# Cooperative Power-Domain NOMA Systems: An Overview
## 1 Introduction  
To make fifth-generation (5G) networks more efficient and beyond fifth-generation (B5G) networks a reality, recently a lot of cutting-edge concepts have been proposed and examined. Small cells, device-centric architecture, beamforming, full-duplex technology, massive multiple-input multiple-output (MIMO), millimeter (mm) and terahertz (THz) waves, non-orthogonal multiple access (NOMA), and reconfigurable intelligent surface (RIS) are some of the major enabling technologies that have been taken into account for 5G and B5G systems. The bandwidth of millimeter waves can be ten times greater than that of the entire 4G cellular band [1]. The idea of ultra-massive MIMO (UM-MIMO), which uses plasmonic materials for transceiver construction to achieve THz band capacity, has emerged [2]. In B5G IoT, various resources of the vertical industries will require the ability to support a huge number of connections, motivating fundamental research on how to increase the system's spectral efficiency. In this concern, one of the key enabling technologies is NOMA [3]. Additionally, RIS-assisted NOMA is further considered to be a promising new technology that can reconfigure the surroundings for wireless transmission utilising software-controlled reflection to fulfill the needs of 5G and B5G communication [4]. In the last few decades, mobile communication technology has progressed from the audio-centric first generation (1G) to the data-centric recent generations to cater for wide range of applications. In different generation of mobile communication, MA methods permit users to utilise the time and/or spectrum resource simultaneously, and this may be conducted without compromising the users' quality-of-service (QoS). In Figure 1, MA techniques such as frequency-division multiple access (FDMA), time-division multiple access (TDMA), orthogonal frequency-division multiple access (OFDMA), code-division multiple access (CDMA), and non-orthogonal multiple access (NOMA) show how various users (represented by different line designs) are assigned spectra. The analogue 1G system, for instance, uses FDMA to enable voice communication only [5]. By using FDMA [6], frequency bands are separated into distinct channels and hence the channels are then assigned to different users in 1G. TDMA became increasingly feasible with progress in digital communication systems, and it is used in the second-generation (2G) systems for text and voice applications [7]. In TDMA, each user is authorised to transmit on the same frequency band, but only within a specified time period. The third-generation (3G) systems use CDMA to offer video, voice, and data services [8]. CDMA [6] provides consistent bandwidth and transmit power where users can send data at the same time utilising the same frequency range, employing pseudo-code of sorts. To enable rich multimedia broadband services that desire low latency and high data rates, the fourth-generation (4G) long-term evolution-advanced (LTE-A) [9] was introduced, which uses OFDMA and single-carrier FDMA for downlink and uplink, respectively. In OFDMA [10], MA is achieved by distributing portions of subcarriers to each individual. 4G technologies included carrier aggregation (CA), heterogeneous network (HetNet) deployment, coordinated multi-point (CoMP), improved broadcasting and multi-casting services, and full-dimension multiple-input multiple-output (FD MIMO) [11]. The peak data throughput requirements for LTE-A are 1 Gbps for downlink, with 5 ms latency and a mobility of up to 350 km/h [12]. However, sophisticated albeit important future applications such as tactile communication, augmented and virtual reality, ultra high-definition video quality, and online remote control monitoring are some of the challenging performance objectives. Hence, these aspects generated the need for 5G and B5G systems [13].