Reliable and Secure Wireless-Powered Communications via Hybrid Active-Passive Double-RIS

This paper investigates the reliability and security of a hybrid double-reconfigurable intelligent surface (HDRIS) aided wireless-powered communication (WPC) system in the presence of eavesdroppers, where one active/passive RIS (RIS-1) is deployed between the power station and the information user, and the other passive/active RIS (RIS-2) is deployed between the information user and the access point. We propose two modes of HDRIS aided WPC, i.e., HDRIS-I with passive RIS-1 and active RIS-2, and HDRIS-II with active RIS-1 and passive RIS-2. Based on the two modes, we analyze the outage probability (OP) from the perspective of reliability and intercept probability (IP) from the perspective of security, and derive their accurate and asymptotic expressions, respectively. Moreover, a joint metric is proposed, i.e., reliability and security probability (RSP), to reveal the superiority of HDRIS compared to pure double-RIS (PDRIS). The results show that compared to PDRIS, the proposed HDRIS-I and HDRIS-II have better OPs than PDRIS-I with two passive RISs, but worse OPs than PDRIS-II with two active RISs. Both HDRIS-I and HDRIS-II have worse IPs than PDRIS-I, but better IPs than PDRIS-II. Interestingly, in HDRIS-I and HDRIS-II, the diversity gain for legitimate users is proportional to the number of elements of the RISs, while the diversity gain for eavesdroppers is only 1, indicating that HDRIS provide greater benefits for legitimate communications. In Particular, HDRIS-I achieves the best RSP under high transmission power, while HDRIS-II achieves the best RSP under low transmission power, demonstrating the superiority of HDRIS.