  Reconfigurable intelligent surface (RIS)-based communication networks promise
to improve channel capacity and energy efficiency. However, the promised
capacity gains could be negligible for passive RISs because of the double
pathloss effect. Active RISs can overcome this issue because they have
reflector elements with a low-cost amplifier. This letter studies the active
RIS-aided simultaneous wireless information and power transfer (SWIPT) in a
multiuser system. The users exploit power splitting (PS) to decode information
and harvest energy simultaneously based on a realistic piecewise nonlinear
energy harvesting model. The goal is to minimize the base station (BS) transmit
power by optimizing its beamformers, PS ratios, and RIS phase
shifts/amplification factors. The simulation results show significant
improvements (e.g., 19% and 28%) with the maximum reflect power of 10 mW and 15
mW, respectively, compared to the passive RIS without higher computational
complexity cost. We also show the robustness of the proposed algorithm against
imperfect channel state information.
