  Unmanned aerial vehicles (UAVs) as aerial relays are practically appealing
for assisting Internet of Things (IoT) network. In this work, we aim to utilize
the UAV swarm to assist the secure communication between the micro base station
(MBS) equipped with the planar array antenna (PAA) and the IoT terminal devices
by collaborative beamforming (CB), so as to counteract the effects of collusive
eavesdropping attacks in time-domain. Specifically, we formulate a UAV
swarm-enabled secure relay multi-objective optimization problem (US2RMOP) for
simultaneously maximizing the achievable sum rate of associated IoT terminal
devices, minimizing the achievable sum rate of the eavesdropper and minimizing
the energy consumption of UAV swarm, by jointly optimizing the excitation
current weights of both MBS and UAV swarm, the selection of the UAV receiver,
the position of UAVs and user association order of IoT terminal devices.
Furthermore, the formulated US2RMOP is proved to be a non-convex, NP-hard and
large-scale optimization problem. Therefore, we propose an improved
multi-objective grasshopper algorithm (IMOGOA) with some specific designs to
address the problem. Simulation results exhibit the effectiveness of the
proposed UAV swarm-enabled collaborative secure relay strategy and demonstrate
the superiority of IMOGOA.
