Energy Efficient NOMA for Mixed FSO-RF Communication System with IRS-Aided SLIPT

This paper proposes a novel relay-based mixed free space optical-radio frequency (FSO-RF) communication system utilizing non-orthogonal multiple access (NOMA) to assist two users over RF-hop. Assuming non-availability of direct FSO-link, we consider the deployment of an optical intelligent reflecting surface (OIRS). Moreover, we deploy an RF intelligent reflecting surface (RIRS) in the vicinity of far user to improve its performance. We adopt simultaneous lightwave-information and power transfer (SLIPT) technology to harvest energy at relay node, which is utilized to forward the information signal over RF-hop and thus, makes the signal-to-noise ratio (SNR) of RF-hop dependent on the FSO-channel coefficients. For the proposed system with generalized Malaga ( $\mathcal {M}$ )-distributed FSO-link with pointing error and Nakagami- $m$ faded RF-link, we evaluate the system's performance in terms of outage probability (OP), coverage probability (CP), and throughput (TP). We also derive asymptotic outage at high SNR and obtain diversity order analytically. Moreover, we propose an efficient algorithm for designing of reflection coefficients of RIRS elements to maximize the downlink SNR of far user. Numerical results are provided to see the impact of system and channel parameters including imperfect SIC conditions, imperfect CSI of far-user channel, and power allocation coefficients on system's performance. Simulations are given to show the extension of the proposed system to multiple user scenario. The integration of OIRS and RIRS exhibits significant improvement in the performance of the considered NOMA-based system.