Performance Analysis And Optimization Of MAC Protocol For Vehicular Ad Hoc Networks

A vehicular ad hoc network(VANET)is a special network that applies the mobile ad hoc network(MANET)to traffic scenarios.In this case,VANET has extra characters and more transmission problems than MANET,including real road scenes,high-density distribution of nodes,and rapidly moving nodes.These characters directly impair the VANET's transmission performance.A focus of research in recent years is to propose an appropriate medium access control(MAC)protocol to improve the VANET's transmission performance.This dissertation studies the TDMA(Time Division Multiple Access)based and contention based MAC protocol in vehicular ad hoc networks.In the TDMA based MAC protocol,a game based time slot reservation allocation algorithm is proposed to deal with the time slot reservation conflict problem on the control channel in the high density network.A cooperative transmission MAC protocol is proposed to solve the problem that the idle nodes may occupy the time slots on the service channel.Considering the existence of multiple transmission modes in heterogeneous vehicular network,a new cooperative transmission MAC protocol is proposed which insists both V2 V and V2I/I2 V transmission.In contention based MAC protocol,considering the different delay requirements of multiple types of services in the network,an adaptive backoff algorithm is proposed to differentiate the priority of services and optimize the system total delay.The main achievements of this research are as follows:(1)Considering the different delay requirements of multi-type data communication in vehicular ad hoc networks,an adaptive transmission backoff algorithm for vehicular ad hoc networks is proposed.The algorithm optimizes the backoff parameters according to different delay requirements and the number of vehicles in oeder to avoid drastic changes in the size of competition window caused by emergencies.A two-dimensional Markov chain is established to analyze the performance of backoff algorithm,the expressions of network throughput and delay are derived.The optimization problem aiming at minimizing system delay is constructed and sloved by particle swarm optimization.The simulation results show that the proposed algorithm can reduce the average delay,and all kinds of data are guaranteed to meet the corresponding delay requirements.(2)Considering the time slot reservation conflict on the control channel in vehicular ad hoc networks,a game based time slot reservation algorithm(GAH-MAC)is proposed.This algorithm provides two strategies for the conflicting nodes when the reservation conflict occurs: reserve the original conflict slot again or reserve the new idle time to obtain higher priority.Conflict nodes use game theory to maximize their utility by choosing different strategies(the successful probability of slot reservation).Each node sets a waiting counter to record the number of times it abandons the reservation of the original conflict slot,the waiting counter value is directly related to the utility value of each node.By distinguishing different priorities,nodes will choose different strategies with different probabilities.By establishing Markov model,the speed of reservation slot is analyzed,and the theoretical upper and lower bounds of reservation slot speed are deduced.The simulation results show that the algorithm can reduce the problem of continuous reservation conflict,improve the success rate of slot reservation,reduce the packet loss rate in high-density network,and effectively improve throughput.(3)Considering the idle nodes occupying time slot on the service channel in vehicular ad hoc networks,a TDMA based cooperative transmission MAC protocol is proposed.This protocol can insist multi-mode cooperative transmission.Vehicle nodes will dynamically change the mode of cooperative transmission according to the relay node and their own storage queue length.The queuing models of normal nodes,cooperative nodes and relay nodes are analyzed differently.One-dimensional Markov chain is used to construct the queuing model of ordinary nodes,one-dimensional state-dependent queuing model is used to construct the queuing model of cooperative nodes,and two-dimensional state-dependent queuing model is used to construct the queuing model of forwarding nodes.According to the queuing model,the distribution of storage queue length is deduced iteratively and the expressions of throughput and packet drop rate are obtained.The simulation results show that the protocol can effectively increase network throughput,reduce packet loss rate and transmission delay.(4)In order to reduce the data delay of secure services in heterogeneous vehicular networks with multi-type data communication,a hybrid transmission protocol for vehicular networks is proposed in this dissertation.This protocol can insist multi-mode transmission,cooperative transmission is provided by the base stations to optimize the transmission delay of V2 V data.The queuing model of vehicle nodes and base stations is modeled and analyzed differently,the queuing model of V2 V adopts one-dimensional Markov model,the V2 I queuing model uses one-dimensional queuing model with priority,the queuing model of base station is analyzed by two-dimensional queuing model with priority,and the expressions of data average queue length are obtained.The simulation results show that the protocol can effectively reduce the transmission delay and packet loss rate of V2 V data,which can improve the performances of safe driving.