Medium Access Control Protocol Design And Implementation For Different Data Types In VANET

With the rapid development of vehicular ad-hoc network(VANET)technology,its role has evolved from a simple means of information exchange between vehicles to a tool of large-scale traffic management.From safety-related applications to infotainment applications,a variety of vehicle networking applications come out one after another.These applications put forward higher requirements on MAC layer protocol design of vehicle networking.The reliability and timeliness of information are closely related to the life safety and user experience of drivers and passengers.How to provide vehicles with high-throughput and low-latency wireless access service has become a problem which need to besolved urgently.In this paper,according to the characteristics of high mobility and rapid change of network topology in vehicular network environment,we propose a data type based wireless access MAC protocol and build a V2 I communication system.This paper first describes the mobility based contention window adjustment algorithm.IEEE 802.11 p protocol uses the traditional binary exponential backoff algorithm,the node will resend data after a random period of time if a collision happens.If the backoff time is too small,the collision may become serious,while if the backoff time is too long,channel resource will be wasted.Therefore,it is crucial to choose the proper contention window size.We propose an algorithm named MCWA(Mobility based Contention Window Adjustment Algorithm)to dynamically change the initial contention window size in the back-off process to match the intensity of the channel competition and obtain the best network performance.Firstly,we use the two-dimensional Markov model to model the back-off process,look for the relationship between the size of the optimal contention window and the number of nodes,and then study the movement of the vehicle,estimate the vehicle density and the number of competing vehicles.Finally,based on the above analysis,the calculation method of suitable contention window size is given,and the performance of the algorithm is simulated by using SUMO and NS-3 synthetically.The simulation results of the highway scene and the urban traffic scenario show that the MCWA algorithm can effectively reduce the collision probability of the message,improve the throughput and communication quality in the high-density environment compared with the original IEEE802.11 p protocol.This paper goes on to detail the design of our data type based MAC protocol.The WAVE protocol divides 75 MHz spectrum into 7 channels,of which control channel(CCH)is used to transmit safety and control messages and service channels(SCHs)are used to transmit nonsafety messages,but no specific channel usage plan is given.As the channel access in WAVE is all based on the contention-based CSMA / CA mechanism,when there are many vehicles that need to send information,the probability of collision of messages increases dramatically,the performance of the network drastically declines,the timeliness and reliability of the security messages can not be guaranteed,seriously affecting traffic safety.Therefore,we propose a protocol named SPA-MAC(Spectrum Penetration Assisted MAC protocol),SPA-MAC protocol takes full advantage of the flexibility of competing schemes and the reliability of distributed schemes.By means of registration and reservation mechanism,we provide non-competitive channel access service for security information and service information to avoid the serious collision of messages in high-density environment,and reduce the message delay.In addition,the use of spectrum penetration mechanism solves the pain point that the safety message can not be sent in the SCH time slot.In view of the characteristics of high mobility of vehicles,the protocol introduces the information of vehicle coordinates for channel assignment optimization.We compare the SPA-MAC protocol with other protocols in terms of packet delivery rate,delay,and throughput in simulation and verify The validity of SPA-MAC.Finally,the paper introduces our V2 I communication system based on Labview.With the help of software defined radio equipments,we develope an OFDM technology based communication system and verify our protocol on it.In addition,the platform not only simulates the bus network in the vehicle,but also has WIFI,Bluetooth,3G / 4G and other external means of communication.The system can not only provide support and experimental tools for related research,but also lay the foundation for further development.