  This letter considers the unmanned aerial vehicle (UAV)-enabled relay system
to deliver command information under ultra-reliable and low-latency
communication (URLLC) requirements. We aim to jointly optimize the blocklength
allocation and the UAV's location to minimize the decoding error probability
subject to the latency requirement. The achievable data rate under finite
blocklength regime is adopted. A novel perturbation-based iterative algorithm
is proposed to solve this problem. Simulation results show that the proposed
algorithm can achieve the same performance as the exhaustive search method, and
significantly outperforms the existing algorithms.
