Research On Application-Oriented Multichannel MAC Protocol In VANETs

Vehicular Ad Hoc Networks(VANETs)utilize the information interaction between vehicles and vehicles and between vehicles and infrastructures,to improve the safety and comfort of vehicles on the road.The reliable Media Access Control(MAC)strategy is a guarantee,which meets the different transmission requirements of various messages in specific applications.However,due to highly dynamic topology,variable traffic density and different transmission requirement of applications,it is difficult for VANETs to meet the Quality of Service(QoS)requirements of various applications.According to the requirements of different applications in VANETs,this thesis models and analyzes the multichannel MAC protocol,and the multichannel MAC protocol design and optimization of VANETs are thus illustrated.Therefore,the key issues such as efficient transmission of various types of messages,multichannel coordination and resource allocation in different traffic density environments are solved.The main contributions are as follows:(1)According to the unique features of VANETs,such as high-speed mobility of vehicles,rapid changes in topology,and different application requirements,a multichannel integrated model of VANETs is proposed,which includes the environmental model,the physical layer model of IEEE 802.11p and the multichannel model of IEEE 1609.4,and it describes unsaturated network conditions,virtual collision,hidden terminal,channel fading and channel switching in detail.The proposed model analyzes packet transmission delay,packet reception rate and throughput in detail,which is closer to the real communication environment.This model provides the foundation for deep understanding of the problems in VANETs' multichannel MAC protocol,analyzing various performance indicators,optimizing and improving the protocol under vehicular environment.(2)A multichannel MAC protocol for VANETs(AOCM)is proposed based on statistics.Accoding to real-time statistics and prediction for different types of messages coming from upper application layer,the AOCM protocol could derive the optimal Control Channel(CCH)interval to ensure the timely and reliable transmission of safety messages on the CCH.On the Service Channels(SCHs),the proposed protocol relies on a reservation mechanism to achieve contention-free transmission of non-safety messages,so as to improve the throughput of SCHs.Therefore,the AOCM protocol solves the limitation of IEEE 1609.4 protocol in supporting delay-or throughput-sensitive applications.The proposed protocol was suitable for both saturation and unsaturation conditions,and has low computational complexity and high scalability.Extensive simulation results demonstrate the superiority of the AOCM protocol compared with other existing protocols.(3)A Reliable,Efficient and QoS supported multichannel MAC(REQ-MAC)protocol is proposed.On control channel,under the coordination of the road side unit,each node can reserve the time slot for transmitting the safety messages without exchanging additional information,and the contention-free transmission and the bounded transmission delay of safety messages can be achieved.The contention-free transmission of safety messages can leave more time for non-safety messages to perform service channel negotiation and reservation.The introduction of forwarding mechanism ensures the reliable transmission of coordination information.On service channel,non-safety messages can be transmitted throughout the whole synchronization interval,which solves the problem of low channel utilization and waste of channel resources.Each node dynamically adjusts the contention window size of the WSA(Wireless Access in Vehicular Environments Service Advertisement)package according to the traffic density,and it can thus achieve the predefined throughput ratio between non-safety packages of different priorities,which solves the problem of different QoS communication requirements for non-safety applications under high vehicle density changes.Theoretical model analysis and extensive simulation results show that the REQ-MAC protocol can guarantee reliable and timely transmissions of safety messages,and support QoS differentiation based on ensuring high saturation throughput and low transmission delay for non-safety applications.