Research On MAC Layer Access Technology For VANETs

The fifth generation of mobile communications(5G)has proposed a new network architecture,bringing a broader world of the internet of things(IoT),and stimulates the coordinated development of artificial intelligence(AI),cloud computing,big data and other fields,and opens a new era of ubiquitous perception and connectivity of the IoTs.As one of the largest application scenarios in the 5G era,the internet of vehicles(IoV)is an important part of the construction of intelligent transportation and smart city,and has become one of the hot spots in the academic and industrial research.The potential commercial application capability of IoVs has greatly promoted the development of technologies.Especially,the mobile communication technology will improve and correct from the aspects of physical layer,access layer,network layer in order to deal with problems in IoV netwoeks,which caused by the rapid movement of vehicles.For access technologies in IoV,vehicles must quickly discover adjacent vehicles and establish communication links with them to exchange information.However,the high mobility of vehicles leads to the rapid change of the network topology and the instability of the communication link,and thus causes sharp decreases in communication reliability and effectiveness.In particular,the communication technology in the IoVs not only provide video or audio entertainment services,but also must provide safety-related services to deal with the problems such as rear-end collision or safety warning in complex driving environment,which further demands lower communication delay and higher reliability.Therefore,we must design a reasonable access mechanism to enable vehicles to acquire channel resources more effectively.This paper mainly studies the access technology from the following three aspects:Firstly,aiming at the low utilization rate of communication resources caused by uneven distribution of nodes and frequent topological changes in VANET,this paper proposes a media access control(MAC)protocol based on adaptive frame structure to improve the utilization rate of resources.Firstly,the CCH frame is divided into adaptive safety message broadcast period and service channel negotiation period,which based on time division multiple access and backoff competition respectively.Secondly,the service negotiation period is further divided into service request sub-period and service reply sub-period.The adaptive process of CCH and SCH is realized by adjusting the time slot number of broadcast period and designing channel switching mechanism respectively.Compared with the traditional scheme,the time slot utilization can be improved by about 20%and the frame duration of the SCH can be improved by 70%.Secondly,in order to meet the needs of vehicles to broadcast Beacon messages periodically and to broadcast emergency messages in emergencies,this paper designs an efficient broadcast MAC protocol and proposes a frame structure based on listening time division multiple access(TDMA).By improving the traditional TDMA frame structure,the protocol allows Beacons and emergency messages to share the same channel,so as to avoid allocating separate channels for beacons and emergency messages.By means of listening-based access,the MAC protocol can greatly reduce the message collision problems caused by vehicle movement,such as access collision and merging collision.Compared with the traditional TDMA mechanism,it can improve the access rate by about 20%and reduce the number of access collision and merging collision by dozens of times.In addition,by designing a preemptive emergency message transmission mode,the transmission delay of emergency messages less than 5ms can be achieved while satisfying the periodic Beacon transmission.Thirdly,the limited base station capacity is difficult to meet the nodes uplink transmission requirements in a large-scale vehicle networking environment.Therefore,this paper studies the uplink transmission strategy based on non-orthogonal multiple access technology(NOMA).In the scenerio of vehicle clustering,an incremental power allocation scheme is proposed.Through the hierarchical management for cluster head and cluster members,the reasonable power allocation is realized,and a semi-authorized transmission mechanism is adopted to distinguish the transmission process of cluster head and cluster members,so as to reduce the transmission delay of the network and effectively improve the neteork capacity.The key performance indexes such as transmission outage probability and system capacity based on NOMA scheme are analyzed in detail.