Energy efficiency optimization for a RIS-assisted MISO SWIPT system with discrete phase shifts

This paper investigates an energy efficiency (EE) optimization method for a discrete phase shift reconfigurable intelligent surface (RIS)-assisted multi-user multiple-input single-output (MISO) simultaneous wireless information and power transfer (SWIPT) system. The system employs a RIS controlled by PIN diode circuits to realize discrete phase shifts, with the goal of maximizing the system’s energy efficiency. This work fully considers the power consumption differences caused by the ON/OFF states of different RIS reflecting elements, as well as multiple system constraints, including the base station’s maximum transmit power limit, each receiver’s minimum data rate requirement, and minimum energy harvesting (EH) requirement. The power allocation at the receivers, the ON/OFF states of the RIS elements, and the power splitting (PS) ratios are jointly optimized. To efficiently solve this non-convex joint optimization problem, it is decomposed into three subproblems and addressed using an alternating optimization approach. The simulation results confirm the effectiveness of the proposed alternating optimization algorithm. Under the same number of RIS reflecting elements, the proposed scheme achieves about a 6% EE improvement over the baseline, which strongly demonstrates the significant role and great potential of RIS in enhancing the EE of SWIPT systems.