The Research On Strategies And Protocols For Vehicular Ad Hoc Network Communication

As the society grows and develops, traffic safety has increasingly become one of the world's most concerned problems. With combination of wireless sensor networks and wireless Ad hoc network characteristics, the Vehicular Ad hoc Network (VANET) [1] has been proposed for the road traffic system. It aims to reduce rate of traffic accidents and improve the efficiency of transport system. VANET communication comprises of two parts, vehicle-to-vechicle (V2V) communications and vehicle-to-roadside (V2R) communications [2]. Due to rapid changes of dynamic topology caused by high-speed movement of nodes in VANET, intermittent communication links, a sharp decline of network performance become more serious. When an accident occurs, the traditional 802.11 series and wireless sensor network protocols in the agreement can not be achieved on both fast and long-distance transmission of emergency messages. Therefore, it is needed to design dedicated protocols of VANET for high-speed mobile environment and special applications.This thesis focused on communication strategy and protocol technologies for VANET, and four key issues were studied. First of all, for the VANET with a fast-moving feature of the terminals, thesis theoretical analyzed network throughput with various protocols. Then, with characteristics of directivity and the spatial reuse using directional antennas, a MAC protocol was proposed to improve the network capacity. Next, the performance of signals based on the Black-burst priority access algorithm was evaluated for channel contention in a dense district. Finally, for delay tolerant characteristics of VANET, a routing protocol was improved, in order to achieve a reliable transmission.The main contributions of this thesis include:1. Due to the terminal characteristics of high-speed mobility, based on time Slot ALOHA and Reservation ALOHA throughput analysis, a SR-ALOHA protocol was proposed in a vehicular high-speed environment. It adapted to intermittent communication links with a dynamic protocols switching method. A high throughput performance and a low end-to-end transmission delay in the dynamic environment could be achieved. At last, the proposed SR-ALOHA was proved feasible through our simulation.2. Directional antennas can increase the wireless channel spatial reuse, it can improve the directional antenna gain, expanding the scope of the antenna signal transmission and so as to reduce numbers of hops required in multi-hop communication. With these advantages of directional antennas compared to traditional omni-directional antennas, the local antenna beam table scheme was proposed to improve the performance of multi-channel MAC protocol in dense traffic environments. Results indicated that using directional antennas in VANET a better performance could be achieved than other multi-channel MAC protocols.3. With directional antennas, spatial reuse of the wireless channel and a higher gain can increase the capacity of VANET. As a further transmission range achieved, a problem how to select the furthest node for multi-hop communication makes a remarkable significance. A nonlinear Black-burst scheme was proposed for dealing with this problem. Road inside the transmission range is divided into segments nonlinearly, and each segment corresponds to certain time slots in contention period. Numerical results show that our improvement gives a higher success probability of selection of the furthest node with less iteration in highway scenario.4. The static network structure of Vehicular Delay-tolerant Network was introduced. For such a network characterized by intermittent connectivity, an analysis was obtained using the node store and forward mechanism to ensure reliable transmissions. An information-aware routing protocol, which was based on prediction theory of Kalman filter, was proposed to optimize the selection of forwarders using prediction of local historical information. Simulation results indicated a reliable transmission and a significantly decrease of network overhead with the proposed routing protocol using Kalman filter theory.This thesis focused on several key issues that urgent solutions were needed in VANET, which made an exploratory study in improvement and tantalizations of protocols. Researches of this thesis are helpful for VANET in wireless communications, and a forward-looking with a certain theoretical and practical value could be achieved in reality.