Study On The Model And Optimization Methods Of Information Broadcasting In VANET

Road safety and traffic efficiency are two crucial problems in current transportation systems. Vehicular ad hoc networks (VANET), as an application of mobile ad hoc networks (MANET) in Intelligent Transportation Information Systems, is developed to provide people with safer and more efficient mode of transportation. With its remarkable ability to extend the horizon of drivers and on-board devices, VANET is expected to reduce the dramatically high number of accidents and their fatal consequences and thus to improve road safety and traffic efficiency.Research institutes and automobile industries in EU, US, Japan and other developed countries have made various plans to conduct wide and in-depth researches on VANET applications in Intelligent Transportation Information Systems. The design of effective broadcast protocols has become the most important area in VANET research for its effectiveness in the distribution of traffic information such as warning of danger, cooperative driving, and briefing on traffic flow.In this dissertation, the characteristics and application areas of VANET are first briefly introduced, then through analyses and comparison, the characteristics, performance, and application areas of various broadcast protocols are discussed and finally, in accordance with the characteristics of VANET and application requirements of transportation information dissemination in highways, four key topics of broadcasting protocols are proposed through theoretical modeling and Numerical Analysis.(1) In the third chapter, a discrete markov analytic model has been proposed for performance evaluation of IEEE 802.11 broadcasting protocol under saturation condition for vehicular ad hoc networks in highways. Through analyses, we can study the relationship between two groups of parameters, namely, the number of vehicle nodes, the contention windows, the hidden/exposed vehicle nodes and the communication reliability, the system throughput, the broadcasting delay. So, how to optimize networks throughput and enhance communication reliability is one of key issues for designing effective broadcasting protocol.Numerical analysis shows that the difference of throughput depends on the network size and that there is an optimum contention window size which maximizes broadcast throughput performance. Therefore, we deduce the relationship between the optimum contention window and the number of network broadcasting nodes, for broadcast communication can be used to derive a maximum throughput solution which can be used to design broadcasting protocol in VANET.(2) In the fourth chapter, the author firstly analyzes some factors which affect the reliability of broadcasting in highways. To enhance the reliability of broadcasting transportation information by IEEE 802.11 broadcasting protocol, backoff contention windows may be set large enough. However, scaling up the contention window may negatively impact the system throughput, leading to the so called reliability-throughput tradeoff. So, based on multi-object optimization scheme, the Adaptive reliability single-hop broadcast protocol (ARSB) is proposed, and simulation results show that ARSB not only enhances broadcast reliability and satisfies the broadcasting information real-time'needs, but also optimizes broadcast throughput by adapting backoff contention windows of broadcasting nodes.(3) Multi-hop broadcasting of safety messages is one of the fundamental services in VANET. Broadcast protocols have to guarantee high reliability and low propagation delay of the information all over the network. Therefore, in the fifth chapter, the author firstly analyzes network characteristics of VANET in highways and QoS of transportation information. Then, the author proposes a multi-hop forward and carrier protocol (MH-FC) similar to UMB for highway VANET. It basic idea is that the broadcasting nodes try to select the furthest node in the broadcast direction to set up point-to-point communication scheme, so that broadcast storm, hidden node and reliability problems in multi-hop broadcast may be solve. At the same time, a carry and forward scheme is proposed to solve"informantion isolated point".To evaluate performance of MH-FC, theoretical modeling is proposed based on applied probability. Then, by simulations and Numerical Analysis, the author evaluates the broadcast protocol performance and compares it with other algorithms. The author concludes that MH-FC not only ensures QoS of transportation information broadcasting, but also improves scalability and Robustness of multi-hop protocol.(4) In urban and highways settings, vehicular ad hoc networks hierarchy may enhance efficiency of networks resources and protocol. Previous research results show that the stable clustering methods can reduce the networks of communication relay and provide for a more efficient hierarchical network topology. Chapter six proposes weight-based stability clustering algorithm, in which stability neighbor sets are considered as a vehicular node weighted factor and an index of clustering re-affiliation. Meanwhile, to ensure networks load-balancing performance, by applying optimized algorithm, an optimized clusterhead (CH) sets is obtained. Simulation experiments are conducted to evaluate the performance of our algorithm in terms of the number of CH, re-affiliation frequency and dominant set updates. Simulation results show that our algorithm performs better than previous research work.