Secure two-way transmission via untrusted UAV Relay: Joint path design and slot-pairing strategy

This paper proposes a novel two-way secure communication system that allows numerous pairs of ground users (GUs) communicate with one another using an untrusted UAV relay (UUR), where the UUR can eavesdrop on information while helping GUs perform two-way information transmission. We propose a safe scheme using slot pairing and joint path design to ensure the security of communication. Specifically, according to the physical-layer network coding (PNC) protocol, the collected information from GUs is cached in the UAV's shock absorber before being transmitted as the UAV approaches optimal channel conditions. By matching the channel status of two-way transmission with the slot-pairing approach, our goal is to optimize the system's secrecy sum rate (SSR). Due to the hybrid integer non-convex optimization problem, the formulated problem is difficult to address. As a result, we decompose it into three subproblems and propose an effective iterative algorithm that uses the block coordinate descent (BCD) approach and the sequential convex approximation (SCA) technique. The convergence and efficacy of our proposed scheme are demonstrated by numerical results.