## Research Base  
References in this cluster focus on two keywords: security and blockchain. As intelligent transportation systems benefit both consumers and manufacturers, traffic management, road safety, security, and privacy threats become important issues. Researchers find blockchain-based technology to be the potential solution to this problem [21; 22; 23; 69; 70; 71]. In a decentralized trust management system for vehicular networks based on blockchain, a rating from each neighboring vehicle is calculated and packed into a block, and then the total value of the blocks is evaluated [71]. On the one hand, researchers wish to ensure data sharing security. [PERSON] et al. propose an approach of consortium blockchain and smart contract technologies to ensure the security of vehicular edge computing servers [22]. [PERSON] and his colleagues also propose a two-stage soft security enhancement solution to improve the existing delegated proof-of-stake consensus scheme, which is used to establish a blockchain-enabled Internet of Vehicles (IoV) [23]. On the other hand, to protect users' privacy, researchers are making many attempts. [PERSON] et al. propose a blockchain-based architecture that increases the security of vehicular networks [21]. [PERSON] et al. propose an anonymous reputation system (BARS) that is used to establish a trust model for VANETs [69]. [PERSON] et al. propose CreditCoin, an announcement network that allows users to post announcements anonymously to share traffic information [70]. References in this cluster are from literature within the past 5 years, indicating the research direction is relatively novel. Research Frontier  
The development of the technology of concern seems to have had certain achievements, so the newest articles are mostly literature reviews. Researchers conduct comparative studies on blockchain-based models in security, privacy prevention, and trust management systems [25], authentication schemes in the IoV and VANETs [26], authentication schemes with 5G and 5G-SDN, and blockchain technologies [72]. As can be seen, blockchain-based solutions are favorable in the VANET scenario [73], wireless technologies such as IEEE 802.11p and 5G V2X are adopted [29], and the vehicular network environment is shifting from VANET to the IoV, a new concept that is likely to realize the application of V2X [29; 74]. In addition, researchers classify authentication schemes according to security mechanisms, requirements, limitations, attacks, countermeasures, and so on [28], classify related literature according to research directions and blockchain-based IoV implementations [27], and summarize ways of attacking as well as unsolved issues in the field. Certainly, researchers will not forget the open issues. They propose a consortium blockchain technology, as 5G appears, that enables traceable and anonymous V2V data sharing and privacy prevention in IoV [75]. Moreover, data storage environment and security are studied, and solutions such as practical byzantine fault tolerance (PBFT) are proposed, which allows consortium blockchains to audit publicly, store data sharing, and record consensus processes [76]. Citing articles in this cluster were published in 2021 and 2022, meaning researchers are currently concerned about vehicular trust management, security, and privacy systems. Cluster 2: learning-based resource allocation; machine learning  
**Research Base**  
As vehicular networking is strongly demanded, machine learning, one branch of artificial intelligence (AI), is expected to achieve intelligent transportation [30; 31]. Besides, the development of communication technologies, including cellular networks, LTE, and 5G, has made great progress in vehicular communication [32; 33; 77]. Therefore, researchers are devoted to building more reliable and convenient vehicular networks. From 2017 to 2020, researchers established a solid research foundation on vehicular networks based on communication technology.