Performance Study Of Multiple Access Protocol In VDES Maritime Communication System

In recent years,the VDES(VHF Data Exchange System)has received widespread attention and admiration in the international maritime field and has been identified as a data exchange platform for the core system of e-Navigation strategy in the future.Compared with the existing AIS(Automatic Identification System),VDES has more channel links and more diversified service of business carrying capacity,which is an effective solution to the increasingly overload of AIS link load.SOTDMA(Self-organized Time Division Multiple Access)protocol is the key protocol of AIS system.However,the research on MAC access protocol of VDES system at home and abroad is still in its infancy.Therefore,this study is aimed at theoretical analysis and simulation of the performance of the SOTDMA protocol.Then an improved MAC multiple access protocol(the Improved-SOTDMA protocol,ISOTDMA)was proposed.The improved protocol can effectively reduce the slot reservation conflict rate,and it provides an important reference value for the standard development of the access protocol VDES in the future.In this thesis,the massage types of VDES system was summarized and classified,and the message priority levels were set for them,respectively.Starting from the generation principle of slot reservation conflict,the theoretical value of slot reservation conflict rate of existing SOTDMA protocol is obtained through theoretical analysis and simulation by Matlab.In this thesis,ISOTDMA protocol is proposed specific to the new channels and message types of VDES system,according to the movement state of the ship at sea,the network model of VDES is established by using OPNET simulation platform,and the relationship between message reporting rate,network capacity and slot reservation conflict rate is analyzed.Then the rationality and correctness of the VDES network model established by OPNET are verified by comparing the OPNET simulation value with the theoretical value of Matlab.In addition,to resolve the problem that the slot reservation conflict rate of ISOTDMA protocol is still high in the scenarios of large and crowded network of vessels,an improved scheme of intelligent selection of the relatively idle channel is proposed,and the mechanism is simulated in the VDES network model built by OPNET.The experimental results show that the slot reservation of the improved ISOTDMA protocol in large scenarios is much lower than the conflict rate of traditional SOTDMA protocol.