## 4 Discussion  
### Development Trajectory of RFID and Clusters  
In addition, relatively special applications were found in the clusters, i.e., RFID applications in privacy and safety, as recommendations for scholars to study the direction of RFID technology applications. RFID applications in privacy and safety are the key-route main path of RFID applications, indicating that this research topic is influential, but the relevant academic literature appears in the global main path. In this study, the overall development direction of studies on RFID was determined through main path analysis of the critical path. To guarantee consistency between the cluster and development direction and to evaluate and analyze the consistency between a cluster and its corresponding cluster, a technical analysis of text and data as well as a key-route main path analysis of the critical path were used to divide the citation network into various clusters. First, this study analyzed the development path for the field of RFID. Academic development generally occurs along multiple channels. According to the paths developed through the key-route main path analysis of the critical path, one main paper were source points leading to one branch (Figure 6). The branch, which included the one connecting line on the yellow dotted frame, contained 33 studies of RFID ([PERSON], 2006). Subsequently, key-route main path analyses of the critical paths of the clusters were performed. In the main path of a critical path, each node represents a study, and connecting lines represent citation relationships between studies. Figure 6: The development trajectory of academic papers on RFID research. Figure 7: Themes, number of papers, keywords, word cloud and growth curve of each group. Group 5: Privacy and Safety (Period: 2006-2018)  
[PERSON] [86] examined the privacy protection and comprehensive protection measures for RFID systems proposed by scientists and considered the social and technical contexts of this research. [PERSON] et al. [87] researched the application of RFID protocols in tag readers for mutual identity authentication. [PERSON] et al. [88] proposed the key array authentication protocol; a logistical, safety, and performance analysis of the protocol determined it to be reliable in RFID systems. [PERSON] [89] proposed an RFID mutual authentication protocol based on elliptic curve cryptography and discussed its application in security problems. Subsequently, [PERSON] [90] described the shortcomings of the protocol proposed by [PERSON], indicating that the protocol could not ensure tag privacy, forwarding secrecy, identity authentication, or server authentication and was unable to protect against simulated, tag cloning, and location-tracking attacks. He and [PERSON] proposed an RFID identity authentication solution for medical environments [91]. [PERSON] et al. [92] employed the elliptic curve Diffie-Hellman protocol to generate a shared key and encrypt transmitted data. To solve identity authentication and privacy protection problems in campus security monitoring, [PERSON] et al. [93] employed RFID to improve campus security authentication and established a system for mutual authentication among tags, readers, and the backend database. This system effectively blocks simulated, location-tracking, synchronized, eavesdropping, and denial-of-service attacks. [PERSON] et al. [94] calculated the offloading capacity of IoT devices, analyzed the power consumption of different components in a mobile RFID system, and proposed a model for reducing the power consumption of similar systems. In IoT applications, the power supply and computational capability of sensors are limited. Therefore, future studies can focus on reducing power consumption. One of the major concerns in many applications is the security and privacy of data. The main factor is the fast expansion and Internet diversification in the IoT and related services, such as used protocols and mutual authentication protocol.