altitude \(h_{U}=150\) m, BS height \(h_{A}=5\) m, mean building height and standard deviation \(E[H_{B}]=30\) m and \(\sigma[H_{B}]=10\) m, respectively, block width and length of \(b_{w}=b_{l}=100\) m. As one may  
Figure 5: UAV blockage probability as a function of BS height and UAV altitude. (a) Various BS heights; (b) Various UAV altitudes. Figure 6: UAV blockage probability as a function of building block height parameters. (a) Various mean heights; (b) Various standard deviations. drastically affect the considered metric of interest. However, the effects are different.