Research On Dynamic Time Slot Allocation Mechanism For VANET Under Urban Scenario

Vehicular ad-hoc network(VANET)is an important branch of Intelligent Transportation System(ITS).VANET can improve the traffic safety and efficiency by exchanging vehicle movement information and road condition information through onboard unit.Most of recent channel access mechanisms in MAC layer cannot guarantee satisfied communication performance of VANET,due to the high dynamic topology and the regularity of vehicle movement.Therefore,the research of channel access mechanisms in MAC layer become investigative emphasis in the research fields of VANET.Due to the larger amount of users and the complex road condition,it proposes the higher performance demand of channel access mechanisms in MAC layer under urban scenario.So,the investigation of channel access mechanism of VANET,which is suitable for urban scenario,is significant.Aiming at solving the problems of existing channel access mechanisms in VANET,such as the weak adaptability and low time slot utilization,we focus on the investigation of distributed TDMA mechanism and carry out the research of dynamic time slot allocation mechanism.Firstly,a modified distributed adaptive TDMA scheduling mechanism(MDATS)for urban scenario is proposed,named MDATS mechanism.In this mechanism,the competitive area is divided into several logical segments according to the number of idle slots.The-vehicle occupies the time slot based on the corresponding relationship between the logical segment and the idle time slot.MDATS mechanism reduces channel collisions between vehicles which access the network simultaneously through the decentralization of idle time slots in space.Simulation results show that the performance of the MDATS mechanism has been improved in terms of the success rate of channel access,access delay and so on.Secondly,in order to further improve time slot utilization and user fairness,an adaptive multichannel TDMA scheduling mechanism based on SD-TDMA is proposed,called SD-TDMA mechanism.SD-TDMA mechanism redistributes channels according to the geographic location,and divides the superframe into the user detection interval and the data transmission interval.In the user detection interval,vehicles access channels in the distributed manner,based on the location-time slot mapping table.Owing to the uniqueness of vehicle location,most vehicles can access channels without collisions,and participate in the time slot allocation process successfully.Only few vehicles have transmission collision in the user detection interval,and they can compete to send request packets on the corresponding secure channel to participate in the time slot allocation process again.Simulation results show that SD-TDMA mechanism can significantly improve the utilization of time resources and higher data transmission fairness than existing similar mechanisms.