| VANET is the main means to achieve intelligent transportation system, which will release the crowded traffic and reduce the road accident. To promote the development and application of VANET rapidly, IEEE has promulgated IEEE802.11 p and IEEE1609 series standards. These standards regulate the system architecture and work pattern of wireless access in vehicular environment. The IEEE1609.4 standard classifies channels into control channel(CCH) and service channels(SCH), CCH is used to exchange safety messages and control messages, while SCH is used to exchange entertainment and service messages. The shortcoming of the IEEE1609.4 standard is that it cannot adapt the network environment neatly. Therefore, it is important to optimize and improve multi-channel transmission so as to improve the transmission performance. This thesis mainly accomplishes some research work as follows in view of transmission mechanism and time interval distribution.Firstly, the multi-channel work pattern of alternating switch between control channel and service channel is presented. For the synchronized collisions generated at the start of transmission interval, this thesis improves the transmission scheme to make sure safety messages transmit in proirity and provide contention-free access to SCH by reservation on control channel. Self-adapted channel-load dissemination algorithm designed in the CCH transmission interval enables nodes to determine whether transmitting messages immediately or not in accordance with the channel condition. The simulations by the EstiNet show that it can effectively reduce the packet collisions, obviously increase the packets delivery rate and decrease transmission delay.Secondly, the IEEE1609.4 standard defines fixed channel switch period, which is not quite reasonable for the complex traffic condition of VANET. This thesis puts forward an optimizational time interval allocation algorithm, which is adjustable for the transmission interval flexible. The process of channel competing among nodes is described by Markov chain model. The optimal time interval distribution theoretically can be deduced through this model. Node density and DATA packet length are the main factors of varying optimal time interval. Theoretical analysis and simulation results show that the optimal time interval distribution algorithm enhances the throughput of service channel and the packets delivery rate significantly; meanwhile the entire safety message can be transmit reliably. In general, the performance of this algorithm is better than IEEE1609.4. |
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