Research On Backoff Algorithm In VANET

With the rapid development of traffic and the great improvement of living standard,vehicles become the necessities for people.Although the vehicles provide convenience for us,they also bring non-negligible pressure.Due to this pressure and the limited traditional traffic resources,the traffic system can't effectively work,even resulting in system paralysis on account of such rapid increasing number of vehicles.Hence,vehicular ad hoc network(VANET)comes into being.VANET is a specific wireless ad hoc network and it is also as a foundation of traffic intelligence achievement.VANET mainly adopts two communication modes.The one is the communication between vehicle and vehicle(V2V)and the other one is between vehicle and infrastructure(V2I).It is also responsible for the security and control data transmission,absolutely including the normal data,so as to provide accurate real-time road condition and secure traffic service for divers.However,VANET faces some challenges in terms of communication by the reason of its particularity.Especially in the MAC(Media Access Control)protocol,the backoff algorithm which the vehicles adopt when they compete for the channels plays a significant role for the transmission performance.Hence,this paper studies the backoff algorithm of IEEE 802.11p MAC protocol in the broadcast mode,the mainly work as follows:The paper analyses the scalability requirements of MAC protocol in VANET and summary the disadvantages of common backoff algorithm.Then it proposes a new contention window adjustment based on the improved MARKOV model.This method adds a new status which represents the idle state to the traditional MARKOV model in order to make the model be more adaptive for the various communication scenarios.Meanwhile,using vehicular density to measure traffic congestion,this method combines the stationary distribution and taylor formula to deduce the relationship of the vehicular contention window size and vehicular density.Finally,exploiting the stationary contention window in the 802.11p,the paper designs new contention window adjustment.Specifically,the window will dynamically update and improve the performance of the 802.11p.The simulation shows that the new method provides low collision probability and maintains small delay.Based on the various communication environment and vehicular particular characteristics which are different from other communication nodes,this paper proposes more adaptive backoff scheme:the backoff method based on classification and estimation mechanism.In order to classify the vehicle and remove redundant,firstly it presents the classification strategy which establishes the attribute sets using the number of vehicular neighbor,velocity and stop time as the criteria.Meanwhile,introducing the feedback scheme makes the attribute sets updating with the ever-changing scenarios so as to guarantee the accuracy of classification.Then based on the strategy above,this method calculates the contention window corresponding to the attribute sets and establishes the window reference table.Finally,using the history vehicular states and the HMM estimation mechanism,the window size at the next moment can be predicted and the backoff time also can be ascertained.This scheme possessing adaptivity and predictability comprehensively considers the characteristics of vehicles and the variously restricted communication environments.The simulation proves that this method can greatly optimize the performance of broadcast.