# RIS-Assisted Fixed NOMA: Outage Probability Analysis and Transmit Power Optimization
###### Abstract  
Reconfigurable intelligent surface (RIS)-assisted non-orthogonal multiple access (NOMA) has the ability to overcome the challenges of the wireless environment like random fluctuations, shadowing, and mobility in an energy efficient way when compared to multiple input-multiple output (MIMO)-NOMA systems. The NOMA system can deliver controlled channel gains, improved coverage, increased energy efficiency, and enhanced fairness in resource allocation with the help of RIS. RIS-assisted NOMA will be one of the primary potential components of sixth-generation (6G) networks, due to its appealing advantages. The analytical outage probability expressions for smart RIS-assisted fixed NOMA (FNOMA) are derived in this paper, taking into account the instances of RIS as a smart reflector (SR) and an access point (AP). The analytical and simulation findings are found to be extremely comparable. In order to effectively maximize the sum capacity, the formulas for optimal powers to be assigned for a two-user case are also established. According to simulations, RIS-assisted FNOMA surpasses FNOMA in terms of outage and sum capacity. With the aid of RIS and the optimal power assignment, RIS-AP-FNOMA offers \(\approx\)62% improvement in sum capacity over the FNOMA system for a signal-to-noise ratio (SNR) of 10 dB and 32 elements in RIS. A significant improvement is also brought about by the increase in reflective elements. order to secure sustainable green communication [3]. Massive MIMO employs an array of antennas at both the base station (BS) and the user equipment (UE) to provide unrivaled capacity improvements. Higher radio frequency (RF) bands, on the other hand, make electro-magnetic (EM) waves more susceptible to obstruction, particularly in metropolitan areas [4]. With today's fifth-generation (5G) wireless technologies, universal coverage and massive connectivity are not possible. As a result, new technologies are needed to enable data-intensive and energy-intensive applications. RIS is a new emerging hardware technology that reduces energy usage, while artificially controlling and changing the propagation environment to create a superior wireless channel. The RIS is built up of a collection of low-cost, passive reflecting components that reconfigure incident EM waves [5; 6]. Using RIS, the uncontrollable wireless environment is transformed into a semi-controllable wireless environment. RIS can be simply installed on a variety of surfaces, including building facades, vehicle doors, roadside billboards, indoor walls, unmanned aerial vehicles (UAVs), pedestrian clothing, and so on. Tall buildings can obstruct the line-of-sight (LoS) between BS and UEs, reducing throughput significantly. The judicious placement of RIS can create a virtual LoS between the BS and the UEs, improving the signal-to-interference plus noise ratio (SINR). RIS can passively beamform signals collected from a transmitter to a desired receiver [7]. Unlike traditional amplify-and-forward (AF) and decode-and-forward (DF) relays, power amplifiers are not required for RIS [8; 9; 10]. Further, energy sources are also not required to encode, decode, retransmit, and process RF signals [4]. As a result, RIS installation is both energy efficient and environmentally benign. RIS can be integrated with other emerging technologies, such as NOMA, simultaneous wireless information and power transfer (SWIPT), UAV-aided communication, physical layer security (PLS), cognitive radios, autonomous cars, etc., due to its appealing features [11]. Power-domain NOMA can be coupled with RIS to improve spectral efficiency and the enormous connectivity. In NOMA, the signals of different users are overlaid in the power domain so that the spectrum is exploited more efficiently. Based on the channel conditions, users are explored opportunistically. Further, it is also compatible with full-duplex relays, index modulation (IM), etc. Because of these advantages, RIS provides unlimited potential to the 6G wireless technologies.