# 3D Global Path Planning Optimization for Cellular-Connected UAVs under Link Reliability Constraint
## 4 Results and Discussions
### Reliable Aerial Cellular Coverage  
The dataset contains 8457 samples of the data of four BSs on the UKM campus, in which RSRP and RSRQ values were measured along different flight paths and heights. In the following, to demonstrate the status of a signal based on its RSRP and RSRQ level, we refer to the classifications presented in Table 6.  
Figure 5 depicts the box plots of distance, angle, RSRP, and RSRQ for overall data as well as four considered BSs (the overall data are a combination of data from all the considered BSs). The results show that the samples are primarily located in a radius of 10 to 593 m and an angle of 0.5 to 56 degrees. The distribution of the measured data is predominantly influenced by parameters, such as network density, network design, the radiation pattern of antennas, and handover. Results in Figure 5c,d show that the received signals are in a wide range of strength and quality, from \"excellent\" to \"weak\". A closer look into the results of different cells reveals that, within the considered radius and elevation angle, the RSRP values between the first and third quartile are almost in \"good\" status. Meanwhile, RSRQ shows more variations in the considered scenarios and varies from the \"good\" to the \"medium\" range. For a more detailed analysis, Figure 6 shows box plots of RSRP and RSRQ for Route I at different flight heights. The results show that increasing flight height decreases the median values of RSRP and RSRQ. In the conducted measurement, the drone was flying mostly in the LOS of BSs. Hence, the main factors of RSRP degradation are path loss and antenna gain reduction. The main reason for the antenna gain reduction is that the BS antennas are down-tilted; thus, based on the antenna radiation pattern, by increasing the flight height, the chance of serving the drone with the main lobe decreases. This consequently degrades the received signal energy at the drone side and enhances the uncertainty in the communication link. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Signal Strength/Quality** & **RSRP** & **RSRQ** \\ \hline Excellent & \(-\)60—70 dBm & \(>\)\(-\)6 dB \\ Good & \(-\)70—80 dBm & \(-\)6—10 dB \\ Medium & \(-\)80—90 dBm & \(-\)10—15 dB \\ Weak & \(-\)90—100 dBm & \(\lesssim\)\(-\)15 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Signal status based on RSRP and RSRQ values [56]. On the other hand, RSRQ indicates the received signal quality in a network as well as the effect of interference from adjacent BSs. As the results in Figure 6b show, in higher altitudes, the drone considerably experiences lower link quality. The main reason is that, besides the desired signal power level degradation, in higher altitudes, drones can see a larger number of BSs. Therefore, the probability of receiving signals from adjacent/interfering BSs increases, and consequently, the drone receives higher interference energy at higher altitudes. Another parameter that impacts the reliability of a cellular communication link is handover. Handover is considered a required procedure in cellular communications to maintain the connectivity and quality of a link between users and the network. However, the multiplicity of handovers in a short time interval shows the uncertainty of the communication link in an area. Table 7 demonstrates the impact of flight height on the number of handovers. Note that there was no continuous coverage for Route I at a height  
Figure 5: Box plot of (**a**) distance, (**b**) elevation angle, (**c**) RSRP, and (**d**) RSRQ of the overall dataset and considered BSs. Figure 6: Box plot of (**a**) RSRP and (**b**) RSRQ for Route I at different heights, 65, 85, 105, and 125 m.  
of 125 m, that is, the BSs could not provide aerial coverage for the entire path at that height. Therefore, the number of handovers was reduced. As can be seen, the number of handovers in the three routes increased significantly as the flight height increased.