Research On VDE-SAT Downlink Channel Modeling And Its Application

Sea transportation is the main carrier of world trade,reflecting the trend of the global economy.The increasing frequency of sea transportation drives the rapid growth of the number of ships in the world.The VHF band of the original automatic identification system(AIS)has been very crowded and the system is heavily loaded.In this context,International Association of Lighthouse Authorities(IALA)first proposed the concept of VHF data exchange system(VDES).As a supplement and enhancement of AIS,VDES system greatly enhances maritime communication capability,among which VDE-SAT is the core part of VDES.VDE-SAT broadens the coverage of communication services and is of great significance to the development of digital and high-speed maritime communications,with broad application scenarios.At present,the formulation of VDES related technical standards is still in the initial stage,and a large number of protocol drafts need further discussion and demonstration.By constructing the channel model,the real communication channel can be described approximately,which is convenient for the subsequent simulation and testing work,and can provide important theoretical support for the subsequent VDES system design scheme planning and formulation.Therefore,this paper conducts the following research on VDE-SAT downlink channel modeling and its application.The scattering characteristics of sea surface lead to the complex and changeable channel model of VDES maritime communication.In the VDE-SAT downlink,low elevation communication leads to more serious multipath effect caused by wave reflection.To solve this problem,this paper proposes a modeling method of VDE-SAT downlink channel based on spherical coordinate system.Firstly,the influence of the curvature of the earth on Satellite-Ship signal propagation is considered,and the Satellite-Ship scattering spherical coordinate system is established according to the geometric relationship between the satellite and the ship station.Then,the effective diffuse reflection region is calculated according to the scattering characteristics of sea surface.Based on this,the signal multipath propagation model of Star-Ship communication link is established considering the large scale fading and multipath effect.Finally,the accuracy and effectiveness of the proposed channel modeling method are verified by simulation analysis of the PDP spectrum,QQ plot and signal BER of VDE-SAT downlink.The G1139 protocol published by IALA stipulates that the VDES system adopts Turbo codes as its forward error correction codes and provides detailed coding process and parameters,but the protocol does not specify the decoding algorithm of Turbo codes.Therefore,based on the established VDE-SAT downlink channel,this paper analyzes the performance of Turbo codes decoding algorithm by using external information transfer characteristic curve,EXIT graph and BER curve.Simulation results show that Log-MAP algorithm has better performance.At the same time,the BER of the signal receiving characteristics under different sea conditions is simulated and analyzed.The results show that the channel is mainly specular reflection in good sea conditions,and diffuse reflection in bad sea conditions.