Outage Performance Analysis of RIS-FA-Assisted NOMA Systems Over Nakagami-m Fading Channels

Fluid antenna (FA) is an emerging technology in recent years to switch the physical locations of antennas in a predetermined small space. This article proposes a reconfigurable intelligent surface (RIS)-cooperative framework with FA-assisted nonorthogonal multiple access (NOMA) system, namely, FA-RIS-NOMA. Targeting multi-interference scenarios in urban environments, the considered system incorporates a base station (BS) equipped with a conventional antenna and users each equipped with an FA. Additionally, the RIS is utilized to forward signals between BS and users blocked by obstacles. The non-line-of-sight (NLOS) multipath fading characteristics of the RIS-assisted link are modeled as a Nakagami-m channel. To overcome the multiuser interference and improve the system performance, we develop the NOMA technique for resource allocation. Meanwhile, to reduce the signal processing complexity at the receiver, a group optimization greedy detection (GPGD) method is proposed. To evaluate the system's reliability, the outage probability for each user is analyzed by using the copula function. This method enables the derivation of the cumulative distribution function (cdf) and probability density function (pdf) for the equivalent user-side channel, from which closed-form outage probability expressions are obtained. The numerical results demonstrate that the proposed framework achieves signal-to-noise ratio (SNR) gains of approximately 9 and 8 dB over fixed-antenna and relay-assisted systems, respectively. Furthermore, the proposed detection method reduces computational complexity by over 75% with less than 0.5-dB performance degradation.