Rapid development and deployment of vehicular ad-hoc networks (VANETs) require an efficient and scalable media access control (MAC) protocol to support high-priority safety applications and infotainment requirements. This paper proposes SATMAC, a self-adaptive time division multiple access (TDMA)-based MAC protocol for VANETs. In order to improve the stability of the time slot scheduling in VANETs, a slot status updating strategy is carefully designed, which utilizes accurate information of the two-hop neighbors and the rough information of the three-hop neighbors to detect the potential packet collisions and avoids potential collisions by adjusting the occupied time slot. Besides, an adaptive frame length (the number of time slots contained in a frame) approach is proposed on the basis of the slot adjustment to support various densities of vehicles, where the frame length between neighbors can be inconsistent. We conduct theoretical analysis and extensive simulations in a realistic VANET environment to evaluate SATMAC. Simulation results show that compared with IEEE 802.11p and LTE-V2X PC5 Mode 4, SATMAC significantly improves PDR of beacons over 60%. Moreover, our SATMAC design is further implemented and validated on our FPGA-based testbed.

In the development of geographic information-based applications for mobile devices, achieving better access speed and visual effects is the main research aim. In this paper, we propose a new geographic data display method based on Geohash, namely GeohashTile, to improve the performance of traditional geographic data display methods in data indexing, data compression, and the projection of different granularities. First, we use the Geohash encoding system to represent coordinates, as well as to partition and index large-scale geographic data. The data compression and tile encoding is accomplished by Geohash. Second, to realize a direct conversion between Geohash and screen-pixel coordinates, we adopt the relative position projection method. Finally, we improve the calculation and rendering efficiency by using the intermediate result caching method. To evaluate the GeohashTile method, we have implemented the client and the server of the GeohashTile system, which is also evaluated in a real-world environment. The results show that Geohash encoding can accurately represent latitude and longitude coordinates in vector maps, while the GeohashTile framework has obvious advantages when requesting data volume and average load time compared to the state-of-the-art GeoTile system.

In recent years, a number of routing protocols have been proposed for urban vehicular ad-hoc networks (VANETs). Yet, a common issue in these protocols is that the building shadow effect at intersections is not carefully considered. As a result, the performance of these routing protocols may degrade as the data transmission at the intersections can be severely affected by the building shadowing. To solve this problem, in this paper, we propose MBR, a map-based relaying algorithm in the MAC layer, to enhance the performance of the routing protocols in urban VANETs. To improve the reliability of data transmission at intersections, MBR novelly utilizes the digital map and the Geohash coding system to relay the packets (frames) in the MAC layer, and selects the optimal relay node based on the relative position of the nodes and the intersection. In this paper, we propose two versions of the algorithm: MBR-S utilizes a single IEEE 802.11p radio; and MBR utilizes a collaboration of the IEEE 802.11p channel and the Sub-1GHz channel, which has border communication range in the intersections than that of MBR-S. Both our real world experiments and extensive simulations demonstrate that MBR-S can significantly improve the performance of the state-of-the-art VANET routing protocols, which can be further enhanced with the collaboration of two channels.

Many researchers have introduced blockchain into the Internet of Vehicles (IoV) to support trading or other authentication applications between vehicles. However, the traditional blockchain cannot well support the query of transactions that occur in a specified area which is important for vehicle users since they are bound to the geolocations. Therefore, the querying efficiency of the geolocation attribute of transactions is vital for blockchain-based applications. Existing work does not well handle the geolocation of vehicles in the blockchain, and thus the querying efficiency is questionable. In this paper, we design a rapid query method of regional transactions in blockchain for IoV, including data structures and query algorithms. The main idea is to utilize the Geohash code to represent the area and serve as the key for transaction indexing and querying, and the geolocation is marked as one of the attributes of transactions in the blockchain. To further verify and evaluate the proposed design, on the basis of the implementation of Ethereum, which is a well-known blockchain, the results show that the proposed design achieves significantly better-querying speed than Ethereum.

Mobile applications based on geospatial data are nowadays extensively used to support people’s daily activities. Despite the potential overlap among nearby users’ geospatial data demands, it has not been feasible to share geospatial data with peer wireless devices directly. To address this issue, we designed a scheme based on vector tiles to organize spatial data and proposed a system named GeoTile for geospatial data caching and sharing. In GeoTile, a tile request from the mobile client relies on multihop communication over intermediate nodes to reach the server. Since GeoTile enables all network nodes to cache and process geospatial data tiles, requests may be handled before they actually reach the server. We implement the GeoTile prototype system and conduct comprehensive real-world experiments to evaluate the performance. The result shows that the GeoTile system can serve vector tiles for users conveniently and friendly. In addition, the caching mechanism based on vector tiles can substantially reduce the response time and network throughput under the wireless multihop scenarios.