A vehicular ad hoc network (VANET) is a mobile ad hoc network in which network nodes are vehicles—most commonly road vehicles. VANETs present a unique range of challenges and opportunities for routing protocols due to the semi-organized nature of vehicular movements subject to the constraints of road geometry and rules, and the obstacles which limit physical connectivity in urban environments. In particular, the problems of routing protocol reliability and scalability across large urban VANETs are currently the subject of intense research. Clustering can be used to improve routing scalability and reliability in VANETs, as it results in the distributed formation of hierarchical network structures by grouping vehicles together based on correlated spatial distribution and relative velocity. In addition to the benefits to routing, these groups can serve as the foundation for accident or congestion detection, information dissemination and entertainment applications. This paper explores the design choices made in the development of clustering algorithms targeted at VANETs. It presents a taxonomy of the techniques applied to solve the problems of cluster head election, cluster affiliation, and cluster management, and identifies new directions and recent trends in the design of these algorithms. Additionally, methodologies for validating clustering performance are reviewed, and a key shortcoming—the lack of realistic vehicular channel modeling—is identified. The importance of a rigorous and standardized performance evaluation regime utilizing realistic vehicular channel models is demonstrated.

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A Comparative Survey of VANET Clustering Techniques

IEEE 802.11p based Dedicated Short Range Communication (DSRC) has been considered as a promising wireless technology for enhancing transportation safety and traffic efficiency. However, with limited literature available, there is lack of understanding about how IEEE 802.11p performs for vehicleto-vehicle (V2V) communications in urban environments. In this paper, we conduct intensive statistical analysis on V2V communication performance, based on the empirical measurement data collected from off-the-shelf IEEE 802.11p-compatible onboard units (OBUs). We have several key insights as follows. First, both line-of- sight (LoS) and non-line-of-sight (NLoS) durations follow power law distributions, which implies that the probability of having long LoS/NLoS conditions can be relatively high. Second, the packet inter-reception (PIR) time distribution follows an exponential distribution in LoS conditions but a power law in NLoS conditions. In contrast, the packet inter-loss (PIL) time distribution in LoS condition follows a power law but an exponential in NLoS condition. Third, the overall PIR time distribution is a mix of exponential distribution and power law distribution. The presented results provide solid ground to validate models, tune VANET simulators and improve communication strategies.

An Empirical Study on Urban IEEE 802.11p Vehicle-to-Vehicle Communication

In this work, the authors are interested in periodic beacons transmission, the main cause of the Control Channel (CCH) congestion and the major obstacle delaying the progress of safety messages dissemination in vehicular ad hoc networks (VANETs). In order to offload the network, solutions that range from transmit rate to transmit power adaptations including hybrid solutions have been proposed. Although some of these solutions have managed to successfully reduce the load on the wireless channel, none, to the best of the authors knowledge, have considered the impact of the applied adaptation scheme on the overall level of awareness among vehicles and its quality. ETSI TS released a technical specification stating a limit for the minimum beacons transmit rate in order to maintain a good level of awareness among vehicles and ensure a certain accuracy in VANET applications. In this paper, the authors propose to jointly adapt both transmit rate and power in a new smart way that guarantees a strict beaconing frequency as well as a good level of awareness in closer ranges, while maintaining a marginal beacons collision rate and a good level of channel utilisation. First, the transmit rate is adapted to meet the channel requirements in terms of collision rate and channel load; then, once the minimum beacon transmit rate, set by ETSI, has been reached, transmit power is adapted in a way that guarantees a good level of awareness for closer neighbours. The simulation results show a significant enhancement in terms of the quality as well as the level of awareness.

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Smart adaptation of beacons transmission rate and power for enhanced vehicular awareness in VANETs

It is foreseeable that in the few upcoming years, real time traffic information, including road incidents notifications, will be collected and disseminated by mobile vehicles, thanks to their plethora of embedded sensors. Each vehicle can thus actively participate in sharing the collected information with the other peers forming an infrastructure-less self-organising network of vehicles. However, the fast development of applications in ITS field may result in an excessive load on such a network; therefore an efficient use of the available bandwidth is highly required. Not only should the size of the data inserted in the network be properly controlled, but also the extent of each message should be accurately defined. In this paper, we propose a distributed dissemination protocol for safety messages in urban areas, dubbed "Road-Casting Protocol (RCP)", which is based on a novel cooperative forwarding mechanism. Moreover, an accurate definition of the Region of Interest (RoI) (i.e. the geographical scope) of each broadcasted safety message is also devised to ensure better control of the network load. We have evaluated the efficiency of the RCP along with the proposed RoI definition using realistic simulations, based on an accurate propagation loss model for urban vehicular ad hoc network communications, and the obtained results show a substantial improvement, compared to state of the art schemes, in terms of enhanced packet delivery ratio up to higher than 95%, lower end-to-end delay and reduced network load.

