Optimization And Performance Research Of IEEE 802.11p Protocol In Dense VANET Scenarios

VANET is the application of MANET in the field of road traffic and plays an important role in ITS.With the emergence of VANET,people can better control vehicles and surrounding environments in assisting driving,safety warning,and vehicle control,thereby avoiding many traffic accidents.However,the nodes in VANET have the characteristics of fast movement speed and dynamic topology structure,which has high requirements for the timeliness of message transmission.Especially in the vehicular dense network,the contention based CSMA/CA mechanism aggravates the collision between vehicle nodes,causes serious transmission delay,and then causes significant economic losses and casualties to the transportation system.Therefore,ensuring efficient communication in dense VANET scenarios is a critical issue.To address the above problems,this study considers optimizing and improving vehicle communication in two different ways in dense network environments and establishes two-dimensional Markov chain models to analyze the performance of the improvement mechanisms.The specific work is as follows:(1)In the network with dense vehicular nodes,the existing IEEE 802.11 p protocol cannot guarantee effective data transmission efficiency due to the aggravation of collisions between nodes.In order to ensure the efficient transmission of data packets in cluster-based networks,this paper first proposed an improved IEEE 802.11 p protocol combined with TDMA mechanism for dense scenarios.Secondly,the improved mechanism is modeled by a two-dimensional Markov mathematical model,and steady-state probability expressions,system transmission rate,and delay performance evaluation indicators are derived for each state.Finally,the improved mechanism is simulated and evaluated,and the influence of the number of nodes and cluster heads on the system communication performance is studied.To some extent,it can guide the deployment of roadside facilities and the optimization of cluster heads in VANET.(2)In VANET,a fixed contention window is adopted considering the traditional CSMA/CA mechanism.This leads to an increase in collision rate and channel occupancy,which seriously affects key performance such as successful delivery ratio and transmission delay.The existing two technologies,DSRC and C-V2 X,cannot guarantee efficient data transmission.Therefore,an IEEE 802.11 p mechanism based on adaptive primary and secondary window partitioning is proposed to reduce the collision rate of vehicle nodes and increase data transmission rate.The backoff process is divided into two stages: primary and secondary backoff,and the sum of the primary and secondary backoff windows is a fixed value,but the ratio is controlled by parameter BP.Secondly,to accurately analyze the influence of parameter BP and MAC layer parameters on the communication protocol,a Markov chain model is established,and analytical expressions for system average delay and transmission success rate are derived.The influence of system parameters such as BP on system performance is analyzed by simulation,and compared with the classical mechanism,the improved mechanism can obtain better network performance regardless of whether the traffic density is sparse or dense.Finally,a multi-objective optimization model with minimum delay and highest reliability is established to optimize the proposed mechanism and obtain balanced system performance.