Research On Beacon Message Transmission Scheme In Vehicular Ad Hoc Networks

Vehicular Ad Hoc Network(VANET),which applies wireless communication to transportation system,has attracted extensive attention in global academic and industry.It is considered as one of the key technologies to protect traffic safety,improve traffic efficiency,and solve the energy and pollution problems in the Intelligent Transportation System(ITS).In VANET,beacon message is the most basic and important message type.The beacon message contains the driving state and the communication state of the communication unit,including vehicle unit and roadside unit.Communication unit connect with each other through the periodic beaconing.However,the variety of vehicle density,network topology and wireless channel brings more challenges to the assurance of the reliability and real-time requirement of the beaconing,and this will directly affect the performance of VANET.Concerning these challenges,kinds of studies have been proposed.Researchers try to find proper information dependencies and optimization approaches to improve the beaconing performance.Existing studies focus on a simple VANET environment without considering the cluster-based environment and the environment with RSUs.Moreover,how to quickly adapt to the great change of the vehicle density,how to reduce the energy consumption of the beaconing,and how to improve the sluggishness of the existing scheme,are worth exploring for the promotion of the beaconing scheme.This thesis focuses on the optimization of the beaconing scheme and tackles the aforementioned issues as follows:1.We study a quick number evaluation method of the cluster members(CMs)in the cluster-based VANET environment.The evaluation result can be used as the information dependency required by a beaconing scheme for decision making.The traditional method evaluates the number of the CMs through checking the member list of the cluster head(CH).It is time consuming and even fails to work when the vehicle density is high.To tackle this problem,we proposed a time slot occupation-based method which can evaluate the number quickly.Specifically,1)we introduce the basic principles of the time slot occupation-based method and the detailed method to determine reasonable parameters.The validity and the accuracy of the method are also discussed.The efficiency of our method has also been proved;2)we propose an adaptive evaluation method which can further reduce the evaluation time by introducing an interrupt mechanism;3)we analyze the effects of the channel state for the proposed'evaluation methods.By choose the reasonable parameters,we can guarantee the validity,accuracy and efficiency of the evaluation in the non-ideal channel.2.With considering the energy consumption,we study a new beaconing scheme in the VANET environment with RSUs.For a RSU,it transmits the beacon messages periodically with the traditional beaconing scheme.When the vehicle density is low,it can be thought as a waste of energy since most of these beacon messages are not received by the vehicles.As the existence of the Markov property of the presence of vehicle has been validated,the problem of determining the optimal beaconing interval to improve the energy efficiency and to combine the decreased energy consumption and the decreased data rate is well motivated.Specifically,1)we present the state transition probabilities for the Markov model by using the maximum likelihood method.Meanwhile,we calculate the quantity requirement of the samples with which the modeling can guarantee a certain accuracy based on the central limit theorem;2)we obtain the relationship between the beaconing interval and the expectation of a valid transmission opportunity discovery interval,and then we present the optimal beaconing interval with different initial states;3)we introduce a satisfaction degree based on the Sigmoid function to combine the decreased energy consumption and the decreased data rate effectively.Then the optimal beaconing interval is determined by balancing the tradeoff among all the system parameters.3.We study an adaptive beaconing scheme based on the short-time traffic environment parameters prediction in VANET.The traditional method that adapts the future beaconing scheme based on existing parameters is thought to be delayed as the topology of VANET changes fast.To tackle this problem,we propose an adaptive beaconing scheme based on the traffic environment parameters prediction.It can improve the beaconing performance effectively.Specifically,1)we propose a new average vehicle quantity estimation method.With considering the fair utilization of the channel,we propose a judgment process about whether to initiate a beaconing scheme adaption or not;2)according to the Autoregressive Integrated Moving Average Model(ARIMA),we build the traffic patterns based on the historical traffic environment parameters.By matching the traffic pattern,the future parameters prediction is achieved;3)we propose an adaptive beaconing scheme by combining the beacon message generation frequency and beaconing power control with predicted parameter.