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Reconfigurable intelligent surfaces (RISs) achieve high passive beamforminggains for signal enhancement or interference nulling by dynamically adjustingtheir reflection coefficients. Their employment is particularly appealing forimproving both the wireless security and the efficiency of radio frequency(RF)-based wireless power transfer. Motivated by this, we conceive andinvestigate a RIS-assisted secure simultaneous wireless information and powertransfer (SWIPT) system designed for information and power transfer from a basestation (BS) to an information user (IU) and to multiple energy users (EUs),respectively. Moreover, the EUs are also potential eavesdroppers that mayoverhear the communication between the BS and IU. We adopt two-timescaletransmission for reducing the signal processing complexity as well as channeltraining overhead, and aim for maximizing the average worst-case secrecy rateachieved by the IU. This is achieved by jointly optimizing the short-termtransmit beamforming vectors at the BS as well as the long-term phase shifts atthe RIS, under the energy harvesting constraints considered at the EUs and thepower constraint at the BS. The stochastic optimization problem formulated isnon-convex with intricately coupled variables, and is non-smooth due to theexistence of multiple EUs/eavesdroppers. No standard optimization approach isavailable for this challenging scenario. To tackle this challenge, we propose asmooth approximation aided stochastic successive convex approximation (SA-SSCA)algorithm. Furthermore, a low-complexity heuristic algorithm is proposed forreducing the computational complexity without unduly eroding the performance.Simulation results show the efficiency of the RIS in securing SWIPT systems.The significant performance gains achieved by our proposed algorithms over therelevant benchmark schemes are also demonstrated.

Secrecy Rate Maximization of RIS-assisted SWIPT Systems: A Two-Timescale Beamforming Design Approach