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A fast, reliable and lightweight distributed dissemination protocol for safety messages in Urban Vehicular Networks

Periodic beacon messages are one of the building blocks that enable the operation of vehicular ad hoc networks (VANETs) applications. In vehicular networks environments, congestion and awareness control mechanisms are key for a reliable and efficient functioning of vehicular applications. In order to control the channel load, a reliable mechanism allowing real-time measurements of parameters like the local density of vehicles is a must. These measurements can then serve as an input to perform a fast adaptation of the transmit parameters. In this paper, considerable efforts have been directed in the recent years toward designing flexible yet robust protocols solving this problem; yet, very few have considered a proactive adaptation of the transmit parameters as a preventive measure from channel load peaks. To this end, we take the opportunity to introduce prediction and adaptation algorithm (PA and up to 10% and 20% increase in busy ratio compared to our previous scheme and the ETSI schemes, respectively) as well as the achieved level of awareness (higher coverage with higher transmission rate and power in dense scenarios, and up to 8% and 55% improvement in density perception accuracy compared to our previous scheme and the ETSI schemes, respectively).

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An Altruistic Prediction-Based Congestion Control for Strict Beaconing Requirements in Urban VANETs

Accurate modelling of the radio channel is often the most difficult aspect of a Vehicular Ad Hoc Network (VANET) simulation due to the large variability present in vehicular terrain CORNER is a propagation model that calculates path-loss in an urban terrain with a large concentration of buildings, based on the position of the transmitter and receiver on a street map This paper proposes additions to the CORNER propagation model to take selective multi-path fading into account and investigates the performance of the GPSR routing protocol under the CORNER propagation model in a realistic city environment

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Studying the Impact of the CORNER Propagation Model on VANET Routing in Urban Environments

CORNER is an urban propagation model which simulates the presence of buildings in city scenarios and models radio propagation as a series of reflections and diffractions around buildings. CORNER was validated in the original publication with a series of packet-delivery-ratio measurements. However, the accuracy of these measurements is limited by interference from nearby networks. This paper independently evaluates the CORNER model using signal strength measurements across three separate sites in Sydney and Wollongong. The measurements are analysed and compared with the predicted analytical estimates. The fading model is also analysed with direct measurements. A new CORNER link classification algorithm is also proposed in this paper.

Experimental validation of the CORNER urban propagation model based on signal power measurements in a vehicular environment

A method of approximating the Rician K-Factor with considerations of the local human-built environment is proposed for urban VANETs. The model is validated experimentally on a busy street in Australia, in the presence and absence of other vehicles. The model is found to accurately predict actual channel measurements in close-range communications scenarios.

Dynamie environmental fading in urban VANETs

https://opus.lib.uts.edu.au/bitstream/10453/32273/1/main%20%281%29.pdf

Including general environmental effects in K -factor approximation for rice-distributed VANET channels

The CORNER propagation model, first proposed in 2010, has been previously validated and found to be a reasonably accurate representation of propagation scenarios in urban Vehicular Ad Hoc Networks (VANETs). This paper considers the impact of the propagation environment on routing performance and reveals a pressing need to consider more accurate propagation models when designing urban VANET routing protocols. A greedy routing protocol, which uses CORNER's propagation estimates for neighbour selection, is then presented. The new protocol, named Corner Propagation Stateless Routing (CPSR) is compared to GPSR, a benchmark protocol for VANETs, showing between 87% and 300% improvement in packet delivery ratio at higher network loads.

Abhinay Mukunthan

Papers

Studying the Impact of the CORNER Propagation Model on VANET Routing in Urban Environments

Experimental validation of the CORNER urban propagation model based on signal power measurements in a vehicular environment

Dynamie environmental fading in urban VANETs

Including general environmental effects in K -factor approximation for rice-distributed VANET channels

Leveraging the Propagation Model to Make Greedy Routing Decisions in Urban Environments