UAV-enabled reliable mobile relaying under the time-varying Rician fading channel

This paper adopts an unmanned aerial vehicle (UAV) relay to forward data from a source sensor to a remote base station (BS). To ensure reliable data transmission, our objective is to minimize the average outage probability along the three-dimensional (3D) UAV trajectory under the UAV mobility and source/relay power allocation constraints. The UAV mobility causes the time-varying Rician fading channel, thus the outage probability function considers both the distance-based large-scale fading and a time-varying Rician factor. To tackle our formulated non-convex problem, we approximate the outage probability by its first-order asymptotic form, based on which we propose a block coordinate descent (BCD) iteration algorithm by alternatively solving the 3D UAV trajectory and power allocation subproblems. The successive convex optimization (SCO) method is applied to deal with the 3D UAV trajectory subproblem. To reveal the advantage by designing the 3D UAV trajectory, we also consider joint optimization of the two-dimensional (2D) UAV trajectory and power allocation to minimize the average outage probability. Extensive simulations verify the accuracy of the approximate outage probability, and the results show that considering the time-varying Rician fading channel, the 3D UAV trajectory helps to obtain a lower link outage probability than the 2D UAV trajectory.