## 5 Overview of 5G-Enabling Technologies
### Key 5G-Enabling Technologies
#### 5.1.10 The 5G Non-Orthogonal Multiple Access (NOMA) Principle  
Another essential 5G wireless system component is the NOMA principle, which allows several users to exchange data in a small cell. It is highly advantageous for heterogeneous networks. On the most basic level, it works by allocating the same resources in terms of time, area, and frequency to different users. NOMA is separated into code-domain NOMA and power-domain NOMA [103]. While code-domain NOMA can enhance 5G connectivity by enhancing the spectral efficiency of massive MIMO, power-domain NOMA is more commonly used in 5G wireless networks due to its ability to integrate with a wide range of wireless communication techniques, including MIMO, beamforming, space-time coding, network coding, full-duplex and cooperative communication, among others [9].  
Figure 9: Result of power consumption of MBS and microwave backhaul hub. The figure was reproduced from a paper [101] titled: _Energy Efficiency of Backhauling Options for Future Heterogeneous Networks_; published in Advances on Computational Intelligence in Energy: The Applications of Nature-Inspired Metaheuristic Algorithms in Energy, Green Energy, and Technology; Springer, 2019; pp. 169–194 ISBN 978-3-319-69889-2.  
NOMA can benefit a variety of uses, including massive machine communication, machine-to-machine, and Ultra-Dense Networks (UDN). This ground-breaking technology also has high spectrum efficiency, high-speed massive connectivity, high reliability, and low latency. Furthermore, when resources for bandwidth are assigned to users with minimal Channel State Information (CSI), the traditional Orthogonal Frequency-Division Multiple Access used by 3 GPP in 4G LTE networks results in unusually low spectral efficiency. This issue was resolved by employing NOMA [9; 104], which provided users access to all subcarrier channels. This increased spectral efficiency by allowing users with high CSI to use bandwidth resources formerly reserved for users with low CSI. Despite the many advantages of NOMA, there are some disadvantages as well. For instance, NOMA requires a lot of Computer Processing Unit (CPU) resources from many consumers at high data rates to execute the SIC algorithms. Secondly, managing power allocation optimization when users leave networks is a challenging problem in NOMA [105].  
A look at a basic NOMA model with two users, which clarifies the fundamental principles of the model, was provided by [106]. Their research was then broadened to encompass an overall architecture, including multicarrier support on each sub-carrier, an arbitrary number of consumers. Resource sharing and multiple-input-multiple-output NOMA are investigated in performance evaluation using current methodologies. The authors included the essential components of NOMA as well as potential research requirements. They examined the two-user SC-NOMA and the multi-user MC-NOMA designs to demonstrate the fundamental strategies and conventions of NOMA. The key features and challenges of NOMA technology were also discussed. The authors [107; 108] investigated non-orthogonal multiple access (NOMA) from its inception to recent advances. They contrasted NOMA approaches with traditional OMA techniques in the context of information theory. The NOMA schemes were formally discussed as belonging to the power and code domain, including design concepts, operational principles, and features. Performance, spectrum efficiency, and receiver complexity are utilized as comparative criteria. The expected outcomes of NOMA and how it would meet the essential needs of 5G mobile communication systems, such as enormous connection and reduced latency, were highlighted.