# Secure Cognitive Radio Vehicular Ad Hoc Networks Using Blockchain Technology in Smart Cities
## 2 Literature Review  
The development in vehicular communication is evidence of the advancement in technology. In the early 2000s, when the VANETs were introduced, it was considered a remarkable milestone as it completely transformed the vehicular communication setup. But two issues currently prevail in VANETs. One is the short nature of the spectrum that was dedicated to vehicular communication, and the second is security. CR-VANET [3] has been introduced as a promising solution to overcome the issue of spectrum scarcity. Also, different security-based methods have been proposed in the literature [4] to provide trust-based communication in VANETs. To detect malicious nodes, the scheme ensured security and established trustworthiness. Using cognitive technology, the authors demonstrated an improvement in delay, delivery ratio, and throughput. [PERSON] et al. [5] proposed a resource allocation scheme that considered the driving route of autonomous vehicles. They developed a blockchain-based setup to provide a resource and route to a specific node for a specific time-period based on tokens. Their future direction is to consider vehicle-to-vehicle communication and integrate it into their approach. In a similar fashion, a concept for a cognitive internet of vehicles was developed in [6] to enable autonomous vehicles to share cognitive data based on consensus using cognitive engines. A credit evaluation mechanism of blockchain technology was used to resist malicious nodes. However, designing an incentive mechanism was declared by the authors as a future project. [PERSON] et al. [7] implemented a scheme using smart contracts (digital contracts written in solidity language where the conditions to work in the blockchain environment are described). The scheme explains the implementation in the areas of vehicle identification, user authentication, and communication. Once a vehicle registers itself on the blockchain network using the contract, the vehicle is identified and the user receives unique public and private key pairs to communicate within the zone. The vehicle can also communicate with vehicles in other zones if they have cellular networks. The thing that is not addressed in this study is how it will deal with the increasing number of vehicles registered on the network. Another scheme, TOPSIS [8], that implemented the cognitive concept in blockchain was introduced to prevent malicious nodes by allowing vehicles to track both legal and illegal activities. Parameters such as delivery time and overhead consumption in sensing were cited as future work by the authors. Similarly, an offline blockchain framework [9] was introduced to store the information of participating nodes and make it available for future use. As the scheme was implemented offline, no real-time delay was considered. Furthermore, overhead consumption was not considered. To secure data in vehicular communication [10], an RSU was considered as a fog node that provides decentralized data privacy using blockchain technology. Latency and overhead were used as performance metrics when evaluating the scheme. Despite this, the vehicular scheme was conventional and did not consider spectrum issues. BBSF (blockchain-based secure weather forecasting) [11] is a routing scheme that takes into account the sharing of critical information from the source to the destination node. Because it is weather-related information, the authors secured it using blockchain technology. But, like the previous scheme, it also did not consider spectrum issues. To prevent malicious attacks on VANETs, [PERSON]. [12] suggested a multi-hop routing scheme that relies on accumulating trust. RSU calculated the current trust value of the vehicle based on the number of times it forwards packets and then added this value to the blockchain to create an accumulating trust. To assess the routing algorithm, a basic numerical simulation was performed.