  Short-packet communication (SPC) and unmanned aerial vehicles (UAVs) are
anticipated to play crucial roles in the development of 5G-and-beyond wireless
networks and the Internet of Things (IoT). In this paper, we propose a secure
SPC system, where a UAV serves as a mobile decode-and-forward (DF) relay,
periodically receiving and relaying small data packets from a remote IoT device
to its receiver in two hops with strict latency requirements, in the presence
of an eavesdropper. This system requires careful optimization of important
design parameters, such as the coding blocklengths of both hops, transmit
powers, and the UAV's trajectory. While the overall optimization problem is
nonconvex, we tackle it by applying a block successive convex approximation
(BSCA) approach to divide the original problem into three subproblems and solve
them separately. Then, an overall iterative algorithm is proposed to obtain the
final design with guaranteed convergence. Our proposed low-complexity algorithm
incorporates robust trajectory design and resource management to optimize the
effective average secrecy throughput of the communication system over the
course of the UAV-relay's mission. Simulation results demonstrate significant
performance improvements compared to various benchmark schemes and provide
useful design insights on the coding blocklengths and transmit powers along the
trajectory of the UAV.
