## 2 Existing Surveys and Identified Limitations  
A review of prior works on 5G-enabling technologies from 2010 to 2022 focused solely on review papers. The main academic databases considered for this review were IEEE Explore, Google Scholar, MDPI, Science Direct, Hindawi, and Springer. These databases contain credible and excellent peer-reviewed research reports in journals, conference proceedings, and review articles. The search phrases 5G and \"enabling technologies\" were used to query these research databases to obtain relevant papers. A total of 50,679 articles were obtained from the specified databases, all of which were published in English. The articles were reduced to 100 after evaluating their topics, then to 70 on the basis of the relevancy of their abstracts, and finally to 29 after reading through the full texts. The number of articles that satisfy the inclusion criteria retrieved from the various databases is shown in Table 1.  
The frequency of occurrence of 5G enablers in the papers considered is presented in Figure 1. The current survey across the various databases shows that extensive research and analyses have been conducted on the various 5G-enabling technologies. However, these assessments examined only a subset of the various technologies. Most of the critical 5G technologies were not presented in earlier works comprehensively. Thus, in this work, the shortcomings identified in earlier works have been addressed by conducting an extensive survey to identify and combine the various technologies for easy reference.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**S/N** & **Article Source** & **URL** & **Number of Articles** \\ \hline
1 & Google Scholar & [[https://scholar.google.com/](https://scholar.google.com/)]([https://scholar.google.com/](https://scholar.google.com/)) (accessed on 15 September 2022) & 23 \\ \hline
2 & IEEE Explore & [[https://ieeexplore.ieee.org/](https://ieeexplore.ieee.org/)]([https://ieeexplore.ieee.org/](https://ieeexplore.ieee.org/)) (accessed on 15 September 2022) & 1 \\ \hline
3 & Springer & [[https://www.springer.com/gp](https://www.springer.com/gp)]([https://www.springer.com/gp](https://www.springer.com/gp)) (accessed on 15 September 2022) & 3 \\ \hline
4 & Science Direct & [[https://www.sciencedirect.com/](https://www.sciencedirect.com/)]([https://www.sciencedirect.com/](https://www.sciencedirect.com/)) (accessed on 15 September 2022) & 0 \\ \hline
5 & MDPI & [[https://www.mdpi.com/](https://www.mdpi.com/)]([https://www.mdpi.com/](https://www.mdpi.com/)) (accessed on 15 September 2022) & 1 \\ \hline
6 & Hindawi & [[https://www.hindawi.com/](https://www.hindawi.com/)]([https://www.hindawi.com/](https://www.hindawi.com/)) (accessed on 15 September 2022) & 1 \\ \hline Total & & 29 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The number of articles retrieved from the various databases.  
Even though virtually all of these enabling technologies are required for the seamless and satisfactory implementation of 5G, it was discovered that some of them, such as mMIMO and mmWave, are critical to the development of 5G wireless networks, as more than half of the reviewed works presented these technologies. This is not surprising given that they are both critical technologies that enhance, assure, and satisfy the key 5G requirements for huge bandwidth. Next, enablers such as Device-to-Device (D2D), Software Defined Networks (SDN), HetNet, Cloud-Based Radio Access Networks (C-RAN), Network Function Virtualization (NFV), and Network densification, have a large impact on 5G adoption, while others are less prominent. Therefore, more research efforts could be directed toward the technologies that have received less attention from researchers, even though they are equally important to the launch of 5G technology. Table 2 summarizes all the literature reviewed, including the various enabling technologies covered.  
Figure 1: A Summary of 5G Enabling Technologies.