Research On Efficient Message Dissemination Protocol In Urban Vehicular Networks

As a fundamental component of intelligent transportation system(ITS),vehicular ad hoc networks(VANETs)play an important role in helping drivers avoid potential hazards in a timely manner and improving the driving experience.Message dissemina-tion is one of the most important applications in VANETs,in which vehicles can deliver safety messages to region of interest(ROI)quickly and accurately and also share non-safety data that improves driving comfort.Therefore,this thesis mainly focuses on the research on efficient message dissemination protocols in urban vehicular networks.In order to deliver messages to a wider range,the existing safety message dissemi-nation protocols employ a multi-hop broadcast schemes.By selecting part of vehicles to forward messages can greatly reduce the probability of message collision and improve the channel utilization.The existing protocols mainly consider factors such as distance,vehicle density,speed of vehicles when selecting candidate forwarders.However,sig-nal propagation experiences severe attenuation from obstacles especially at intersec-tions.Therefore,how to comprehensively describe the impact of physical channels and deal with the message dissemination at intersections faces great challenges.When dis-seminating non-safety messages with large amount of data,the existing protocols take advantage of the huge bandwidth resources provided by millimeter-wave frequency band.However,how to achieve efficient beam alignment in the dynamic vehicular networks,how to optimize network throughput through multi-hop point-to-multipoint message dissemination,and how to carry out concurrent transmission opportunistically among vehicles still face many challenges.In order to efficiently disseminate messages in VANETs,an adaptive link-quality based safety message dissemination scheme is pro?posed and mmWave/DSRC is used to realize the dissemination of massive non-safety messages.(1)Considering the complexity of the urban environment,a comprehensive phys-ical channel connectivity calculation method is proposed for accurately estimating the connectivity probability among vehicles.Combined with large-scale free-space loss,mesoscale shadow effect and small-scale fading effect,the wireless channel is described in an all-round way.A score-based priority allocation mechanism for candidate for-warders(CFs)is proposed to coordinate the contention among CFs.Finally,this thesis calculates the minimum waiting time and contention window size for each vehicle in CFs,With regard to message dissemination at intersections,an alternate polling chan-nel access mechanism is proposed to ensure that vehicles located in different streets access wireless channel fairly.Simulation results show that our proposed scheme can effectively reduce the time delay while ensuring higher message receiving rate.(2)The core problem in millimeter-wave(mmWave)vehicular communications is how to increase the network throughput.Most of the existing protocols in mm Wave vehicular communications only focus on one-hop point-to-point message dissemination and adopt a fixed beamwidth.Although they also considered the concurrent transmis-sion of messages,there is still room for further improvement of network throughput.In this thesis,a multi-beamwidth configuration is adopted at the sender,which dynami-cally selects the best beam according to the distribution of its neighbors.In order to in-crease the coverage area of the network,relay vehicles are used to forward messages and the possibility of concurrent transmissions among several point-to-multipoint mm Wave links are investigated at the same time.Finally,the network throughput can be further increased by using Device-to-Device(D2D)message transmission as a supplementary mechanism.The simulation results show the superiority of our proposed protocol.