## 2 Literature Review Methodology
### Development of the Review Protocol
#### 2.2.3 Systematic Approach Used in Extracting and Analyzing Studies  
Table 3 provides a framework for extracting, analyzing, and classifying research studies. It categorizes studies into key domains: channel estimation, adaptive modulation, and modulation recognition. Consequently, it enables comparative assessments of methodologies and performance metrics. Additionally, Figure 6 presents statistical data on research articles, organized by publication year. \begin{table}
\begin{tabular}{l c c c} \hline
**Search Term** & **IEEE** & **Elsevier** & **Springer** \\ \hline Underwater Acoustic Modulation Classification & 52 & 419 & 299 \\ \hline Underwater Acoustic Modem Machine Learning & 6 & 107 & 43 \\ \hline Underwater Acoustic Modulation Machine Learning & 42 & 382 & 243 \\ \hline Underwater Acoustic Modulation Deep Learning & 76 & 407 & 276 \\ \hline Underwater Acoustic Modulation Recognition Machine Learning & 16 & 199 & 128 \\ \hline Underwater Acoustic Modulation Recognition Deep Learning & 28 & 222 & 147 \\ \hline
**Total Articles from Databases** & **6983** & **18,882** & **12,396** \\ \hline
**Sum of All Articles** & & **38,261** & \\ \hline
**Additional Google Scholar Articles** & & **3000** & \\ \hline
**Final Grand Total (All Sources)** & & **41,261** & \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 5: Stepwise selection and screening procedure for research article inclusion. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **ML/DL Technique** & **Optimizer** & **Training Examples** \\ \hline
[17] & LR & GD & Measured Data \\ \hline
[20] & LSTM, BiLSTM, SBULSTM & — & Simulated (BELLHOP) and Measured Data \\ \hline
[21] & ABiGRU & Adam & Measured and Simulated \\ \hline
[57] & AttLstmPreNet & — & Simulated Data (From [25]) \\ \hline
[29] & UACC-GAN & Adam & Measured Data (From [26]) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Overview of ML/DL-based channel estimation in SC-UWA communication. \begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Item** & **Corresponding Details** \\ \hline
1 & Citation Data & It includes the title, author(s), publication year, publisher, and research type (journal or conference). \\ \hline
2 & Overview & A concise summary outlining the fundamental proposal and primary objective of the research \\ \hline
3 & Results & Findings obtained from the analyzed research, highlighting key insights and conclusions \\ \hline
4 & Data Collection & Specifies whether the study employs quantitative or qualitative data gathering methods \\ \hline
5 & Assumptions & Identifies any underlying assumptions made to support and validate the research findings \\ \hline
6 & Validation & Describes the methodology used to verify the accuracy and reliability of the proposed study \\ \hline
7 & Channel & Overview: Tables 4 and 5 \\  & Estimation & Characteristics: Tables 6 and 7 \\  & Techniques & Comparison: Tables 8 and 9 \\ \hline
8 & Adaptive & Overview: Tables 10 and 11 \\  & Modulation & Characteristics: Tables 12 and 13 \\  & Techniques & Comparison: Tables 14 and 15 \\ \hline
9 & Modulation & Overview: Table 16, Characteristics: Table 17 \\  & Recognition & Comparison: Table 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Systematic process for extracting, analyzing, and classifying research studies in ML/DL-based UWA communication: outlining key data collection methods, evaluation criteria, and classification approaches. Figure 6: Year-Wise Distribution of Selected Research Articles from W6S-Indexed Journals (2020–2025), Illustrating Publication Trends and the Evolving Focus on ML/DL Applications in UWA Communication. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref** & **Bandwidth (KHz)** & **Tx-Rx Distance (km)** & **Modulation** \\ \hline
[17] & 2 & 0.205 & BPSK \\ \hline
[20] & 4 & 2 to 3 & BPSK, DS-SS \\ \hline
[21] & 14 to 18 & 1 & QPSK \\ \hline
[57] & 10 \& 5 & 1 & — \\ \hline
[29] & 10 to 18 & — & FH-SS \\ \hline \hline \end{tabular}
\end{table}