Hybrid Time-Switching and Power-Splitting EH Relaying for RIS-NOMA Downlink

Reconfigurable intelligent surface (RIS) offers a potential performance boost for next-generation wireless networks, with significant improvements in spectral and energy efficiencies. In this paper, considering a two-user multiple-input single-output non-orthogonal multiple access (MISO-NOMA) downlink, we propose a RIS-aided cooperative transmission scheme using hybrid simultaneous wireless information and power transfer (SWIPT) and transmit antenna selection (TAS) protocols. Particularly, in the proposed scheme, the cell-center user servering as a relay helps the cell-edge user with the support of hybrid time-switching (TS), power-splitting (PS) SWIPT protocols and RIS passive beamforming. To evaluate the network performance, we first derive the best- and worst-case of closed-form expressions in terms of outage probability. For gleaning further insights, we also investigate the diversity order and the high signal-to-noise ratio (SNR) slope for the relevant outage probability. Simulation and numerical results demonstrate the achievable performance improvement for our proposed scheme with respect to schemes of orthogonal multiple access (OMA), non-cooperative NOMA and cooperative NOMA without RIS. In addition, the proposed scheme achieves a higher diversity gain with more transmit antennas and RIS elements.