Heterogeneous vehicular communications: A comprehensive study

Abstract

Vehicular communications have developed rapidly contributing to the success of intelligent transportation systems. In VANET, continuous connectivity is a huge challenge caused by the extremely dynamic network topology and the highly variable number of mobile nodes. Moreover, message dissemination efficiency is a serious issue in traffic effectiveness and road safety. The heterogeneous vehicular network, which integrates cellular networks with DSRC, has been suggested and attracted significant attention recently. VANET-cellular integration offers many potential benefits, for instance, high data rates, low latency, and extended communication range. Due to the heterogeneous wireless access, a seamless handover decision is required to guarantee QoS of communications and to maintain continuous connectivity between the vehicles. On the other hand, VANET heterogeneous wireless networks integration will significantly help autonomous cars to be functional in reality. This paper surveys and reviews some related studies in the literature that deals with VANET heterogeneous wireless networks communications in term of vertical handover, data dissemination and collection, gateway selection and other issues. The comparison between different works is based on parameters like bandwidth, delay, throughput, and packet loss. Finally, we outline open issues that help to identify the future research directions of VANET in the heterogeneous environment.

Introduction

Millions of persons are killed every year around the world in the road accidents. According to the World Health Organization (WHO) reviews fact sheet on road traffic injuries (9 May 2016), around 1.25 million people die as a result of road traffic crashes each year (3400 deaths per day) [1]. Furthermore, the forecasts are even worse; it is estimated that by 2020 road traffic crashes are predicted to increase to become the 7th leading reason for death [2]. This obviously demonstrates that it has been a challenge to stop these accidents, which mean urgent actions and intensive efforts are required to prevent and reduce car accidents as well as improving road safety.

In order to save lives, money, time, and the environment, the Intelligent Transportation System (ITS) has been recently attracted both academia and industry attention. It is the hope of such technologies that countries such as Japan and Sweden have publicly announced an objective of reaching “zero traffic fatality” societies by 2020 [3]. Intelligent Transportation Society of America (ITSA) summarizes its mission declaration as "vision zero" meaning its objective is to decrease the fatal accidents and delays as much as possible [4]. Recent development in automobiles and wireless communication technologies have enabled the evolution of ITS which addresses numerous vehicular traffic issues like information dissemination and traffic congestion.

Vehicular Ad-hoc Network (VANET) is an integral element of ITS in which moving vehicles are connected and communicate wirelessly. Wireless communication technologies play an essential role in assisting both Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure communication (V2I) in VANET. V2V communication allows vehicles to communicate with each other and to share information regarding their state (e.g., position, velocity, acceleration, etc.) or information about the traffic (e.g., state of traffic lights, accidents, traffic jams, the line works, etc.). However, V2I communication allows the cooperation between road infrastructure and vehicles.

Moreover, VANETs are used to support safety-critical applications and non-safety infotainment or entertainment based applications. Safety applications such as collision avoidance, pre-crash sense or lane changing are aimed to minimize road accidents by using traffic monitoring and management applications. Non-safety applications enable passengers to access various services like interactive communication, internet access, payment services, online games and information updates when vehicles are on the move.

Research on VANET led to the approval of the IEEE 802.11p standard [5], [6], [7] as an amendment to the well-known IEEE 802.11. The enhanced version IEEE 802.11p forms the standards for Wireless Access in Vehicular Environments (WAVE) at a frequency of 5.9 GHz. WAVE protocols (IEEE 802.11p/1609) provide interoperability between wireless devices On-Board Unit (OBU) of vehicles and infrastructure situated near the roads, Road Side Unit (RSU). Thus, V2V and V2I communications can be established in the vehicular network. Despite the significant research that has been done on IEEE 802.11p, it suffers from scalability issues, limited coverage area, and unbounded delay.

On the other hand, cellular networks have been developed in recent decades. Some of the disadvantages related to IEEE 802.11p, and the potential for the use of existing networks, have motivated researchers to investigate the possibility of using cellular networks in vehicular applications instead of IEEE 802.11p. By reviewing the characteristics of different access technologies, it is concluded that the cellular network is the best option as an alternative to IEEE 802.11p for supporting vehicular applications. Some of the distinguishing characteristics associated with LTE are high data rate, high spectral efficiency, and low latency in the control plane [8].

In order to ensure vehicles to access the network, even in places uncovered by RSUs, existing radio access networks such as cellular networks (3 G/LTE) and Wi-Fi may be employed to improve vehicular communications. The potential impact of heterogeneous wireless networks has been confirmed by an ever-increasing amount of mobile internet traffic, which cannot solely be absorbed by cellular data communication networks. Then, it can form heterogeneous vehicular networks that can be a combination of VANETs and cellular networks for vehicular communications.

