Research On Media Access Control And Information Broadcast In Vehicular Ad Hoc Network

Vehicular ad hoc network (VANET) enabling vehicles to communicate with each other, and providing safety services to reduce traffic accidents on the road as well as multimedia services to supply information and entertainment to traveling people, has become hot research area in wireless communications. As an indispensable part of intelligent transportation system (ITS), VANET can operate autonomously in self-organized, distributed mode without the support of roadside wireless communication equipments. However, the lack of infrastructure support, high mobility of vehicles, dynamic topology changes, and hostile wireless communication environment make the wireless resource allocation of VANET extremely challenging. In addition, exposed terminal, hidden terminal, fairness, reliability problems on the MAC layer, and broadcast storm problem on the network layer, make VANET difficult to guarantee quality of service (QoS) requirements of various applications. For instance, safety related services demand quick and reliable message delivery, while multimedia services usually require high throughput and good fairness performance. Therefore, it is very important to design efficient protocols to meet different QoS requirements of vehicular applications in VANET. The contributions of the thesis are concluded as follows:(1) Propose a multi-channel token ring media access control (MAC) protocol MCTRP.The traditional Ad Hoc networks adopt the distributed coordination function (DCF) of IEEE802.11, and the packet collisions increase when the node density goes up, which prolongs the channel access delay of data packets. Therefore, DCF is diffucult to guarantee the QoS requirements of safety services, which have stringent delay requirement. Although most current research works aim to reduce the channel access delay of safety messages at the cost of decreasing the efficiency of channel resource, they cannot guarantee high throughput for multimedia services and low delay for safety services simultaneously. Based on the characteristics of VANET and QoS requirements of its services, we design a multi-channel token ring MAC protocol MCTRP. The protocol adopts the IEEE802.11p multi-channel structure, and through adaptive ring coordination and dynamic channel scheduling, vehicles are autonomously organized into multiple rings operating on different service channels. In addition, a Markov chain model is developed to dynamically keep track of the number of nodes in a ring, and further evaluate the performance of MCTRP in terms of the average full ring delay, emergency message delay, and ring throughput, etc. Extensive simulations are conducted to validate the analytical model, and performance comparisons with other MAC protocols are also performed. Analysis and simulation results show that MCTRP can quickly and effectively deliver safety messages, improve the throughput for multimedia services, and finally guarantee the QoS requirements of various services in VANET.(2) Propose a single relaying metric multi-hop broadcast protocol PMBP based on geographical position.In order to make safety messages be received by remote nodes as fast as possible in VANET, a position based on multi-hop broadcast protocol (PMBP) is proposed to deliver safety messages, and thus achieve cooperative driving in VANET. PMBP adopts a cross-layer approach on the MAC layer, and selects the neighboring node with the farthest distance from the source node in the message propagation direction as the next relaying node, which ensures emergency messages can be delivered to remote nodes with low time latency。 Furthermore, a mathematical model is developed to analyze the performance of PMBP, such as number of receiving nodes at each hop, end to end delay, ratio of informed vehicles to broadcasting vehicles, etc. Compared with the mechanism of DCF in IEEE802.11, PMBP is able to overcome several challenging problems of multi-hop information broadcast, for instance, message redundancy, unreliable link, hidden terminal, broadcast storm problems, and greatly improve the performance of the network. (3) Propose a mutiple relaying metrics multi-hop broadcast protocol CLBP based on channel conditions, moving velocities, and geographical location.In VANET, hostile communication environment, pass loss of signal, and fast movement of vehicles increase the packet error rate (PER). The transmitting node always repeadly transmits data packets to increase the message reliability due to the lack of acknowledgement, which prolongs the delay on the link layer. Therefore, under bad channel conditions, the farthest neighboring node with high PER is not the ideal relaying node for multi-hop information broadcast. To adapt the characteristics of physical channel in VANET, a cross layer broadcast protocol (CLBP) is proposed to deliver safety messages. A novel composite relaying metric is firstly designed for next hop relaying node selection, which jointly considers geographical locations of vehicles, physical layer channel conditions, and moving velocities of vehicles. Based on the designed relaying metric, a distributed relay selection scheme is proposed to assure a unique relay is selected to reliably forward the safety message in the information propagation direction, and IEEE802.11e EDCA MAC is further revised to guarantee QoS provisioning to safety services. In addition, a mathematical model is developed to study the performance of the proposed CLBP, and performance metrics PER, relay selection delay, emergency message access delay are thus calculated. Mathematical analysis and simulation results show that CLBP can not only reduce the broadcast message redundancy, but also quickly and reliably deliver safety messages in VANET.