Research On Cooperative Downloading Strategies Of VANET In Expressway Scenarios

With the development of Internet, there is increasing need for people to be connected. People expect to be able to connect to the network anytime and anywhere, even in a moving car. For this reason, the VANET (vehicular ad-hoc network) is developed and now getting more and more attention. However, VANET is still in a preliminary stage at present, both correlation technique and infrastructure construction are immature. The sparse AP/RSU distribution and the restricted communication coverage are far away from the demand of users on the network under expressway VANET environment.To solve this problem, those existing VANET network models and typical cooperative downloading strategies are lucubrated and a vehicle-string-based cooperative downloading strategy, VsCoDS is proposed in order to providing better network quality of service for users based on existing network facilities condition of VANET. In addition, given the development tendency of VANET wireless communication technique, a multi-channel cooperative downloading strategy (CSCoDS) that applies to IEEE802.11p/WAVE is proposed. In VsCoDS, a vehicle string including the one that needs data is formed with the homodromous vehicles so to leverage the characteristic of less relative velocity and better connection quality, and also, the performance in TCP multi-hop communication is considered. Then, to select a set of suitable opposite vehicles to carry data user needed for cooperative downloading. The method analyzes the problems of existing cooperative downloading strategy, utilizes the vehicle string to extend communication time between the opposite assistant vehicles and the target vehicle effectively, thereby it can make the selection strategy simplified and be adapted to varied traffic density. Besides, the thesis analyzes the multi-channel cooperative downloading communication model based on IEEE802.11p/WAVE, in which the channel fading problem caused by fast vehicles is involved. And then the CSCoDS is proposed. It takes full advantage of characteristics of this model, determinates the assistant vehicles that offer cooperative downloading for the target vehicle by the change of channel status.At last, simulation experiments are carried on. The results show that both of the cooperative downloading strategies proposed in this thesis can reduce communication delay in dark area effectively and improve network downloading throughput in single-channel and multi-channel network communication model respectively.