Data Dissemination In Vehicular Ad Hoc Network

Vehicular ad-hoc network (VANET) is a very promising direction in the development of intelligent transportation system (ITS) and attracts many researchers and organizations. It has become a hot research topic of wireless ad hoc network and ITS. The information in VANET is very large and the requirements of Quality of Service (QoS) in applications are varied because of the rapid development of sensor technology and the variety of applications. For example, the messages should be broadcast promptly and reliably to the relevant vehicles in the safety related applications, such as pre-crash sensing, chain collision prevention and intersection collision warning; the messages should be transmitted efficiently to a specific geographic area in the non-safety related applications, such as commercial ad dissemination, parking space reservation and public information query. As a result, data dissemination is a fundamental and important issue in VANET.Due to the rapid movement, the varied node densities and the varied communication scenarios, the topology of VANET is highly dynamic and the wireless channel of VANET is unstable and unreliable. Therefore, multihop transmission in VANET faces a great challenge and conventional data dissemination schemes for traditional mobile ad hoc network are not fit for VANET. How to enhance the network performance by disseminating the data with different requirement effectively in VANET has been attracting the interest from both academic and industrial communities.The dissemination schemes of the emergency data and the non-emergency data are investigated for highly dynamic and unreliable VANET. The contributions of the thesis are summarized as follows:1. Based on the rapid changing topology and the lossy wireless channel of VANET, a sender-designated opportunistic broadcast protocol (SOBP) for VANET is proposed. The average number of transmissions in a successful broadcast is analyzed and the retransmission strategy to enhance the reliability is presented. Two problems existing in highway VANET, i.e., the slow reaction problem (SRP) and the local broadcast storm problem (LBSP), are investigated. To solve these problems, a sender designates a fixed number of candidate forwarders and assigns priorities to them by using a metric depended direction and distance before broadcasting a packet. Simulations show that SOBP injects fewer packets into the network and saves the bandwidth. It is able to effectively solve the SRP and the LBSP, and consequently enhances the reliability of broadcast.2. A multi-hop multiple candidate relays opportunistic broadcast (MCROB) protocol for safety related applications in VANET is proposed. Based on the analysis of the transmission speed, a candidate relay selection metric called expected transmission speed (ETS) for the MCROB is proposed with the derived packet delivery ratio (PDR). A priority rule for selecting a proper candidate relay and an adaptive algorithm for forwarding timers of candidate relays are also presented in the work. Simulations show that MCROB can be adaptive to the rapid changing of network conditions. It reduces the delivery latency effectively and keeps a low communication overhead introduced by the broadcast. MROB outperforms the existing protocols and meets the requirement of broadcasting emergency messages.3. A transmission scheme that uses network coding with the dynamic sliding window and the lower triangular matrix coding method is proposed for delay jitter sensitive data transmission in VANET under the unreliable wireless channel. By adjusting the coded window size according to the network status, it transmits the packets coded with Random Linear Network Coding (RLNC) and improves the end to end throughput effectively. The scheme uses the lower triangular matrix coding method to smooth the decoding interval in the receiver. Simulations show that the scheme is able to increase the throughput and decrease the delay jitter efficiently compared to conventional generation-based schemes. It is especially appropriate for delay jitter sensitive multimedia applications in VANET.4. A network coding based data distribution scheme which disseminates information to a specific geographical area for VANET is proposed. The main idea of the proposed scheme is the local response service (LRS) that all the requests from the nodes located in the service area can be serviced by any surrounding neighbors. To provide services for the request, all the distributing packets coded with RLNC are cached in the vehicles within the service area randomly. Every vehicle overhears the transmissions from others to cope with the case that two vehicles have some dependent data. A concept called "neighbor data utility" is proposed to calculate the proper number of the packets that should be transmitted when the server responds the request. The calculate method avoids the useless transmission and saves the bandwidth. Simulation results show that the scheme, without multihop transmission, can reduce the distribution delay effectively and consequentially improve the message delivery ratio.