Energy Efficiency Maximization for Secure RIS-Assisted Integrated Sensing and Wireless Power Transfer in IoT Networks

Abstract

This paper investigates an intelligent reflecting surface (RIS)-assisted integrated sensing and communication (ISAC) system for Internet of Things (IoT) deployments, where a large number of low-cost and energy-constrained devices require reliable connectivity, environment awareness, and sustainable powering. We consider multiple energy-harvesting receivers (EHRs) that can be interpreted as batteryless IoT nodes and may potentially eavesdrop, while the system simultaneously serves multiple communication users, wirelessly powers multiple EHRs, and detects multiple targets. To enhance physical-layer security in such dense IoT settings, a friendly jammer transmits artificial noise (AN) to confuse the EHRs. Furthermore, a signature sequence (SS) sensing scheme is employed at the base station (BS) to distinguish different targets. On this basis, we formulate a joint optimization problem to maximize the system’s energy efficiency (EE) by designing the BS’s active beamforming, the jammer’s AN precoding, and the RIS phase shifts, while satisfying constraints on the communication quality-of-service (QoS), desired detection accuracy, wireless power transfer requirements, and total transmit power. The resulting problem is highly non-convex and challenging to solve directly. We therefore develop an efficient alternating optimization (AO) algorithm that decomposes the original design into two subproblems. Specifically, for the active beamforming and AN precoding subproblem, semidefinite relaxation (SDR) combined with successive convex approximation is employed to tackle the non-convex constraints. For the RIS phase-shift subproblem, we adopt an SDR-based lifting to cope with the unit-modulus constraint and solve the resulting non-convex program via successive convex approximation (SCA) and the convex–concave procedure (CCCP) within the AO framework. Finally, simulation results corroborate the effectiveness of the proposed scheme and demonstrate notable EE gains, highlighting the potential of jointly integrating RIS-assisted SWIPT and AN-based security into ISAC-enabled IoT networks.