Robust beamforming design for STAR-RIS assisted SWIPT systems

A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted simultaneous wireless information and power transfer (SWIPT) framework is proposed. More particularly, an STAR-RIS is deployed to assist in SWIPT from a multi-antenna access point (AP) to multiple single-antenna information users (IUs) and energy users (EUs). Due to the near-passive operation of the STAR-RIS, a more practical setup under the assumption of imperfect channel state information is investigated. The max-min fairness optimization problem is formulated to maximize the minimum power harvested by EUs, subject to the signal-to-interference-plus-noise ratio (SINR) constraints for IUs. To tackle this non-convex problem, an alternating optimization (AO) based algorithm is proposed for robust beamforming design. We first approximate the semiinfinite inequality constraints with S-procedure, then the AP active beamforming and the STAR-RIS passive beamforming are alternatively designed, where a penalty based approach is leveraged for STAR-RIS reconfiguration. Numerical results demonstrate that: i) the significant performance gains can be achieved by the proposed scheme over the baseline schemes; and ii) more STAR-RIS elements and higher SINR requirements weaken the robustness of the EU performance in terms of energy harvesting.