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This paper studies the application of simultaneous wireless information and power transfer (SWIPT) to downlink non-orthogonal multiple access (NOMA) system. A novel cooperative NOMA protocol for high communication reliability and user fairness is proposed, where a near NOMA user acts as a full-duplex (FD) energy-harvesting relay to help transmission from the source node S to the far NOMA user. The power splitting (PS) architecture is adopted at the relay to perform the SWIPT. The aim is to maximize the data rate of the near NOMA user while satisfying the QoS requirement of the far NOMA user and the energy causality condition of the near NOMA user. The formulated problem is a non-convex fractional programming. By jointly optimizing the power allocation factor, the PS ratio, the receiver filter, and the transmit beamforming, we propose alternative optimization (AO)-based algorithm to obtain an optimal solution. In addition, a low-complexity suboptimal scheme is proposed and the semi-closed form solution is derived to characterize the performance of our proposed design. The simulation results verify the correctness of theoretical analysis and show performance gain of our proposed protocol over the existing transmission protocols. INDEX TERMS Non-orthogonal multiple access (NOMA), simultaneous wireless information and power transfer (SWIPT), power splitting, full-duplex relaying, alternative optimization (AO), suboptimal scheme.

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