Research On Message Dissemination Technology For Vehicular Ad Hoc Networks

Vehicular Ad Hoc Networks(VANET),which is regarded as a significant component of intelligent transport system,plays an important role in improving road safety,traffic efficiency and driving experience.However,due to the varying network density,channel fading,rapid change in network topology and high mobility in VANET,it is challenging to guarantee the reliability of message dissemination.For example,the broadcast of periodic message may cause message congestion and the forwarding of message could lead to intense collision.The transmission can also be interrupted during the multi-hop broadcast.As a result,for both theoretical research and practical implementation,it is of great value to investigate how to disseminate message with high reliability in VANET.The main work and contributions in this thesis are as follows,1.An improved Markov chain model was proposed to accurately model the periodic broadcasting under different network density;adaptive Media Access Control(MAC)protocols,which were based on network busy factor and Bayesian network,were proposed to improve the packet delivery ratio for periodic broadcasting message.Firstly,an improved Markov chain model that can depict periodic broadcasting message both in low and high density network was proposed.This model can differentiate the message arrival situation during the busy slot and idle slot.It can also describe the periodic broadcasting more accurately,so packet collision probability was analyzed more precisely.Secondly,network busy factor,which is used to determine network communication state,was defined according to this model,and the adaptive MAC protocol based on network busy factor was proposed.In order to utilize vehicle internal information,external environment information and network performance parameter to more accurately depict network communication state in complicated urban environment,Bayesian network was introduced to infer network communication state according to the aforementioned context-aware information,and the context-aware adaptive MAC protocol based on Bayesian network was proposed.Finally,simulations were presented to verify the performance of the proposed two kinds of adaptive MAC protocols.Simulation results showed that the proposed enhanced Markov chain model can be used to accurately model the periodic broadcasting under different network density.Besides,the two kinds of adaptive MAC protocols outperformed the basic protocol in terms of packet delivery ratio,satisfying the requirement of low delay and high reliability for periodic broadcasting message.2.Since line-of-sight(LOS)transmission is obstructed in VANET,a decentralized cooperative relay selection scheme was proposed to overcome the challenges arising from intense collision resulting from inherently synchronized relays.The proposed scheme can effectively alleviate packet collision;a mathematical analytical model was designed for the proposed scheme,and the closed-form expression for end-to-end packet delivery ratio and the analysis expression for throughput were derived.Firstly,a new score function was designed to prioritize the relays based on their reception quality from source and channel conditions towards intended destination.Then,the waiting retransmission time was calculated based on the sorting results.Secondly,three kinds of time-out probabilities were deduced according to the waiting retransmission time.Thirdly,the closed-form expression for end-to-end packet delivery ratio and the analytical expression for throughput were deduced with consideration of wireless channel conditions and MAC operation.Finally,the proposed scheme was validated by simulations.The analyses and simulation results showed that the proposed scheme outperformed the benchmark scheme in term of packet delivery ratio and throughput,and it is suitable for reliable retransmitting message when LOS is obstructed.The analytic expressions can be used for the evaluation of the designs related to MAC and physical layers in VANET.3.A relay selection scheme based on Vehicle-to-Vehicle(V2V)multi-hop broadcast was proposed,where channel fading,high mobility and the reliability requirement of safety message dissemination were taken into account,and packet loss ratio was decreased effectively;a mathematical model was designed for the proposed scheme,and the closed-form expressions for average packet loss ratio and rebroadcast distance were also derived.Firstly,combined the advantages of the sender-oriented relay selection protocol and receiver-oriented relay selection protocol,the best candidate relay zone was decided according to the direction of message dissemination and the best principle(which means that the average packet loss ratio in broadcast zone is the lowest).Waiting retransmission time of the candidate relay was allocated uniformly and randomly afterwards.Secondly,the probability for each candidate relay becoming the relay was derived with taking into account wireless channel conditions,MAC operation and the mobility of vehicle.Thirdly,the closedform expressions for average packet loss ratio and rebroadcast distance were deduced with consideration of the number of candidate relays and relay locations,the joint distribution of inter-vehicle distance and the relay probability.Finally,the proposed scheme was validated by simulations.The analyses and simulation results showed that the proposed scheme outperformed the benchmark scheme in term of average packet loss ratio and rebroadcast distance.The analytic expressions can be used to decide the best number of candidate relay and contention window size,or design the related parameters for physical layers when the aforementioned best parameters are given in VANET.