UAV-Enabled Reliable Mobile Relaying Based on Downlink NOMA

Unmanned aerial vehicle (UAV) relaying is an efficient solution to provide wireless access for emergency communications due to the high flexibility. The system reliability is usually constrained under limited bandwidth and power resources. Downlink non-orthogonal multiple access (NOMA) can improve the system reliability through a higher resource utilization. To this end, we introduce downlink NOMA to a UAV-enabled mobile relaying system and investigate a scenario where a fixed-wing UAV flies in a circular trajectory to serve as a mobile decode-and-forward (DF) relay in an emergency situation. Since guaranteeing a reliable link is necessary in an emergency situation, our formulated problem is to minimize the maximum outage probability among all links, taking into account the bandwidth and power allocations based on downlink NOMA. Specially, the condition for successful successive interference cancellation (SIC) is also considered. By making change of variables and introducing slack variables, we reformulate our problem into a more tractable form, then we propose an iteration algorithm to solve our problem based on the successive convex optimization (SCO) technique. The optimized bandwidth and power allocation schemes as well as the min-max outage probability along the UAV trajectory are obtained, respectively. Two benchmarks are designed to reveal the performance of our proposed algorithm, and the reliability gain can be obtained by comparing the min-max outage probability and the overall average outage probability.