Cross-Layer Design For Adaptive VANET Safety-Related Information Broadcast Protocol Based On Multi-Objective Optimization

With the rapid development of wireless communication technologies and modern car manufactures, Intelligent Transportation System emerges as the times require. As the key technology of Intelligent Transportation System, VANET (Vehicular Ad hoc Network) has attracted more and more attentions. In the traffic safety-oriented applications, broadcast is the major means of transmitting periodical beacons and event-triggered warning messages in VANET and it is important to ensure a real-time and reliable delivery. However, in the special environment of VANET, the fast speed of vehicles, frequent changes of network topology and unstable wireless channel create new challenges to the delivery of safety-related messages. Design of medium access control layer to ensure a reliable broadcast then becomes the focus of VANET.In order to enhance the reliability and stability of safety-related messages deliveries in VANET, a vehicle’s safety-related messages broadcast mechanism is proposed in the thesis, which uses time division multiple access and carrier sense multiple access to access channels and considers rode side units as a controller. Moreover, cross-layer design between medium access control layer and physical layer is applied. The main content and contribution of this thesis are as follows:(1) Based on deep analyses research on medium access control layer, optimization of physical layger parameters and cross-layer design about safety-related information broadcast in VANET, problems needed to be solved further have been explored. And then we establish a network communication model of vehicle’s safety-related information broadcast.(2) Targeted at ensuring the delivery of safety-related messages, we make full use of both carrier sense multiple access and time division multiple access mechanism and propose CA-MAC (Cross-layer Adatptive Medium Access Control). In the design, centralized and distributed fusion mechanism is adopted. Rode side units, considered as centralized controllers, are in charge of slots’allocation and scheduling and adjust frame lenghs adaptively based on the vehicle densities. Vehicles under the communication coverage of rode side units broadcast safety-related messages periodically by time division multiple access mechanism and handle the cooperative optimized calculation for physical layer control paramters distributedly. And carrier sense multiple access is adopted to delivery emergency warning messages and time slot applications.(3) Under the framework of time division multiple access mechanism and on the basis of cooperative differential evolution algorithm, CA-MAC transmission parameters’multi-objective cooperative optimized algorithm is proposed. In this algorithm, transmission rate and power of physical layer are taken as decision variables, network delay, packet loss rate and goodput are considered as optimization objections. Weights of these object functions will be changed dynamically based on the signal to interference and noise ratio from physical layer. Meanwhile, periodical safety-related messages will be used to delivery rate and power results and receive others’, in order to realize the cooperative optimized process of physical layer parameters.(4) The proposed protocol takes highway as the application scene. The simulation scene of safety-related information broadcast in VANET is built by SUMO and NS2. Parameters of the CA-MAC cooperative optimized algorithm are dicussed and decided based on the simulation. In addition, performance of the proposed protocol is verified and analyzed under highway scene.