# Physical Layer Authentication in Wireless Networks-Based Machine Learning Approaches
## 4 Research Methodology
### Phase 2. Conducting the Review  
In this subsection, we describe each step in choosing papers and the search process based on SLR methods, as depicted in Figure 3. This approach to identifying relevant papers has four steps. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**Research Questions** & **Motivations** \\ \hline RQ1: What machine learning approaches are applied in physical layer authentication to secure the wireless network? & To assess whether machine learning approaches in physical layer authentication models enhance the performance of wireless network security. \\ RQ2: What are the existing physical layer authentication techniques for handling a wireless network’s security problems? & To identify what techniques have been used in physical layer authentication. \\ RQ3: What are the existing key challenges, open issues, and future trends in wireless network security based on physical layer authentication solutions? & To identify gaps in the physical layer authentication literature on wireless network security and to suggest directions for future research. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Research questions and motivations. **Step 1. Selecting of primary studies:**  
This step aims to find as many papers related to our research as possible. Then, we will determine which sources to search for and identify the search terms to be applied. _Identify the Search Terms:_ We defined the search terms used for the manual searches in online scientific databases. A general approach is to break down the research questions into individual terms and then make a list of synonyms and abbreviations. We divided the key terms into three categories, each of which included synonyms or variations of the terms relevant to the research question:  
* _Group 1 finds papers related to transmitter identification:_ (\"Transmitter Identification\" OR \"Transmitter Classification\" OR \"Specific Emitter Identification\" OR \"Emitter Classification\") AND (\"Deep Learning\" OR \"Neural Network\" OR \"Radio Frequency Fingerprints\" OR \"RF fingerprinting\" OR \"RF\"). * _Group 2 finds papers related to wireless network security:_ (\"Wireless Networks Security\" OR \"Wireless Device Security\" OR \"Internet of Things Security\") AND (\"Transmitter Identification\" OR \"Transmitter Classification\" OR \"Specific Emitter Identification\" OR \"Emitter Classification\"). * _Group 3 finds papers related to physical layer authentication:_ (\"Physical Layer Authentication\" OR \"Physical Layer Security\") AND (\"Wireless Networks\" OR \"Wireless Device\" OR \"Internet of Things\" OR \"IoT\" OR \"Radio Frequency Fingerprints\" OR \"RF fingerprinting\" OR \"RF\"). **Step 2. Specifying the Studies' Sources:**  
After identifying the search keywords, we determined which online digital libraries and journals to search. We initially searched for related studies on the online platform Google Scholar, based on the search keywords chosen in the first stage. Later, we included more related articles by searching in recognized academic publishers such as IEEE Xplore, ACM Digital Library, Science Direct, Springer, and Hindawi. Conducting the selected studies by database source is depicted in Figure 4.  
**Step 3. Selection of Primary Studies:**  
When potentially relevant primary studies have been obtained, they must be assessed for their actual relevance and whether they may answer the research questions. Selection of primary studies is considered a multistage process. **Stage 1:** This stage involves filtering potential primary papers by scanning their titles, abstracts, and keywords. This stage yielded 248 papers, including journals, chapters, conference papers, books, and other publications that responded to the search terms. The total number of selected publications is shown in Figure 5, and Figure 6 indicates the type of selected publications.