A wide variety of applications for road safety and traffic efficiency are intended to answer the urgent call for smarter, greener, and safer mobility. Although IEEE 802.11p is considered the de facto standard for on-the-road communications, stakeholders have recently started to investigate the usability of LTE to support vehicular applications. In this article, related work and running standardization activities are scanned and critically discussed; strengths and weaknesses of LTE as an enabling technology for vehicular communications are analyzed; and open issues and critical design choices are highlighted to serve as guidelines for future research in this hot topic.

LTE for vehicular networking: a survey

Vehicle-to-anything (V2X) communications refer to information exchange between a vehicle and various elements of the intelligent transportation system (ITS), including other vehicles, pedestrians, Internet gateways, and transport infrastructure (such as traffic lights and signs). The technology has a great potential of enabling a variety of novel applications for road safety, passenger infotainment, car manufacturer services, and vehicle traffic optimization. Today, V2X communications is based on one of two main technologies: dedicated short-range communications (DSRC) and cellular networks. However, in the near future, it is not expected that a single technology can support such a variety of expected V2X applications for a large number of vehicles. Hence, interworking between DSRC and cellular network technologies for efficient V2X communications is proposed. This paper surveys potential DSRC and cellular interworking solutions for efficient V2X communications. First, we highlight the limitations of each technology in supporting V2X applications. Then, we review potential DSRC-cellular hybrid architectures, together with the main interworking challenges resulting from vehicle mobility, such as vertical handover and network selection issues. In addition, we provide an overview of the global DSRC standards, the existing V2X research and development platforms, and the V2X products already adopted and deployed in vehicles by car manufactures, as an attempt to align academic research with automotive industrial activities. Finally, we suggest some open research issues for future V2X communications based on the interworking of DSRC and cellular network technologies.

Interworking of DSRC and Cellular Network Technologies for V2X Communications: A Survey

With the rapid development of the Intelligent Transportation System (ITS), vehicular communication networks have been widely studied in recent years. Dedicated Short Range Communication (DSRC) can provide efficient real-time information exchange among vehicles without the need of pervasive roadside communication infrastructure. Although mobile cellular networks are capable of providing wide coverage for vehicular users, the requirements of services that require stringent real-time safety cannot always be guaranteed by cellular networks. Therefore, the Heterogeneous Vehicular NETwork (HetVNET), which integrates cellular networks with DSRC, is a potential solution for meeting the communication requirements of the ITS. Although there are a plethora of reported studies on either DSRC or cellular networks, joint research of these two areas is still at its infancy. This paper provides a comprehensive survey on recent wireless networks techniques applied to HetVNETs. Firstly, the requirements and use cases of safety and non-safety services are summarized and compared. Consequently, a HetVNET framework that utilizes a variety of wireless networking techniques is presented, followed by the descriptions of various applications for some typical scenarios. Building such HetVNETs requires a deep understanding of heterogeneity and its associated challenges. Thus, major challenges and solutions that are related to both the Medium Access Control (MAC) and network layers in HetVNETs are studied and discussed in detail. Finally, we outline open issues that help to identify new research directions in HetVNETs.

Heterogeneous Vehicular Networking: A Survey on Architecture, Challenges, and Solutions

In this paper, the problem of joint power and resource allocation (JPRA) for ultra-reliable low-latency communication (URLLC) in vehicular networks is studied. Therein, the network-wide power consumption of vehicular users (VUEs) is minimized subject to high reliability in terms of probabilistic queuing delays. Using extreme value theory (EVT), a new reliability measure is defined to characterize extreme events pertaining to vehicles’ queue lengths exceeding a predefined threshold. To learn these extreme events, assuming they are independently and identically distributed over VUEs, a novel distributed approach based on federated learning (FL) is proposed to estimate the tail distribution of the queue lengths. Considering the communication delays incurred by FL over wireless links, Lyapunov optimization is used to derive the JPRA policies enabling URLLC for each VUE in a distributed manner. The proposed solution is then validated via extensive simulations using a Manhattan mobility model. Simulation results show that FL enables the proposed method to estimate the tail distribution of queues with an accuracy that is close to a centralized solution with up to 79% reductions in the amount of exchanged data. Furthermore, the proposed method yields up to 60% reductions of VUEs with large queue lengths, while reducing the average power consumption by two folds, compared to an average queue-based baseline.

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Distributed Federated Learning for Ultra-Reliable Low-Latency Vehicular Communications

As we edge closer to the broad implementation of intelligent transportation systems, the need to extend the perceptual bounds of sensor-equipped vehicles beyond the individual vehicle is more pressing than ever. Research and standardization efforts toward vehicle to everything (V2X), technology is intended to enable the communication of individual vehicles with both one another and supporting road infrastructure. The topic has drawn interest from a large number of stakeholders, from governmental authorities to automotive manufacturers and mobile network operators. With interest sourced from many disparate parties and a wealth of research on a large number of topics, trying to grasp the bigger picture of V2X development can be a daunting task. In this tutorial survey, to the best of our knowledge, we collate research across a number of topics in V2X, from historical developments to standardization activities and a high-level view of research in a number of important fields. In so doing, we hope to provide a useful reference for the state of V2X research and development for newcomers and veterans alike.

