  This article proposes a cooperative friendly jamming framework for swarm
unmanned aerial vehicle (UAV)-assisted amplify-and-forward (AF) relaying
networks with wireless energy harvesting. Due to the limited energy of the
UAVs, we develop a collaborative time-switching relaying protocol which allows
the UAVs to collaborate to harvest wireless energy, relay information, and jam
the eavesdropper. To evaluate the secrecy rate, we derive the secrecy outage
probability (SOP) for two popular detection techniques at the eavesdropper,
i.e., selection combining and maximum-ratio combining. Monte Carlo simulations
are then used to validate the theoretical SOP derivation and to show the
effectiveness of the proposed framework in terms of SOP as compared with the
conventional amplify-and-forward relaying system. Using the derived SOP, one
can obtain engineering insights to optimize the energy harvesting time and the
number of UAVs in the swarm to achieve a given secrecy protection level. The
analytical SOP derived in this work can also be helpful in future UAV
secure-communications optimizations (e.g., trajectory, locations of UAVs). As
an example, we present a case study to find the optimal corridor to locate the
swarm so as to minimize the system SOP.
