  Simultaneous wireless information and power transfer (SWIPT) can enhance the
spectrum and power efficiencies of wireless communications networks.
Line-of-sight (LOS) transmission is a typical SWIPT scenario. However, the
strong channel correlation limits the spectrum and energy efficiencies of SWIPT
in the LOS channel. Due to the orthogonal wavefronts, orbital angular momentum
(OAM) waves can facilitate the SWIPT in LOS channels. With the assistance of
the reconfigurable intelligent surface (RIS), both the energy efficiency and
capacity can be further improved for the OAM-SWIPT systems. In this paper, we
model the RIS-assisted OAM-SWIPT transmission and derive the optimal reflection
coefficients and power splitting ratio for it. We first give the system and
channel models. Then, we propose the transmission scheme. Based on the
transmission scheme, we formulate the capacity and energy harvesting (EH)
trade-off problem. We solve the problem by developing an alternating
optimization algorithm. Simulations validate the capacity and EH enhancements
brought by the RIS for OAM-SWIPT.
