# Blockchain-based Multifactor Authentication for Future 6G Cellular Networks: A Systematic Review
## 2 Methodology  
The study has been conducted systematically in view of different research and experimental studies on the multifactor authentication techniques, preferably including the concepts of multifactor authentication, 6G, and blockchain. We focused on the published journal articles ranging from the year 2015 to the year 2021. A systematic approach is adopted for the review process, which is the type of methodology utilized and applied to identify, examine, and extract needful information from literature relevant to a specific research topic. The review has been done in two different phases. The first phase involves the identification of information resource that was the search engine and keywords that were utilized for the execution of the query. Whereas the second phase includes the application of specific criteria onto the initial generated list so that only the most relevant articles are shortlisted for the final list of the papers reviewed for the systematic literature review. Figure 1 depicts both phases of the review process adopted in this study. We conducted comprehensive research in this review. In the first phase, the keywords and the search engine are identified for searching the articles. The researcher critically evaluated the research published in the English language between January 2015 and December 2021. The reason for focusing on these years is that the 6G technology research emerged in this era. For this study, \"Scopus document search\" and \"Ebsco databases\" are the chosen search engine because of their efficiency in searching through all authentic databases. The search keywords include \"multifactor authentication in blockchains\", \"multifactor authentication in blockchains, and 6G cellular network\". Once the search \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Study** & **Year** & **Systematic Study** & **Blockchain Focused** & \begin{tabular}{c} **MFA** \\ **Focused** \\ \end{tabular} & \
\begin{tabular}{c} **6G** \\ **Focused** \\ \end{tabular} &
\begin{tabular}{c} **Future Trends** \\ \end{tabular} \\ \hline
[9] & 2021 & ✓ & ✓ & \(\times\) & ✓ & ✓ \\
[14] & 2020 & ✓ & ✓ & ✓ & ✓ & \(\times\) \\
[13] & 2021 & ✓ & ✓ & ✓ & \(\times\) & ✓ \\
[20] & 2020 & ✓ & ✓ & ✓ & ✓ & \(\times\) \\
[22] & 2020 & ✓ & ✓ & \(\times\) & ✓ & ✓ \\ This study & 2022 & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison with other similar review articles (✓: Yes, \(\times\): No). query is executed, the initial list of the articles is obtained. Retrieval run on both databases resulted in 299 papers, 191 papers collected from Scopus, and 108 from Ebsco. At the end of the process of overviewing and removing duplicated articles or whose full text could not be retrieved, 124 articles were left of the initial research of 299, 71 papers were collected from Scopus and 53 from Ebsco. After this, the abstracts were reviewed before the complete screening of material. In this stage, a further 41 articles were dropped. The final set of 73 papers was included in the review; 57 were retrieved from Scopus and 16 from Ebsco. All the retrieved references from the various databases included in the study were then added to the software used in this study for reference management, i.e., Endnote Version 9.2. Table 2 shows the papers included in this review in each domain. In the second phase, we analyzed the initial list. We then defined a specific criterion for obtaining the articles that are significantly relevant and more focused on the multifactor authentication in blockchains and 6G cellular network for the final systematic review. Book chapters and survey/review-based articles are excluded to select more focused articles for this study. Additionally, articles are included that propose some multifactor authentication techniques and are written in English.