# Cooperative Power-Domain NOMA Systems: An Overview
## 1 Introduction
### Motivation and Contributions  
The demand for data rate and capacity has increased dramatically due to the change in users' preferences from voice messages to video streaming, and downloading multimedia applications. For example, video streaming in 4K/8K high definition requires a data rate of about 300 Mbps, which strains the present networks' capacity when a large number of  
Figure 1: Pictorial description of different multiple access schemes, i.e., (**a**) TDMA, (**b**) FDMA, (**c**) CDMA, (**d**) OFDMA, and (**e**) NOMA. Figure 2: Evolution of mobile wireless systems from 1G to B5G along with multiple access techniques used. user devices are provided. Hence, there is motivation for the utilization of NOMA in next-generation mobile networks. Cooperative NOMA is possibly the best technique for radio access that improves spectrum efficiency, transmission rate, and latency [27]. Furthermore, NOMA surpasses the present OMA system in terms of total throughput and a performance boost is predicted with the adoption of an improved power allocation technique [28]. However, further research is needed. It was suggested that NOMA is the prime technology to be introduced in the new communication systems [29; 30; 31; 32; 33]. Furthermore, resource allocation has been identified as a critical approach for maximising a number of network performance metrics, e.g., to maximise the sum rate among other key performance indicators (KPIs). Resource allocation in the NOMA system, in particular, has attracted tremendous attention in the literature because it can produce considerable performance gains over the OMA system [34]. However, some of the research in [34] investigated an improvement in the performance of users with bad channel conditions through the integration of NOMA by optimising the resource allocation in MIMO-NOMA networks. A few difficulties that represent open study areas for cooperative NOMA techniques have been addressed in our survey paper. It is worth noting that a some prior articles have offered a similar notion with different issues and approaches. Multiple review articles have provided detailed surveys on PD-NOMA [35; 36; 37; 38; 39]. The authors of [35] covered aspects of this novel technology, from its use in association with MIMO technologies through NOMA and the connection between cognitive radio and NOMA. The authors also examined the state of standardization initiatives for NOMA deployment in LTE and 5G networks. The authors of [36] examined the analysis of capacity, power allocation algorithms, user-pairing schemes, and user fairness in NOMA, as well as the latest advancements of NOMA in 5G networks. Their research also looked at how NOMA works when combined with other well-known techniques for communication such as MIMO, space-time and network coding, and beamforming. Similarly, the authors of review article [37] explained NOMA concepts, important characteristics, advantages and disadvantages, and then compared these features including the spectrum efficiency and complexity of the receiver. Power domain was explained in [38], where the authors discussed two well-known categories of NOMA, i.e., PD-NOMA and CD-NOMA. Moreover, a comparison of the methodology of different NOMA schemes in terms of complexity of the receiver due to successive interference cancellation (SIC) was discussed as well. The authors of [39] presented a comprehensive overview of multiple-access systems, methodologies, and techniques for NOMA optimization. They also reviewed the literature's taxonomy of multiple-access schemes, followed by a full description of NOMA's aims, limits, challenges, and solutions. The decoding algorithms and KPIs utilized in NOMA were also discussed in this work. Our paper, on the other hand, concentrates on cooperative PD-NOMA networks. It is worth noting that only a few studies have looked into NOMA-based cooperative communication to date, which indicates that research in this area is still in its early stages.