Investigation On Cooperative Communication Technology Of Medium Access Control In VANET

As an important part of the intelligent transportation system,VANET has greatly improved the safety and transportation efficiency of vehicles and provided passengers with a wide range of in-vehicle infotainment owing to its many mobile distributed applications.However,due to the characteristics of dynamic topology,channel fading,etc.,the channel conditions in VANET environment are worse than in the general wireless environment.Since the media access control protocol directly controls the transmission and reception of data packets on the wireless channel,how to design an effective and reliable MAC protocol has always been the key to the research field of VANET.In recent years,distributed time division multiple access protocol,becoming a hot topic of inquiry,not only meets the strict requirements of VANET security application for service quality but also achieves better channel utilization,network throughput and fairness than the current IEEE802.11 series.Furthermore,with the increasing scarcity of the spectrum resources,there are many bottlenecks in the process to improve the performance of the network simply by exploring point-to-point wireless channel.Cooperative communication can utilize the broadcast of the wireless channel to improve the reliability of the wireless link through the cooperation between nodes.Therefore,this thesis studies the protocol of MAC layer cooperation in VANET and fully exploits the internal mechanism of distributed TDMA.Through the use of the cooperative node idle time slot,multiplexing the cooperative node's own time slot and multiplexing cooperative target node time slot,the research aims to provide more efficient,robust and reliable communication in VANET.The main work and innovative results are as follows:1)A cooperative relay(CR-DTDMA)method using distributed TDMA idle time slots in VANET is proposed.The main drawback of the distributed TDMA protocol in vehicular ad hoc networks is the failure to fully utilize wireless channel resources,such as node idle time slots.When the relay node fails to relay data^the cooperative relay node in the CR-DTDMA cooperates to relay and forward data by using its own idle time slot,so as to fully utilize the node idle time slot and improve spectrum utilization.CR-DTDMA implements a network handshake protocol to determine cooperative relay nodes,by exploring the intermal acknowledgment mechanism of the distributed TDMA protocol.Moreover,the control information of the network handshake protocol is transmitted by the message piggyback mechanism and no special control package needs to be sent.All operations jfrom cooperation decision to cooperation execution are performed in a distributed manner,independent of the central control node,and can adapt to frequent changes of network topology in the VANET.Moreover,the system is easy to implement and the network overhead is small.At the same time,since only the node's own idle time slot resources are utilized,the CR-DTDMA method does not affect the normal data transmission service in the network.The closed-form expressions of successful packet transmission probability,transmission delay and packet dropping rate of the proposed method are derived.The superiority of the CR-DTDMA method is confirmed by simulation comparison.2)A distributed TDMA based cooperative network coding(DTCNC-MAC)method in VANET is proposed.The spatial,temporal and coding diversities of MIMO-NC coding are flxlly studied.A distributed TDMA cooperative network coding communication method is proposed for uplink application scenarios in VANET,by combining the MIMO technology,NC technology and cooperative communication.The cooperative node uses MIMO_NC technology to encode its own service data and failed data transmitted by other nodes,so as to implement the cooperative retransmission of other services while transmitting its own service data,and multiplex the time slot of cooperative nodes to improve spectral efficiency.The proposed method utilizes the distributed TDMA intrinsic mechanism to confirn the data transmission result,and uses the message piggyback mechanism to exchange relevant control information to avoid sending special control packets.The closed-form expressions of successful packet transmission probability,transmission delay and packet dropping rate of the proposed method are derived.Theoretieal analysis and simulation results show that the proposed method can significantly improve the probability of data reception and effectively reduce the data transmission delay and packet dropping rate.3)A distributed TDMA based NOMA cooperative communication in VANET(DTNC-MAC)is proposed.Due to the special performance in solving spectrum efficiency,system capacity and number of user connections,non-orthogonal multiple access technology(NOMA),which is the key technology of 5G wireless networks,has been highly valued by the academic community and the industry.NOMA can serve multiple users in the frequency domain and time domain,thus greatly improving the fairness and efficiency of spectrum utilization.Theoretical analysis,NOMA multiplexing can reach the upper bound of the capacity of the channel.In this section,combining NOMA and distributed TDMA technology,a new distributed TDMA based non-orthogonal multiple access cooperative transmission scheme is proposed for specfrum sharing.In DTNC-MAC,cooperative nodes transmit coded message in the source node slot,which superposed with the main network and cooperative network messages using NOMA technology,achieving a wim-win situation between the main network and the eooperative network,thus stimulating the cooperative desire of the cooperative nodes.Secondly,the analytical expressions of the outage probability,ergodic sum rate and system throughput of the primary network and the cooperative network are given.The high signal-to-noise ratio asymptotic expression of the outage probability is further derived.Finally,the simulations show the effectiveness of the scheme and the accuracy of the analysis results.