Research On Back-off Algorithm For Beacon Message In VANET

The Vehicular Ad-hoc Networks(VANETs)is based on Mobile Ad hoc Networks(MANET)technology,which aims to reduce traffic accidents and congestion through the interaction of vehicle information.The beacon message is the most basic and important message type in the VANETs,including the driving and communication status of the vehicle node.The VANETs has the characteristics that the topology changes drastically,the vehicle nodes move fast,and the wireless channel environment is complex.This reduces the broadcast performance of beacon messages.Researchers want to solve this problem by looking for suitable beacon message broadcast optimization basis and method.This thesis studies the back-off mechanism of the Media Access Control(MAC)layer of VANETs,and analyzes the problems of the MAC layer protocol of VANETs in detail.The MAC protocol of VANETs is based on IEEE 802.11.Its problem is that the vehicle cannot dynamically adjust the back-off strategy according to the vehicle density,resulting in poor performance of the transmission performance in high-density scenes.Therefore,the main r work of this thesis is as follows:The vehicle density is one of the important indicators for assessing road traffic conditions,and it also has a great impact on the transmission performance of VANETs.After acquiring the density of the surrounding vehicles,the vehicles can develop a corresponding back-off mechanism according to the vehicle density to improve the performance of the in-vehicle network.Therefore,this thesis proposes a vehicle density estimation mechanism.The vehicle can establish and maintain a neighbor node information table according to the received beacon message,and calculate the number of its one-hop and two-hop neighbor nodes through the information in the neighbor node table.The vehicle density estimation model is established according to the random distribution function of the vehicle,and the optimal estimated density is obtained by finding the optimal solution of the estimation model.According to the vehicle density calculated from the vehicle density estimation mechanism,this thesis proposes an adaptive backoff scheme based on vehicle density.An optimal initial contention window selection mechanism is proposed in the scheme,which enables the vehicle to quickly select an initial contention window according to the vehicle density.This mechanism can improve the communication efficiency of the vehicle with different vehicle densities.Based on this,a backoff algorithm based on network busy factor is designed,which enables the vehicle to further adjust the contention window according to the network state after selecting the initial contention window.The simulation results show that the proposed scheme has better performance in the receiving rate of beacon messages.Although the average packet delay of new scheme is slightly larger than that of BEB and DCW,it still less than the broadcast period.Therefore,the proposed scheme can be applied to improve the broadcast performance of VANETs.