Study On Channel Access Mechanism Of VANET Based On Protocol Sequence Under Expressway

The Media Access Control protocol(MAC)of Vehicular ad-hoc network(VANET)impacts directly on the utilization of channel resources and the performance of vehicular network,which is still a challenge due to the high dynamic network topology of VANET..Therefore,it is urgent to realize a real-time,scalable,fair,efficient and reliable channel access mechanism.Channel access mechanism based on protocol sequence is an important branch of VANET channel access mechanism with the advantages of distributed,non feedback,controllable access delay and simple implementation.However,due to the sequence set can not be adjusted for the number of users,the traditional methods result in a long sequence period,low channel utilization and other issues.In this paper,we study on it based on protocol sequence.Firstly,an algorithm named Protocol Sequence Reconstruct and Allocation(PSRA)is proposed.In this algorithm,the superframe is divided into the detection period and the data transmission period.During the detection period,the number of neighbor nodes in the same section and the channel access order are calculated according to the characteristics of the User Irrepressibility(UI)sequences.During the data transmission period,the sequence reconstruction is carried out according to the number of neighbors in the same section.In addition,reconstructed protocol sequences are allocated according to the channel access order.Resultantly,the utilization ratio of time slots and the adaptability of the time slot allocation are both improved.The simulation results show that the PSRA is superior to the traditional method in terms of average throughput,user channel access delay and channel utilization in a relatively sparse network.Secondly,in order to further improve the utilization ratio of slot,a channel access mechanism based on UI-TDMA(UI-TDMA mechanism)for VANET is then proposed,which combines the advantages of UI sequence with Time Division Multiple Access(TDMA).The mechanism also exchanges information according to the UI sequence obtained by the nodes in the same section.Then,it carries out the statistical average distribution of the superframe time slot during the data transmission period according to the number of neighbors.Thus,the slot resource can be fully used during the data transmission period while time slots can be adaptively allocated according to the variation of users.The simulation results show that the performance of the proposed scheme is superior to the PSRA mechanism and the traditional mechanism in terms of average user throughput and user access latency.