  In this paper, a novel concept of three-dimensional (3D) cellular networks,
that integrate drone base stations (drone-BS) and cellular-connected drone
users (drone-UEs), is introduced. For this new 3D cellular architecture, a
novel framework for network planning for drone-BSs as well as latency-minimal
cell association for drone-UEs is proposed. For network planning, a tractable
method for drone-BSs' deployment based on the notion of truncated octahedron
shapes is proposed that ensures full coverage for a given space with minimum
number of drone-BSs. In addition, to characterize frequency planning in such 3D
wireless networks, an analytical expression for the feasible integer frequency
reuse factors is derived. Subsequently, an optimal 3D cell association scheme
is developed for which the drone-UEs' latency, considering transmission,
computation, and backhaul delays, is minimized. To this end, first, the spatial
distribution of the drone-UEs is estimated using a kernel density estimation
method, and the parameters of the estimator are obtained using a
cross-validation method. Then, according to the spatial distribution of
drone-UEs and the locations of drone-BSs, the latency-minimal 3D cell
association for drone-UEs is derived by exploiting tools from optimal transport
theory. Simulation results show that the proposed approach reduces the latency
of drone-UEs compared to the classical cell association approach that uses a
signal-to-interference-plus-noise ratio (SINR) criterion. In particular, the
proposed approach yields a reduction of up to 46% in the average latency
compared to the SINR-based association. The results also show that the proposed
latency-optimal cell association improves the spectral efficiency of a 3D
wireless cellular network of drones.