Seamless handover is the first necessary step when internet connection needs to migrate between heterogeneous networks. The necessity for vertical handover can be initiated for convenience rather than connectivity reasons (e.g., according to user choice for a specific service). Two of the major challenges in vertical handover management are seamlessness and automation aspects in network switching.

Major car companies, governmental organizations, and the academic community have recognized the increasing importance of interworking over VANETs [9]. Many government projects have been implemented in USA, Japan, and the European Union. The federal communications commission has allocated spectrum for Inter-Vehicle Communications (IVC) and similar applications [10]. Governments and prominent industrial companies, such as Toyota, BMW, and Daimler-Chrysler, have started important projects for IVC communications. Advanced Driver Assistance Systems (ADASE2) [11], Crash Avoidance Metrics Partnership (CAMP) [12], Chauffeur in EU [13], CarTALK2000 [14], FleetNet [15], California Partners for Advanced Transit and Highways (California PATH) [16], and DEMO 2000 by Japan Automobile Research Institute (JSK) are few notable projects, which are a significant step for the realization of intelligent transport services.

In the recent years, few studies have surveyed the heterogeneous vehicular communications. Most of the existing survey researchers focus on either the overview of ITS or a single network [8], [38], [39], [40], [41], [42]. Wu et al. [179] addressed the challenges of using Dedicated Short-Range Communication (DSRC) for vehicular communications and proposed solutions. A comprehensive survey of vehicular ad-hoc networks is presented in [20]. The authors in [180] discussed the challenges and the solutions of connected vehicles. Furthermore, the heterogeneity between different networks is essential as each system offers their unique benefits [181]. Since the vehicular network environment is highly dynamic, Viriyasitavat et al. in [48] analyzed the appropriate channel and propagation models for this heterogeneous system. Some studies tackled the different vehicular applications [49], [50], [51]. Additionally, the capability of LTE supporting vehicular applications is briefly assessed in [8]. On the other hand, Worldwide Interoperability for Microwave Access (WiMAX) network has been proposed to cope with the coverage problem in VANETs [38]. Studies in [8] and [39] focused on the use of LTE in vehicular networks over heterogeneous wireless networks. Vinel et al. in [39] compared 3GPP LTE and IEEE 802.11p/WAVE technologies to find which technology can support cooperative vehicular safety applications. Mane and Junnarkar in [52] surveyed the techniques and fundamentals of internet access in VANET-Internet integration scenarios. They dedicated to improve the performance of mobile gateways and data collection for giving priority to emergency messages dissemination. Moreover, the recent study in [40] reviewed V2I communication over heterogeneous multi-tier with diverse Radio Access Technology (RAT) network environments. Shahid et al. in [54] presented a survey about different VANET technologies; whereas, a comparison between UMTS and LTE for vehicular safety communication at intersections shown in [55]. Besides, Mir et al. in [56] presented a hybrid communication system between LTE and WAVE protocol. However, hybrid approaches suitable for heterogeneous vehicular communication combining both LTE and 802.11p were proposed in [44]. Various LTE-VANET collaborations were offered in [45], [46], [47]. The study in [36] by Zheng et al. concluded that the heterogeneous vehicular networking with LTE for V2I communications and DSRC for V2V communications is one of the best solutions for supporting vehicular services.

In the current study, we present a comprehensive overview of vehicular communications in the heterogeneous environment. Therefore, in contrast with existing surveys in the literature, this work focuses on comparing various solutions proposed by the VANET research community in term of handover, data dissemination, gateway selection, QoS and other concerns. The comparison between different works is based on metrics like bandwidth, throughput, delay, communication overhead, channel capacity and packet loss. Besides, we also explore some open issues for future investigations in heterogeneous vehicular communications. Many challenges need to be addressed to make vehicular communications vision a commercially viable reality in 5 G networks and autonomous cars. This work would motivate VANET researchers, automakers and newcomers to develop VANET- heterogeneous wireless networks integration technology.

The remainder of this paper is structured as follows. Section 2 presents VANET architecture and background. The standards for wireless access in VANET are described in Section 3. Section 4 delineates VANET characteristics briefly. Possible heterogeneous vehicular communications scenarios are discussed in Section 5. Section 6 surveys VANET heterogeneous wireless networks in different classifications such as vertical handover, data dissemination and collection, gateway selection and others. An autonomous car review is given in Section 7. Open issues, challenges, and future research directions are discussed in Section 8. Section 9 concludes the paper.