Research On TDMA Based MAC Protocol For Vehicular Ad-hoc Networks

During the last decade,the motor vehicle has evolved from a simple mechanical device to a smart body of sensors that can measure different attributes,enhancing both vehicle safety and the driver experience.However,the traditional transportation system still suffers from major problems.The fast growth of metropolitan areas has been accompanied with an increasing influx of vehicular traffic to and from big cities.As a result,urban roads and highways are plagued by traffic congestions and road crashes,resulting in serious socioeconomic problems.The intelligent transport system(ITS)and intelligent connected vehicles(ICVs)are envisioned by the auto-motive industry as the future intelligent technologies that enhance the safety and comfort level of road traveling.Vehicular ad hoc networks(VANETs)enable transmission of information between ITS-stations(such as vehicles and roadside units),and support various services and applications,which is a key technology of the ITS.One of the most important goals of VANETs is to provide sufficient quality of service for real-time safety-related applications considering one-hop wireless broadcast and multi-hop wireless broadcast technologies.Nodes share common wireless channels with the same radio frequencies;thus,inappropriate use of a channel may lead to transmission collisions or a waste of bandwidth.Hence,channel sharing is a key issue when seeking to provide a high quality of service.Various MAC protocols have been proposed with the goal of guaranteeing efficiency and fair spectrum sharing among nodes.The most famous is the IEEE 802.11 p standard,which is one of the most prominent contention-based MAC protocols.However,hidden terminals have a detrimental impact on the contention-based MAC protocols,which also suffer from unbounded transmission delays and heavy collisions,especially when the trafficdensity is high.Meanwhile,higher saturation collision probabilities occur for broadcast services than unicast services as there is no binary exponential back-off in the broadcast scheme.Various TDMA-based protocols have been proposed to overcome the shortcomings of contention-based MAC protocols.Although the traditional TDMA-based MAC protocols perform better than IEEE 802.11 p when the traffic density is high,these protocols also encounter many challenges.In order to improve the transmission efficiency of basic safety messages and emergency messages,we study the TDMA-based MAC protocol from two aspects,optimizing the traditional protocol and designing new protocol operation mechanism.The main research contents and work are as follows:1)We proposed a novel method to improve the transmission efficiency of basic safety messages in VANETs by distinguishing perceptual regions.Our scheme improved the awareness quality for close range vehicles and accepted a soft degradation for remote vehicles,which adapts to the requirements of practical applications.By sacrificing the awareness quality of remote vehicles,nodes could use the same slot at closer distances when compared to traditional MAC protocols.Therefore,our scheme performed well even in high density networks.Meanwhile,potential merging collision could be pre-detected and eliminated through collaboration between nodes.2)An application suitable time slot sharing MAC protocol,called ASTSMAC,is proposed for VANET.Nodes can access the acquired time slots cyclically;one time slot can be shared by multiple nodes even if they located in each other's communication range.We present constraints on slot sharing and the operational process of ASTSMAC.There are many principles or methods to guide the adjustment of transmission rate,and we provide a density aware and weight related frequency adjustment algorithm(DAFA),which combines the support for an intelligent transport system.VANET has more knowledge about the vehicles,while the intelligent transport system has a better understanding of the road;therefore,more information should be exchanged between VANET and ITS.3)A novel contention based TDMA scheme(SCMAC)is proposed for VANETs.One of the main challenges for the internet of vehicles is the efficient and salable coordination and communication among the devices.An approach of combining sensing and black-burst broadcast is used to address this problem.The nodes reserve the slots before broadcasting the basic safety messages,and they can broadcast messages effectively even if many of them try to access the same slot simultaneously.The time slot is redesigned by dividing into two periods,reservation period and transmission period.The node that uses the current slot to do the messages broadcasting during the TP can be confirmed effectively and reliably using a fixed number of MSs in the RP.No control messages are needed in SCMAC,and there is no need to assign slots to the nodes.Thus,the nodes do not need to wait to broadcast,until the acquired slots are available.Thus,transmission delay can be lower compared with traditional TDMA-based protocols.Meanwhile,because no control messages are packed in the messages,packet losses caused by the poor wireless channel do not affect the operation of SCMAC.In addition,the proposed protocol can be applied in a more realistic and practical environment.4)A protocol called the SPBCP is proposed for emergency messsages dissemination in distributed TDMA-based VANETs.The proposed protocol does not require that nodes have knowledge of surrounding nodes by collecting beacons broadcast by one-hop neighbors.Thus,in contrast to traditional TDMA-based protocols,no beacon is needed in the SPBCP and only emergency messages are broadcast in the network.Time slot was divided into three periods: the area determination period,node determination period,and transmission period.The area determination period is designed to guarantee that nodes located far from the last-hop node can be selected as forwarders.The node determination period is designed to guarantee that only one node acts as forwarder and broadcasts an emergency messsagein the following transmission period.The SPBCP performed well at different traffic densities and in different channel conditions.The proposed protocol enables efficient and reliable emergency message dissemination in distributed TDMA-based VANETs.