V2X Access Technologies: Regulation, Research, and Remaining Challenges

Inter-vehicle communication promises to prevent accidents by enabling applications such as cross-traffic assistance. This application requires information from vehicles in non-line-of-sight (NLOS) areas due to building at intersection corners. The periodic cooperative awareness messages are foreseen to be sent via 5.9 GHz IEEE 802.11p. While it is known that existing micro-cell models might not apply well, validated propagation models for vehicular 5.9 GHz NLOS conditions are still missing. In this article, we develop a 5.9 GHz NLOS path-loss and fading model based on real-world measurements at a representative selection of intersections in the city of Munich. We show that (a) the measurement data can very well be fitted to an analytical model, (b) the model incorporates specific geometric aspects in closed-form as well as normally distributed fading in NLOS conditions, and (c) the model is of low complexity, thus, could be used in large-scale packet-level simulations. A comparison to existing micro-cell models shows that our model significantly differs.

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5.9 GHz inter-vehicle communication at intersections: a validated non-line-of-sight path-loss and fading model

Vehicular safety communication promises to reduce accidents by assistance systems such as cross-traffic assistance. The information exchange is mostly foreseen to be handled via Dedicated Short Range Communication (DSRC). At intersections, DSRC reception is likely to be problematic due to Non-Line-Of-Sight reception conditions. Alternatively, the required information exchange could also be handled via cellular systems. While cellular systems provide potentially better coverage, they impose other performance constraints. This paper analyzes the suitability of UMTS and LTE for cross-traffic assistance as a worst case application in terms of load and latency demands. It investigates capacity and latency characteristics and discusses influence factors on performance as well as operational aspects. The focus is on the random access performance of the uplink channel. While cellular systems might have some advantages over DSRC, the study shows that UMTS will likely suffer from capacity limitations while LTE could perform reasonably well.

A comparison of UMTS and LTE for vehicular safety communication at intersections

Vehicular Dedicated Short Range Communication (DSRC) promises to reduce accidents by enabling assistance systems such as cross-traffic assistance. This application requires movement information from vehicles that are in Non-Line-Of-Sight (NLOS) due to buildings at intersection corners. DSRC is foreseen to use IEEE 802.11p to deliver regular Cooperative Awareness Messages (CAM). Due to the high operational frequency of 5.9GHz, the ability to provide reliable NLOS reception is often put into question.

We performed an extensive field test specifically targeted to measure DSRC NLOS reception quality. As a novelty, tested intersections were methodically selected to reflect typical urban intersections (in Munich) and the test setup was specifically designed to provide comparable and generalizable results. This allowed us to determine the influence of factors like building positions. The collected data shows that NLOS reception is possible. Reception rates stay mostly well above 50% for distances of 50 meters to intersection center with blocked LOS.

Real-world measurements of non-line-of-sight reception quality for 5.9GHz IEEE 802.11p at intersections

Inter-vehicle communication promises to prevent accidents by enabling applications such as cross-traffic assistance. This application requires information from vehicles in Non-Line-of-Sight (NLOS) areas due to building at intersection corners. The periodic Cooperative Awareness Messages (CAM) are foreseen to be sent via 5.9 GHz IEEE 802.11p. While it is known that existing micro-cell models might not apply well, validated propagation models for vehicular 5.9 GHz NLOS conditions are still missing. In this paper, we develop a 5.9 GHz NLOS path-loss and fading model based on real-world measurements at a representative selection of intersections in the city of Munich. We show that a) the measurement data can very well be fitted to an analytical model, b) the model incorporates specific geometric aspects in closed-form as well as normally distributed fading in NLOS, and c) the model is of low complexity, thus, could be used in large-scale packet-level simulations. A comparison to existing micro-cell models shows that our model significantly differs.

A validated 5.9 GHz Non-Line-of-Sight path-loss and fading model for inter-vehicle communication

Vehicular Dedicated Short Range Communication (DSRC) promises to reduce accidents by enabling assistance systems such as cross-traffic assistance. This application needs movement information from vehicles that are potentially in Non-Line-Of-Sight (NLOS) due to buildings at intersection corners. The amount of NLOS is determined by the placements of buildings. From another point of view, buildings influence the DSRC wireless channel by creating radio signal reflections and diffraction. Here, existing NLOS path-loss models for cellular communication with below roof-top base stations hint that the spatial placement of buildings has influence on the NLOS reception quality. This paper analyzes building positioning in the City of Munich with respect to DSRC. In a first step, it is investigated how much the line-of-sight is blocked by buildings. In a second step, the location of buildings is abstracted and the resulting data clustered to find a set of representative buildingposition scenarios. The presented scenario knowledge can be used in future research to configure meaningful simulations that investigate the radio channel at intersections under load and to select representative intersections for radio propagation field tests.

Thomas Mangel

Papers

5.9 GHz inter-vehicle communication at intersections: a validated non-line-of-sight path-loss and fading model

A comparison of UMTS and LTE for vehicular safety communication at intersections

Real-world measurements of non-line-of-sight reception quality for 5.9GHz IEEE 802.11p at intersections

A validated 5.9 GHz Non-Line-of-Sight path-loss and fading model for inter-vehicle communication

Vehicular safety communication at intersections: Buildings, Non-Line-Of-Sight and representative scenarios