## 5 Discussion
The developed interpolation algorithm provides a fast and straightforward solution to convert the time gap between packet arrivals from a line path to an area mapping. The main disadvantage of this method is that it does not consider the presence of buildings, which leads to unrealistic building edge and interior representations. We addressed this problem by performing a manual correction step, seen as the difference between the white semi-transparent area in Figure 9a and the green/yellow regions in Figure 9b. Previous works reported similar interpolation methods applied to experimental RSSI measurements [67]. However, very few provide complete cityscape RSSI maps, resorting, instead, to a few point measurement comparisons against path loss models. The experimentally obtained maximum coverage range of 340 m is in good agreement with previous studies in urban environments [19]. Several strategies could increase the coverage range, such as using a higher spread factor, placing the LoRa gateway outdoors, and raising the gateway vertically to reduce overall signal reflections. However, the experimental conditions used in this work privilege such interactions and thus highlight the relevance of the O-FDTD simulations for low power, mid-range communications. Applications such as indoor, campus, or neighborhood-wide networks may benefit from smaller coverage areas, e.g., to reduce the chance of malicious communication interceptions. As mentioned in Section 4.1, a reliable coverage range prediction (especially if based on simple path-loss equations) for a real-world application should consider the coverage threshold about 10 dB above the receiver decoding sensitivity, lest the actual range should fall short of the expected. We chose not to do this since the highly detailed O-FDTD maps correlated directly with the decoder sensitivity provide a sharper approximation between the O-FDTD simulated RSSI maps and the experimentally-determined signal-loss regions. The overlay between the experimental and simulated coverage areas reveals an area overlaps of 84%. However, most coverage area-prediction models yield good predictions near the gateway, where the coverage can be assumed homogeneous. A prime feature of the O-FDTD approach is the realistic coverage edge prediction, which is a massive advantage compared to simpler path-loss models that only predict average circular coverage ranges. As demonstrated in Figure 10 by overlaying coverage deviations at the edge, even the best-fitting circular model does not take buildings into account, leading to a much larger net RMSD of 41 m, compared to a RMSD of 24 m achieved by the O-FDTD simulations. While we considered this an excellent agreement, several sources of error in the experimental and simulation approaches are discussed. Our simulations did not consider temperature effects, while high temperatures are known to reduce the coverage range [61; 68]. Indeed, the measurements were performed on a sunny day at 32 \({}^{\circ}\)C (89.6 Fahrenheit), leading to several-degree temperature-variations between sun and shade areas. The lack of terrain information may also play a role in the observed mismatches, as the model does not capture out-of-plane signal propagation. This effect may be the case of the regions highlighted in red on the left side of Figure 9b, where the terrain is lower than at the center of the cityscape. Furthermore, the incomplete experimental characterization of the citywide time gap between packet arrivals causes the experimental signal distribution to be more reliant on the simple interpolation algorithm. As discussed above, this introduces errors at the building interfaces, which are particularly noticeable on the top part of the map in Figure 9a, associated with the green areas in Figure 9b. Also, more refined (and more complex) projection models could be used to convert the GPS coordinates from the spherical to the Cartesian plane, thus improving the overlap between the experimental trajectory and the downloaded city maps, which is not perfect.