Towards Green Communication: Power-Efficient Beamforming for STAR-RIS-Aided SWIPT

As a revolutionary paradigm for green communication architecture for the next-generation communication system, reconfigurable intelligent surfaces (RISs) have been considered a holistic solution for simultaneous wireless information and power transfer (SWIPT). Recently, a novel concept called simultaneous transmitting and reflecting (STAR-RIS), has been recently introduced which facilitates both transmission and reflection through the meta-material surface and thus leads to full-space coverage and even better control than conventional RIS. This paper investigates the resource allocation problem in a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided simultaneous wireless information and power transfer (SWIPT) system. Specifically, we focus on the problem of power minimization for multi-user (MU) multi-input multi-output (MIMO) systems via a joint beamforming design at the BS and the STAR-RIS while guaranteeing the minimum rate and energy harvesting requirement for information receivers (R-IRs and T-IRs) and energy receivers, respectively. Owing to the non-convex and NP-hard nature of the formulated problem, we first utilize a minimum mean square error (MMSE) technique to transform the problem into its simplified form, and later utilize an alternating optimization framework which solves the problems of beamforming design at the BS and the STAR-RIS independently in an iterative manner using successive convex approximations. Simulation results confirm that the STAR-RIS can significantly reduce the required transmission power approximately by 15-20% when compared to passive RIS while satisfying given QoS constraints for SWIPT systems.