  This letter considers an unmanned aerial vehicle (UAV)-enabled relay
communication system for delivering latency-critical messages with ultra-high
reliability, where the relay is operating under amplifier-and-forward (AF)
mode. We aim to jointly optimize the UAV location and power to minimize
decoding error probability while guaranteeing the latency constraints. Both the
free-space channel model and three-dimensional (3-D) channel model are
considered. For the first model, we propose a low-complexity iterative
algorithm to solve the problem, while globally optimal solution is derived for
the case when the signal-to-noise ratio (SNR) is extremely high. For the second
model, we also propose a low-complexity iterative algorithm to solve the
problem. Simulation results confirm the performance advantages of our proposed
algorithms.
