Network Connectivity-oriented Research And Design For VANETs

As one of the key technologies of the intelligent transportation system,Vehicular Ad Hoc Networks(VANETs)can provide the ideal and useful communication networking technology for intelligent transportation.It will play an important role to ensure traffic safety,efficiency,commerce and entertainment.Due to the widely application prospect of VANETs,it is the profound significance for the development of traffic informationization to study the VANETs' network design and implementation.At present,research on safety applications in VANETs is currently a hot topic in the field of academia and industry.Network connectivity is the basis and premise of applications of VANETs,and it includes physical connectivity and logical connectivity,involving the physical link connectivity,the effectiveness of MAC protocols and routing protocols,etc.This thesis mainly studies physical link connectivity,MAC layer protocols,and VANET/cluster research and design for VANETs,providing the theoretical reference for the application and implementation of VANETs.The main work and contributions include the following four aspects:(1)Research on the related theoretical basis and key techniques of VANETs.The related theoretical basis and key techniques are studied for network connectivity of VANETs,involving link connectivity in the physical layer and logical connectivity in the MAC layer.With the basis of the analysis of the VANETs network architecture,key technologies in the physical layer are studied and their influences on network performance are analyzed,including channel model,vehicle movement model,cooperative communication and network coding.Then,the key technical characteristics and problems of MAC layer are discussed and analyzed,including multi-channel management,channel access mechanism,and broadcast mechanism.(2)An analysis method of physical link connectivity based on LOS/NLOS(line-of-sight and non-line-of-sight)of VANETs is proposed.Link models and connectivity performance of the PHY layer in VANETs are studied to solve the problems of network topology changes and non-stable physical links caused by the high-speed vehicles' mobility and complex communication environment in VANETs.Considering the influences of wireless channel fading models,vehicle movement models,and the existence of the moving one between two communication vehicles,an 1-hop link connectivity model based on LOS/NLOS(line-of-sight and non-line-of-sight)of VANETs is proposed,derived,and simulated.The results show this proposed method is more conform to the actual influences of vehicles' real communication environment,and is more accurate and more realistic to reflect the specific impacts of communication environment for communication links.(3)A MAC broadcast optimization protocol integrated backoff mechanism and beacon message generation rate is presented.Aiming at problems of high broadcast packet loss rate and delay caused by complex communication environment in VANETs,a MAC broadcast optimization protocol is proposed.And this proposed protocol uses an 1-D saturated markov chain model to describe the messages(including beacon and emergency messages)broadcast backoff mechanism,and uses an adaptive control method of beacon message generation rate to solve channel congestion problem when the traffic density is high.Two performance parameters,packet delivery rate and transmission delay,are derived and verified with simulation.The simulation results show that the optimized protocol can ensure the transmission time slot for emergency messages and the packet delivery rate for beacon messages,and effectively reduce the load impact with the vehicle density increasing,which is an effective solution to ensure broadcast performances of safety messages.(4)A comprehensive design and analysis method for VANET is presented.In VANETs,the repeated recombination of cluster will produce excessive signaling overhead and result in the decrease of the stability and reliability of the cluster.To solve these problems,a comprehensive design and analysis method is proposed,which integrates three important factors into one model: MAC protocol operations,PHY layer wireless channel conditions,and the moveing pattern of vehicles.Closed-form expressions of network performance measures,i.e.,packet loss probability and system throughput,are derived.The proposed analytic model,validated by simulations,is able to accurately characterize and quantize the VANET performance parameters,and can determine key parameters of appropriate cluster size,the scope of a typical network,and the appropriate data flow control in the real vehicular environment.The simulation and analysis results show that this method is a new method to get key efficient parameters quickly in real VANETs environment,and can be a valid support for VANETs design and management and maintaining the acceptable performance.