# Optimization of Clustering in Wireless Sensor Networks: Techniques and Protocols
## 2 Literature Research Process  
This article adopted a mixture of optimization techniques in order to review the available literature on WSN clustering protocols. According to [8], this method has advantages over the narrative style. It can identify areas covered by existing studies and highlight gaps, approach literature from various perspectives and promote new insights. This survey of optimization techniques and clustering protocols used an online database and other resources to find all articles that met specific criteria, entered information concerning each study into a personal database, and summarized the current state of the table. The comprehensive literature review process is summarized in Figure 1.  
* Search query: a generic search query was constructed for the purpose of search uniformity. This generic search query was utilized when searching for studies within our data sources, including terms \"Clustering\", \"Clustering Protocols\", \"Optimization\", \"Techniques\", \"Wireless Sensors Network,\" and \"WSN\"; the query used for each data source was highlighted in detail. Accordingly, all the search terms were consistent. With this, we conducted a uniform search on all the data sources. However, each database had unique interfaces for advanced search with connectors such as OR and AND sometimes being switched depending on the data source used;
* Data sources: four data sources were used, which were IEEE Xplore, Science Direct, Springer Link, and ACM. By utilizing these data sources, all relevant works in the field of research were expected to be retrieved. This study considered these data sources to be the key sources for obtaining any possible related works;
* Time period: the search within each data source was set to retrieve only studies dated from 2010 to 2021. This was done to ensure that up-to-date studies were the only ones included. Additionally, earlier cited studies were included, as long as the study's full text was available;
* Applying exclusion criteria: our research focused on academic articles published in English. We also considered news articles, books, and annual reports touching on optimization clustering protocols and techniques of WSN;
* Data extraction: each paper was recorded based on author of record, year of publication and the journal in which the study was published. Subsequently, each article was  
Figure 1: Literature review process.  
classified according to the method used and whether the analysis covered state-of-the-art WSN clustering optimization protocols/techniques;
* Identifying data synthesis: analysis was performed to identify the optimization of WSN clustering protocols/techniques and recommend future studies.  
For this paper, all scientific papers were accumulated from online resources. Digital databases including IEEE Xplore, Science Direct, Springer Link, and ACM Digital Library were used to obtain scientific articles for this survey. The literature was chosen based on the following keywords used for the search process: optimization of clustering protocols in WSN, clustering in WSN, clustering protocols in WSN, principles, requirements, and challenges. The search was conducted in April 2021 and was based on article titles. Twenty-three studies were selected from the data sources based on their relevance to the research topic and reviewed.