# Low-Altitude Sensing Model: Analysis Leveraging ISAC in Real-World Environments
## 3 Multi-BS Networking with Cellular-like Topology  
Building on the single base station 3D sensing model in the previous section, this section focuses on multi-BS networking with a cellular-like topology. This approach is crucial for achieving seamless 3D sensing in low-altitude sensing networks. We extend the traditional cellular communication topology by elongating and translating in the vertical dimension. This allows us to achieve continuous sensing with a cellular-like network at different heights across horizontal cross-sections. The relationship between the station spacing \(d\) and the coverage radius \(R\) directly affects the sensing continuity and interference in overlapping areas. As analyzed in the previous section, the station spacing should satisfy \(d\leq R\) to ensure seamless coverage. When taking \(d=R\), for example, the networked sensing 3D structure is shown in Figure 5.  
Further, the sensing network conditions at different heights show significant differences, as illustrated in Figure 6. Different colors represent different cells. At the altitude limit, a standard hexagonal cellular topology was adopted to ensure continuous coverage. As the height gradually decreased, the hexagons elongated, and overlapping occurred in the cellular structure. Additionally, the lower the altitude, the more susceptible the sensing network was to the influence of obstacles. Although overlapping areas may lead to resource loss, de-duplication through multi-BS sensing may help reduce the probability of missed detections. Furthermore, from the perspective of multi-BS networked cellular joint sensing, sensing blind spots are most likely to occur at the altitude limit. As shown in Figure 7, at the maximum coverage height, the tower top blind spot of O\({}_{1}\) was covered by the main lobes of three neighboring sectors, O\({}_{3}\), O\({}_{5}\), and O\({}_{7}\). The same color represents the same base station. Taking the main lobe of the O\({}_{1}\)'s sector pointing to the lower-left direction, which covered the blue area between O\({}_{1}\) and O\({}_{2}\) as an example, the vertical FOV \(\alpha\) of O\({}_{1}\) affected the position of point X. Point X was the outer edge of the tower top blind spot at the maximum height. When the vertical FOV of O\({}_{1}\) was larger, X was closer to O\({}_{1}\); when the vertical FOV was smaller, X was closer to O\({}_{2}\). The position of X affected the coverage requirements of the three neighboring stations for the tower top blind spot of O\({}_{1}\). For example, for neighboring station O\({}_{3}\) covering the O\({}_{1}^{\prime}\)s tower top, the antenna's normal direction of the O\({}_{3}\)'s right-pointing sector was along O\({}_{3}\)O\({}_{1}\). When the FOV of O\({}_{1}\) was smaller, the angle between O\({}_{3}\)X and the O\({}_{3}\)'s antenna's normal direction increased. Since the beam gain in the O\({}_{3}\)X direction decreased more significantly as the deviation from the O\({}_{3}\) normal direction increased, multi-BS networking needed to consider the coverage capabilities of the main lobe beam. Specifically, it needed to determine whether the main lobe beam of O\({}_{3}\) could cover the area between O\({}_{1}\) and X to compensate for the tower top blind spot of O\({}_{1}\). Figure 5: Networked sensing 3D model. Figure 6: Low-altitude sensing network topology at different altitudes. (a) the altitude limit height (b) general height floor (c) the base station height. Specifically, under the station spacing \(d\), the angle \(\beta\) by which O\({}_{3}\)X deviates from the antenna's normal direction O\({}_{3}\)O\({}_{1}\), can be expressed as follows:  
\[\beta= \frac{\gamma}{2}-\arctan\frac{\mathrm{d}-\frac{2\cdot h_{1}}{\tan a}}{ \sqrt{3}d}. \tag{11}\]  
In summary, the design of neighboring stations' coverage over the tower top blind spot of the local station needs to consider the vertical FOV of the local station's antenna, the coverage height, the beam gain of the neighboring stations, and the station spacing.