  In this paper, we propose a secure short-packet communication (SPC) system
involving an unmanned aerial vehicle (UAV)-aided relay in the presence of a
terrestrial passive eavesdropper. The considered system, which is applicable to
various next-generation Internet-of-Things (IoT) networks, exploits a UAV as a
mobile relay, facilitating the reliable and secure exchange of intermittent
short packets between a pair of remote IoT devices with strict latency. Our
objective is to improve the overall secrecy throughput performance of the
system by carefully designing key parameters such as the coding blocklengths
and the UAV trajectory. However, this inherently poses a challenging
optimization problem that is difficult to solve optimally. To address the
issue, we propose a low-complexity algorithm inspired by the block successive
convex approximation approach, where we divide the original problem into two
subproblems and solve them alternately until convergence. Numerical results
demonstrate that the proposed design achieves significant performance
improvements relative to other benchmarks, and offer valuable insights into
determining appropriate coding blocklengths and UAV trajectory.
