Research And Implementation Of MAC Layer Protocol In V2V Network System

Nowadays,the urban transportations and automobile industry in our country is developing rapidly.While people are enjoying the convenience brought by vehicles,they are also facing a series of problems such as traffic jam,safety accidents and so on.In order to alleviate or solve the current contradictions,ITS(Intelligent Transportation Systems)came into being.V2X(Vehicle to Everything),a hot research topic in the field of global Intelligent Transportation,which is used to obtain a series of traffic information,such as real time road conditions,road information,pedestrian information,and so on,is one of the key technologies of future ITS,including important communication technologies such as V2V(Vehicle to Vehicle),V2I(Vehicle to Infrastructure),V2P(Vehicle to Person),etc.The V2 V network is entirely self organized by adjacent vehicle nodes,needing no help of the fixed information access point.Therefore,the demand of V2 V network for infrastructure is not high,and the networking of V2 V is flexible and convenient.In the case of the infrastructure not being popularized yet,V2 V has a more urgent and realistic research significance,which has aroused the widespread concern of many researchers.At present,the most widely used protocol standard in V2 V is WAVE(Wireless Access for Vehicular Environments),which is made by the special wireless access technical standard group of American Association of electrical and electronic engineers,mainly containing IEEE 1609 series protocol and IEEE 802.11 p protocol.This paper mainly studies the IEEE 802.11 p protocol applied to the MAC(Media Access Control)layer of V2 V network and the main work is as follows:1)Import the IEEE 802.11 p protocol code developed by Sherbrooke University in the NS2-2.31 environment into NS2-2.34,in order to research on the performance of broadcasting periodic beacon message on CCH(control channel)and unicasting traditional IEEE 802.11 message on SCH(service channel).And finally point out the shortcomings of the IEEE 802.11 p protocol in the V2 V network environment through simulation and analysis.2)In order to optimize the broadcasting performance of V2 V network based on the IEEE 802.11 p protocol,the Markov chain model of broadcast state on CCH is established.And then an existing algorithm referred to optimizing the minimum contention window is discussed,whose purpose is to minimize the collision probability based on the number of competitive nodes.But considering that the Markov chain model and algorithm did not take the influence of hidden terminals and expired messages into account,and the real-time performance of the algorithm is not so good,yet an improved algorithm combined with the idea of RBEB(Reserve Binary Exponentially Backoff)algorithm and an improved algorithm considering the time delay before nodes get chance to access channel are proposed.3)In V2 V unicasting mode,the BEB(Binary Exponential Backoff)algorithm of IEEE 802.11 p protocol could not adapt well to the environment with larger node density.After making an in-depth mathematical analysis on Bianchi's two-dimensional Markov chain model,two adaptive competition window adjustment algorithms are proposed: one of them is based on the times of retransmission and the evaluation number of competitive nodes,another is based on the optimal(2 and the evaluation number of competitive nodes.Both of these two algorithms estimate the network environment to a certain extent and adjust their backoff strategy or initial contention window adaptively,and their comprehensive performances are all better than the original BEB algorithm.