  Emergency communication is extremely important to aid rescue and search
operation in the aftermath of any disaster. In such scenario, Unmanned Aerial
Vehicle (UAV) networks may be used to complement the damaged cellular networks
over large areas. However, in such UAV networks, routing is a challenge, owing
to high UAV mobility, intermittent link quality between UAVs, dynamic three
dimensional (3D) UAV topology and resource constraints. Though several UAV
routing approaches have been proposed, none of them so far have addressed inter
UAV coverage, collision and routing in an integrated manner. In this paper, we
consider a scenario where network of UAVs, operating at different heights from
ground, with inter UAV coverage and collision constraints, are sent on a
mission to collect disaster surveillance data and route it to Terrestrial Base
Station via multi-hop UAV path. Analytical expressions for coverage probability
(Pcov) and collision probability (Pcoll) are derived and minimum (Rmin) and
maximum (Rmax) distance between UAVs are empirically calculated. We then
propose a novel Multi-hop Opportunistic 3D Routing (MO3DR) algorithm with inter
UAV coverage and collision constraints such that at every hop expected progress
of data packet is maximized. The numerical results obtained from closed form
mathematical modelling are validated through extensive simulation and their
trade-off with variation in network parameters such as path loss component,
trajectory divergence etc. are demonstrated. Finally, we obtain empirical
optimality condition for inter UAV distance for the given application
requirement.
