Interference Coordination for Multi-UAV-Enabled Communications Under Probabilistic LoS Channels

This study investigates an interference coordination for multiple unmanned aerial vehicle (UAV)-enabled communications under a probabilistic Line-of-Sight (LoS) channel model. Given that the LoS probability of a wireless channel is determined by the elevation angle between the UAV and ground node (GN), we jointly optimize the 3-D trajectory, user scheduling, and transmit power of the UAVs to maximize the minimum average spectral efficiency (SE) among GNs. We first present an effective lower bound for the channel-state-dependent SE to handle its complex form that inevitably arises from the probabilistic LoS channel model. To solve the nonconvex optimization problem under consideration, we also divide the original problem into four subproblems and apply a successive convex approximation to make each subproblem convex for a relevant optimization variable. We then propose an iterative algorithm based on a block coordinate descent method to efficiently find the optimal solution for each convex subproblem. The results of extensive simulation show that to mitigate interference with other UAV networks, each UAV optimizes its horizontal and vertical trajectories, along with its radio resources, to establish LoS for desired channels and Non-LoS for interference channels. The proposed scheme is also verified to be superior to conventional schemes in terms of average SE by effectively adjusting co-channel interference between different UAV networks.