# Multi-Blockchain Structure for a Crowdsensing-Based Smart Parking System
## 1 Introduction  
A smart city is a planned city that can collect and flexibly use data in accordance with the development of the Internet of Things (IoT) and wireless sensor network (WSN) technologies [1]. In a smart city, these advanced technologies can be applied to provide real-time information, such as up-to-date parking information, to its inhabitants. An example of such a system, which integrates IoT sensors and cloud servers in a parking system to remotely provide parking space information, has already been proposed [2]. In addition to the aforementioned use of installed IoT sensors in a smart parking system, mobile crowdsensing (MCS) technology, i.e., the method of collecting data through the cameras or sensors of mobile devices owned by the public, can also be applied [3]. MCS technology has an advantage in that the service provider can obtain large amounts of real-time data without the initial installation costs of gathering these data. In this paper, we propose a method using mobile crowdsensing technology for a smart parking system that provides parking space information. Meanwhile, because blockchain technology that provides dispersibility, scalability, transparency, security, and safety of data management has recently been attracting attention, blockchain has also been proposed as a smart city platform [4; 5]. Blockchain technology has been applied to the linking and automation of data processing for mobile crowdsensing with user compensation processing [6]. The blockchain architecture can use smart contracts to automate the provision of rewards via leveraging of the transaction records on the blockchain. These data on the blockchain can be shared transparently while integrity is maintained according to the characteristics of the blockchain. A model that utilizes apublic blockchain has also been proposed [6], but this model faces the possibility of privacy problems because the data are shared with everyone, and thus a limit to the authorized management of its functions is imposed. In this study, we use a mobile crowdsensing service structure to build a smart parking system, and propose a multi-structure system of opened public blockchain and private blockchain with restrictions on the participating node for data management. In a public blockchain, sensing data contributors participate in providing data. In a private blockchain, service providers and data users participate in processing the collected data from the public blockchain to provide users with information about parking lots, such as locations of empty parking spaces. A bridge node joins both blockchains to connect and exchange data between the two blockchains. With such a multi-structure, the public blockchain transparently collects information from many participants, whereas the private blockchain processes and stores the information, reducing the risk of personal information exposure. Moreover, through the sharing of processing information among other service providers in the private blockchain, provision of data on the real-time parking situation, as desired by the user, can be made possible. The contribution of this study is the design of a smart parking system, a representative application example for a smart city, with mobile crowdsensing technology and multi-blockchain structure. This structure can easily accommodate a large number of participants; collect data; provide transparency in data management; and enable service providers to lower the risk of privacy exposure and to process, share, and provide information. The proposed multi-structure and bridge node is then implemented with the representative public blockchain Ethereum [7] and the private blockchain Hyperledger Fabric [8]. Afterward, the feasibility of the proposed system is shown via experimentation with the operational performance on the implemented system, and the safety pursued by the proposed system is revealed via security analysis.