Performance of RIS-Aided Fluid Antenna-Enabled Multiuser NOMA Non-Terrestrial Networks

This paper analyzes the outage probability and ergodic capacity of a reconfigurable intelligent surface (RIS)-assisted fluid antenna system (FAS)-enabled non-orthogonal multiple access (NOMA)-based satellite communication system, considering practical impairments such as imperfect successive interference cancellation (SIC) and channel state information (CSI). We model the satellite links with shadowed-rician distribution and the terrestrial links with Nakagami-m distribution while employing maximum ratio transmission (MRT) at the low Earth orbit (LEO) satellite. By leveraging the central limit theorem (CLT) and the moment method, the cascaded RIS-assisted channel is approximated as a Gamma distribution. Additionally, the clayton copula effectively models the correlation between FAS ports, enabling the derivation of approximate outage probability and ergodic capacity expressions for arbitrary fading distributions. The Gauss-Laguerre quadrature approximation is employed to efficiently solve complex integrals in the ergodic capacity analysis. Furthermore, asymptotic outage probability expressions in the high SNR regime are derived, revealing the diversity order and the impact of various system parameters on outage probability. Numerical results highlight the detrimental effects of these imperfections, underscoring their significance in system design. Performance improvements are demonstrated with an increasing number of RIS elements, transmit antennas, and FAS ports in RIS-assisted NOMA satellite communications. Furthermore, the impact of power allocation on outage probability is analyzed, and a trade-off between the number of FAS ports and the width of the fluid channel is established to achieve optimal performance.