Unified Geometric--Kinematic Laws of Mobility in 3D Cellular Networks

Mobility in three-dimensional (3D) cellular networks induces frequent handovers and random sojourn times for aerial users. Analyzing these events is challenging because they jointly depend on user trajectories and random service-boundary geometry. This paper develops a unified geometric–kinematic framework in which service regions form a stationary random tessellation, their boundaries constitute a random closed set, and user motion follows a cruise–turn mobility model. Under the stated stationarity, isotropy, and independence conditions, the handover rate satisfies the exact law $H=(S_V\mathrm{/}2)\bar{v}$, where $S_V$ is the specific surface area and $\bar{v}$ is the long-run mean speed. The forward boundary-distance distribution provides the local geometric input to an effective-traversal approximation for the mean remaining sojourn time. These results link global and local geometric descriptors to complementary mobility metrics. Combining the handover law with link coverage yields a handover-aware effective spectral efficiency for base station (BS) density selection and service-architecture comparison. We then specialize the framework to Poisson–Voronoi single-BS service and Poisson–Delaunay four-BS coordinated multi-point service; under both models, the handover rate scales as ${\lambda }^{1\mathrm{/}3}\bar{v}$. Numerical results corroborate the exact handover expressions, assess the sojourn-time approximations, and show how user speed and interruption duration affect the preferred density and architecture.