Uplink RIS-assisted NOMA/OMA networks: a theoretical study

Abstract In this paper, we have investigated the outage probabilities (OPs) and the ergodic rates (ERs) of uplink reconfigurable intelligent surface (RIS)-assisted non-orthogonal and orthogonal multiple-access (NOMA and OMA) networks. Particularly, we have tackled a challenging scenario in which both user equipments (UEs) need assistance from the same RIS unit to communicate with the base station (BS). We have modeled all the UEs $$\rightarrow$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>→</mml:mo> </mml:math> RIS and RIS $$\rightarrow$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>→</mml:mo> </mml:math> BS links as Nakagami- m variates. To assess the performance of these networks, we have derived the necessary statistics for the effective channel gain of the reflected (UE $$\rightarrow$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>→</mml:mo> </mml:math> RIS $$\rightarrow$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>→</mml:mo> </mml:math> BS) link for the randomly phase-shifted UE, while adopting its already-existing counterpart statistics for the coherently phase-shifted one. On the basis of that, we have derived closed-form expressions for the OPs and the ERs for both RIS-assisted NOMA and OMA schemes. Finally, to get further insights on these results, we have elaborated on the high signal-to-noise ratio (SNR) regime.