Physical layer security in double RIS-aided WPCN systems based on non-cooperative game

Physical layer security issue is investigated in Wireless Power Transfer Communication Network (WPCN) system, and the double Reconfigurable Intelligent Surface (RIS) are introduced to assist the system to improve communication security and reduce transmission power. The system operates in two phases: wireless energy transfer (WET) and wireless information transfer (WIT). First, the WET phase is conducted to address the issue of insufficient transmitter (TS) energy in the WIT phase, followed by utilizing RIS assistance during the WIT phase to achieve secure transmission at the physical layer. In the WIT phase, the presence of eavesdroppers and interference among multiple Internet of Things (IoT) devices, along with obstructed direct links from TS to IoT devices, reduce the system's security rate. Especially, we introduce double RIS deployed above TS and IoT devices, respectively. Compared to a single RIS, double RISs not only improves the communication system's coverage and data transmission rate but also enhances system security and efficiency. In the WET phase, since TS and power station (PS) belong to different service providers, we propose an energy trading and secure communication scheme assisted by double RIS based on Stackelberg game theory. The leader's strategy problem in this scheme is non-convex, hence decomposed into two subproblems. The first subproblem is independently solved using alternating optimization (AO) algorithm. The second subproblem is tackled with a block coordinate descent (BCD) based iterative algorithm, employing successive convex approximation (SCA) to solve each block-level subproblem. Finally, the numerical results demonstrate that the proposed scheme enhances the system's security rate, reduces the time required for wireless energy transfer (WET), and thereby improves overall system efficiency.