Research On Time Division Multiple Access Based MAC Protocol For Vehicular Ad Hoc Networks

Recently,the dramatic penetration of vehicles brought in severe transportation safety problems.Due to the superiority to enhance the on-road safety via supporting vehicle-to-vehicle(V2V),vehicle-to-infrastructure(V2I),and vehicle-to-pedestrian(V2P)communications,Vehicular Ad hoc Networks(VANETs)have been considered as a significant component in future intelligent transportation systems(ITSs).Efficient Medium Access Control(MAC)protocols are one of the key factors to provide real-time and reliable on-road services.However,the existing MAC protocols in the traditional mobile ad hoc networks are not applicable in VANETs due to the highly dynamic nature.Therefore,VANET MAC protocol designing is a highly motivated researching field and attracts much studying interests.Time Division Multiple Access(TDMA)MAC protocols are proven to have great potential for supporting real-time and reliable on-road safety services.However,to fully realize their potential,two problems are needed to be solved.The first one is how to analyze the performance of the TDMA MAC protocols theoretically,and the second one is to design efficient TDMA MAC protocols which are applicable for the dynamic environments of VANETs.To deal with these two problems,this dissertation studies the performance analysis and designing TDMA MAC protocols in VANETs.There are four parts in the dissertation:(1)the analytical model for evaluating the end-to-end latency;(2)the vehicular mobility information based collision avoidance;(3)the flexible frame length based TDMA MAC protocol;(4)the TDMA protocol for the hybrid network architecture.The first part investigates the analytical models for TDMA MAC protocols.The existing analytical models include some impractical assumptions which make them be inconsistent with the mechanisms of the real protocols.The proposed analytical model takes the effects of packet queuing,collision detection,slot acquisition,and re-transmission into account.Thus,it is more practical than the existing ones.In addition,the analytical results provide some insights on parameter configurations,and reveal the directions for the performance improvement.Based on the analytical results of the first part,the collision problems incurred by vehicle mobility severely affect the latency of on-road safety services.Therefore,the second part proposes the mobility information based TDMA MAC protocol.The proposed protocol selects some vehicular nodes to work as the temporal central nodes when a collision happens,and allows these temporal central nodes to allocate time slots based on the obtained vehicular mobility information.Simulation results show that the collision problems are alleviated effectively with the help of the temporal central nodes.The third part focuses on the scenarios with the varying vehicle densities,where the fixed frame length protocols are no longer applicable.Therefore,a flexible frame length based protocol is proposed which can predict the MAC layer end to end mean latency after adjusting the frame length.Based on the prediction results,the protocol makes the choice with the lowest prediction value.Simulation results show the proposed scheme attains lower mean transmission latency.In the last part,a TDMA protocol is designed for the hybrid network architecture where the centralized and decentralized networks co-existed in the scenario.The slot reservation rules are defined in the protocol to avoid the collisions between the nodes in the Road Side Unit(RSU)coverage area and out of the area.In the proposed scheme,the RSU estimates the number of competitors and the vehicle arrival rate in its coverage area in order to obtain the vehicle density.Then,the proposed scheme adjusts all fields of the frame to adapt the current vehicle density.Simulation results show that the protocol can provide a lower handoff latency,and guarantee that the vehicles on-line all the time.Then,the vehicles can broadcast the safety-related messages timely.