Research On IEEE 802.11p-based MAC Protocol For Vehicular Ad Hoc Network

Vehicular ad hoc network(VANET)is one of the most ideal communication and networking technologies.It allows the development of a large amount of safety applications and non-safety applications.As the key technology of VANET,Media access control(MAC)protocol can directly determine the wireless network performance such as channel utilization,time delay,fairness and congestion,etc.It has been the focus of VANET research.However,VANET has the characteristics of high network density,high mobility of nodes,which brings sever challenges for the design and implementation of MAC protocol.IEEE 802.11 p is the one of the most used MAC protocol standard in vehicular network.If the network load is high,its access performances about delay and throughput deteriorate seriously.It cannot solve the unfair problem to access channel caused by different vehicle speed.In addition,because of the shortage of acknowledgement and retransmission mechanisms,its broadcast mode cannot assure the reliability and effectiveness of the transmitted broadcast packets.Therefore,this paper focuses on improving the access performance of vehicular network such as throughput,reliability and fairness,etc.IEEE 802.11 p MAC protocol is deeply studied and optimized.The main works and contributions are as follows:Firstly,VANET and its related MAC layer protocol(IEE 802.11p&IEEE 1609.4)have been researched.The architecture,features,related access technologies,main applications and simulation tools are summarized.The standards of physical layer and MAC layer of IEEE 802.11 p protocol are described in detail.This paper points out the existing problems of the common back-off algorithms,and its improved strategies.Then,the paper introduces describes multi-channel coordination mechanism provided by IEEE 1609.4,illustrates the state information broadcasting mode on control channel and its defects.Secondly,a mobility-based back-off algorithm with optimal window(MBAOW)is proposed to solve the unfair problem for vehicles to access channel due to different speed in V2 I communications.The best contention window can be calculated according to vehicle's own speed and the network sate information,which can be used to contend wireless channel and improve the fairness due to vehicle speed difference.The mathematical analysis and simulation results show that the system throughput can be maximized,the unfairness problem due to the difference of vehicles' speed can be well solved,the realibility can be improved by using the proposed algorithm.Thirdly,a QoS-assurance-and-mobility-based EDCA mechanism(QAM-EDCA)is proposed.As for the communicatioins with different priority traffics,on the premise of meeting the real-time of two high priority traffics,the best contention windows are selected to maximize the throughput of two low priority traffics.Simultaneously,based on the thought of MBAOW and vehicles' mobility,the contention windows are further adjusted.Thus,fairness for two low priority traffics to access channel can be well improved.Both theoretical analysis and simulation results verify the superiority of the proposed mechanism.Finally,areliable broadcast protocol for VANET safety applications is proposed.It consists of three mechanisms: The first one is called geographical-location-based asynchronous time division multiplexing(GLATDM).In it,a road is divided into different sub-areas.Vehicles in different sub-areas try to send broadcast packets in different time.Thus,the hidden and exposed terminal problems can be effectively overcome.The second scheme is named as channel-state-feedback-based back-off(CSFB).Based on it,vehicles state information whether their broadcast packets have been sent successfully can be notified by vehicles which sent packets previously.Thus,the protovol has the acknowledgement and retransmission scheme.The last mechanism is called as adaptive dynamic contention window(ADCW).Using it,the size of CW can be adjusted adaptively according to the nodes number in a sub-area.Thereby,the collision probability declines,the broadcast finish time is decreased greatly.Moreover,the corresponding mathematical model is established.The simulation results show that the shortage of IEEE 802.11 p broadcast mode can be well improved.