Performance Study And Simulation Of SOTDMA Protocol In AIS System

Shipborne Automatic Identification System (AIS), with the advantage of high positioning accuracy and small environmental impact influenced by the weather and geography, can provide a wealth of static and dynamic navigation information to protect the safety of navigation, keep the marine environment clean and improve transportation efficiency importantly and significantly. The performance of the AIS system can be affected by its key technology, namely, Self-Organizing Time Division Multiple Access protocol (SOTDMA). The performance of SOTDMA protocol in AIS System in term of slot reservation collision rate is investigated by theoretical analysis and simulation in this thesis; and an improved slot reservation algorithm is also presented to improve the channel utilization of AIS system.First, the development of AIS system is analyzed and its structure and function is introduced in the paper. Then the network architecture and data link layer technology of the AIS system is discussed in detail. Finally, the in-depth analysis and research of SOTDMA protocol in AIS system are shown as follows:(1) Combined with the actual situation of AIS operation, the existing theoretical model for the slot reservation collision rate performance of SOTDMA protocol is improved, which is used to re-analyze the slot reservation collision rate performance; (2) The simulation scenario for AIS system is built on the OPNET simulation software, which can achieve the goal of autonomous self-organization slot reservation and simulate the process of communication among the vessels at the same sea area. Using the simulation platform, the slot reservation conflict-rate performance of SOTDMA protocol is studied and the realationship among the slot reservation collision rate, the packet report rate and the number of users is analyzed in the AIS network. At the same time, the comparision between the the simulation results and the theoretical analysis is done to verify the correctness of theoretical research and simulation analysis; (3) When the number of ship in unit space or the rate of packet report is high, the slot reservation collision rate of SOTDMA protocol is greater. Therefore, an improved slot reservation algorithm is also presented to reduce the slot reservation collision rate. Moreover, according to the practical share of the number of different ship motion a simulation scenario is built to validate the effectiveness of the improved algorithm further more.