| Vehicular Ad hoc Network,VANET,is a kind of specific mobile ad hoc network,whose nodes are mainly vehicles equipped with electronic and sensing facilities.With the aid of modern communication technologies,Intelligent Transportation System can be established on top of VANET to provide varieties of services,including traffic safety improvement,traffic efficiency enhancement,comfortable driving experience,and even unmanned driving.As an important application field of IoT,it has been highly valued by industry,academic circles and government worldwide.Wireless communication and networking technologies are key to VANET communications,which face great challenges due to its intrinsic characteristics,e.g.high mobility of network nodes,high diversity of node density,dynamitic change of network topology,and intermittent disconnection of network.Short-range radio communication based DSRC and cellular communication based C-V2 X are two complement mainstream technological framework for VANET communications,in which DSRC has laid foundation and some market deployment.The study of the dissertation focuses on VANET data delivery networking technology under the framework of DSRC,with emphasis played on efficient and reliable routing technique and improved multi-channel MAC technique.The research work and its academic contributions include:(1)Study is made on the routing problem for cross street block delivery between vehicle nodes under the urban VANET environment with traffic light control.Targeting to the minimized end to end data delivery delay,a traffic light sensed shortest delivery time routing algorithm STTLAR and its improved version E-STTLAR are proposed.Based on the Manhattan pattern of urban area,the optimization model is established for the target problem.Using the theory of VANET vehicle distribution,the analysis is given of the vehicle node distribution on the road segment.The wireless connectivity probability of the road segment is derived under the control of traffic light.Following the above theoretic study work,the equivalent distance metric of road segment is constructed,considering the integration of road segment wireless connectivity probability,stability of road segment connectivity,and normalized distance between segment exit and the destination.Its evaluation formula is also provided.An algorithm is then designed for the selection of next road segment.Considering the integration of wireless link quality,the length of nodal data transmit queue,and the next hop distance to the current node,the equivalent relay node distance metric is constructed with its evaluation formula.An improved greedy algorithm is designed for the selection of next hop node.The emulation results have shown that the proposed algorithms are superior to some existing VANET routing algorithms in terms of end to end delay and datagram delivery ratio under the condition of different traffic light period,the number of vehicle nodes and datagram transmit rate.The improved algorithm E-STTLAR is better than STTLAR in terms of the performance of end to end delay,network yield and normalized routing overhead.(2)Study is made on the efficient VANET multicast routing technique to solve the problem of data multicast delivery between member vehicle nodes of a group.With the information of the predictable vehicle driving route and location provided by on board navigation and positioning facility,a VANET cost aware multicast routing algorithm CAM with low bandwidth consumption is proposed,involving the multicast relay of trusted member vehicles.Referring to the Manhattan pattern of urban area,a mathematical model is established for the target problem of delivery cost during the process of datagram wireless delivery.The theoretic analytical results are provided of the direct encountering and indirect encountering sets based on the planned vehicle driving route.The vehicle datagram delivery contribution metric is introduced and its evaluation formula is provided.Following the above theoretic research work,a multicast routing tree construction algorithm is designed considering the integration of delivery cost and nodal contribution metric.Analysis is made of the datagram forwarding mode to decrease the delivery cost within the constraint of some delivery performance.An optimized multicast routing tree construction algorithm is designed through minimization of the ratio between delivery cost and nodal contribution metric.The emulation results have shown that the proposed algorithms can effectively decrease the traffic volume of wireless communications by maximally use of carry-and-forward mode within the limit of allowable datagram delivery delay.In comparison to some commonly used VANET broadcast/multicast routing algorithms,the proposed algorithms are superior in terms of average delivery cost,average data delivery ratio and average data delivery delay under the condition of different datagram TTL constraint,the number of vehicle nodes and vehicle moving speed.(3)Targeting to effectively raising the utilization ratio of wireless channel,a two-level distributed multi-channel MAC protocol TLDM-MAC is proposed,which can be used in the application environment with large number of vehicles competing for the limited wireless resources to deliver data.A two-level distributed channel structure is proposed with control periods distributed in both frequency and time domains.Based on some allocation rule,the control periods are placed in disjoint time segments on different channels with increment of timing sequence,effectively raising the channel utilization ratio.The concept of home channel is introduced and homing rule and transmission rule are defined to increase the success ratio of node Rendezvous with no increase of control overhead.Following the above theoretic modeling study work,detailed protocol design is provided.A 4-step transmission resource negociation handshake process is designed with decision making by the sending part.This allows the adjacent nodes to have a consistent resource view in an ad hoc way,ensuring non-competing reliable data transmission without the problems of hidden terminal and exposed terminal.A slot-hopping retransmission mechanism and a slot-hopping revision mechanism are proposed to cope with the situation of reserved resource conflit due to the possible missing overhearing of resource state information,further enhancing the reliability of data transmission.A multicast resource negociation process is defined based on the available transmission resource view of mlticast group members under the coordinated control of the sending part,increasing the V2 V multicast communication efficiency to the maximum extent.Also defined are a selection rule for broadcast transmission resource and a twice broadcasting mechanism for important information,supporting the reliable transmission of broadcast information.An improved backoff mechanism design scheme is proposed to make the backoff time associated with the ratio of reserved data slots of the sending part in case of control slot conflict or data transmission failure,achieving a better fairness of transmission resource usage.The emulation results have shown that the proposed protocol is superior to some existing VANET MAC protocols in terms of channel utilization ratio,network throughput,data delivery delay,and multicast service support. |
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