Cooperative adaptive cruise control (CACC) systems have the potential to increase traffic throughput by allowing smaller headway between vehicles and moving vehicles safely in a platoon at a harmonized speed. CACC systems have been attracting significant attention from both academia and industry since connectivity between vehicles will become mandatory for new vehicles in the USA in the near future. In this paper, we review three basic and important aspects of CACC systems: communications, driver characteristics, and controls to identify the most challenging issues for their real-world deployment. Different routing protocols that support the data communication requirements between vehicles in the CACC platoon are reviewed. Promising and suitable protocols are identified. Driver characteristics related issues, such as how to keep drivers engaged in driving tasks during CACC operations, are discussed. To achieve mass acceptance, the control design needs to depict real-world traffic variability such as communication effects, driver behavior, and traffic composition. Thus, this paper also discusses the issues that existing CACC control modules face when considering close to ideal driving conditions.

A Review of Communication, Driver Characteristics, and Controls Aspects of Cooperative Adaptive Cruise Control (CACC)

Driven by both safety concerns and commercial interests, one of the key services offered by vehicular networks is popular content distribution (PCD). The fundamental challenges to achieve high speed content downloading come from the highly dynamic topology of vehicular ad hoc network (VANET) and the lossy nature of the vehicular wireless communications. In this paper, we introduce CodeOn, a novel push-based PCD scheme where contents are actively broadcasted to vehicles from road side access points and further distributed among vehicles using a cooperative VANET. In CodeOn, we employ a recent technique, symbol level network coding (SLNC) to combat the lossy wireless transmissions. Through exploiting symbol level diversity, SLNC is robust to transmission errors and encourages more aggressive concurrent transmissions. In order to fully enjoy the benefits of SLNC, we propose a suite of techniques to maximize the downloading rate, including a prioritized and localized relay selection mechanism where the selection criteria is based on the usefulness of vehicles' possessed contents, and a lightweight medium access protocol that naturally exploits the abundant concurrent transmission opportunities. We also propose additional mechanisms to reduce the protocol overhead without sacrificing the performance. Extensive simulation results show that, under a wide range of scenarios, CodeOn significantly outperforms a state-of-the-art PCD scheme based on network coding.

CodeOn: Cooperative Popular Content Distribution for Vehicular Networks using Symbol Level Network Coding

In this paper, we survey recent results in vehicular ad hoc networks (VANETs) data dissemination. We describe methods proposed to enforce dissemination scope such as geocast/broadcast and multicast. A growing category consisting of methods designed to achieve disruption tolerance in vehicular networks is presented. We describe the key ideas of representative technologies in each category. In addition, we consider location service and security issues that are crucial for data dissemination in VANET. We conclude by sharing our thoughts on further challenges. Copyright © 2009 John Wiley & Sons, Ltd.

A survey and challenges in routing and data dissemination in vehicular ad hoc networks

A primary goal of intelligent transportation systems is to improve road safety. The ability of vehicles to communicate is a promising way to alleviate traffic accidents by reducing the response time associated with human reaction to nearby drivers. Vehicle mobility patterns caused by varying traffic dynamics and travel behavior lead to considerable complexity in the efficiency and reliability of vehicular communication networks. This causes two major routing issues: the broadcast storm problem and the network disconnection problem. In this article we review broadcast communication in vehicular communication networks and mechanisms to alleviate the broadcast storm problem. Moreover, we introduce vehicular safety applications, discuss network design considerations, and characterize broadcast protocols in vehicular networks.

Broadcasting safety information in vehicular networks: issues and approaches

In vehicular ad hoc networks (VANETs), multi-hop wireless broadcast has been considered a promising technology to support safety-related applications that have strict quality-of-service (QoS) requirements such as low latency, high reliability, scalability, etc. However, in the urban transportation environment, the efficiency of multi-hop broadcast is critically challenged by complex road structure, severe channel contention, message redundancy, etc. In this paper, we propose an urban multi-hop broadcast protocol (UMBP) to disseminate emergency messages. To lower emergency message transmission delay and reduce message redundancy, UMBP includes a novel forwarding node selection scheme that utilizes iterative partition, mini-slot, and black-burst to quickly select remote neighboring nodes, and a single forwarding node is successfully chosen by the asynchronous contention among them. Then, bidirectional broadcast, multi-directional broadcast, and directional broadcast are designed according to the positions of the emergency message senders. Specifically, at the first hop, bidirectional broadcast or multi-directional broadcast conducts the forwarding node selection scheme in different directions simultaneously, and a single forwarding node is successfully chosen in each direction. Then, directional broadcast is adopted at each hop in the message propagation direction until the emergency message reaches an intersection area where multi-directional broadcast is performed again, which finally enables the emergency message to cover the target area seamlessly. Analysis and simulation results show that the proposed UMBP significantly improves the performance of multi-hop broadcast in terms of one-hop delay, message propagation speed, and message reception rate.

A Multi-Hop Broadcast Protocol for Emergency Message Dissemination in Urban Vehicular Ad Hoc Networks

Dedicated short range communications (DSRC) enable novel automotive safety applications such as an extended electronic brake light (EEBL) or intersection collision avoidance (ICA). These applications require reliable wireless communications even in scenarios with very high vehicle density, where these networks are primarily interference-limited. Given the uncertainties associated with current simulation models, particularly their interference models, it is critical to experimentally validate network performance for such scenarios. Towards this goal, we present a systematic, large- scale experimental study of packet delivery rates (PDRs) in a dense environment of 802.11 transmitters. We show that even with 100 transmitters in communication range with a frame size of 128 B and a bit-rate of 6 Mb/s, 1) most receivers can decode over 1500 packets per second (pps) in a saturated network, which corresponds to a PDR of 45% and 2) the mean PDR, for 10 pps per node workload that emulates vehicular safety applications, is about 95%. These results demonstrate that a COTS 802.11 implementation can correctly decode many packets under collision due to physical layer capture (PLC) and can serve as a reference scenario for validation of network simulators.

Experimental analysis of broadcast reliability in dense vehicular networks

Dedicated short range communications (DSRC)-based communications enable novel automotive safety applications such as an extended electronic brake light or intersection collision avoidance. These applications require reliable wireless communications even in scenarios with very high vehicle density, where these networks are primarily interference-limited. Given the uncertainties associated with current simulation models, particularly their interference models, it is critical to experimentally validate network performance for such scenarios. Towards this goal, we present a systematic, large-scale experimental study of packet delivery rates in a dense environment of 802.11 transmitters. We show that even with 100 transmitters in communication range with a frame size of 128 bytes and a bit-rate of 6Mbps, (a) most receivers can decode over 1500 pps in a saturated network, which corresponds to a packet delivery rate of 45% and (b) the mean packet delivery rate, for 10 pps per node workload that emulates vehicular safety applications, is about 95%. These results demonstrate that a COTS 802.11 implementation can correctly decode many packets under collision due to physical layer capture and can serve as a reference scenario for validation of network simulators.

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R. Onishi

Papers

Experimental analysis of broadcast reliability in dense vehicular networks

Experimental Analysis of Broadcast Reliability in Dense Vehicular Networks