Research On Marching-on In-time Scheme And The Fast Algorithm Of Time Domain Integral Equation

In recent years,with the continuous prosperity of maritime trade,the number of ships and routes of passenger and freight has continued to increase.The Automatic Identification System used for ship supervision and route control has disadvantages such as low information transmission rate and limited physical channels.The time slot conflicts of AIS broadcast messages in densely-shipped areas are serious,which leads to a sharp decline in the ability to supervise ships and seriously affect the safety of maritime navigation.The needs of morden maritime ship can no longer be met by the traditional AIS system.A new generation of VDES(VHF Data Exchange System)system emerges at the historic moment.On the basis of keeping the original working mode of the AIS system unchanged,the VDES system adds the ASM channel and the VDE channel,which improves the information transmission rate and expands the data interaction capability.However,the VDES system did not make any improvements to the AIS system in the relevant proposals,and the serious problem of the time slot conflict in the AIS system in densely-vessel areas such as ports,river outlets,and offshore areas has not been resolved.Therefore,an adaptive channel selection method is proposed in this paper.This apporach is based on the classification of navigational safety messages,which uses logical channels in ASM and VDE that have not yet allocated specific content to send low-security AIS broadcast messages,thereby effectively reducing the probability of timeslot collisions.The use of aircraft and satellites to receive ship broadcast signals,grasp the real-time dynamic information of ships in key areas and even the world,and provide guarantees for the safety of various types of ships is an important research and application direction now and in the future.However,the requirements of frequence offset and sensitivity in the spaceborne cannot be met by using the existing receivers.Under this background,this paper studies the reception technology of multiple modulation methods of the VDES physical layer.Then,an anti-large frequency deviation and high sensitivity VDES signal receiving algorithm which is suitable for the spaceborne scenario is purposed.In addition,this algorithm is implemented and tested on the hardware platform.The specific work is as follows:(1)In view of the current time slot conflict problem of the AIS system,the broadcast messages are classified according to the degree of relevance to navigation safety.The ability of the AIS system terminal to monitor the broadcast channel is used to set the time slot conflict threshold.An adaptive channel selection scheme is proposed.For application scenarios such as single-dense cell,shore-based reception,and satellite-based reception,the adaptive channel selection scheme proposed in this paper is simulated and verified.The results show that this method can effectively reduce the probability of timeslot collision without introducing additional overhead.It has important reference significance for the formulation of future VDES system access protocol standards;(2)Aimed at the shortcomings of traditional signal detection and frame synchronization methods,a signal detection algorithm based on phase variation against large frequency deviations is proposed.Through frequency and phase estimation,combined with an improved ML coherent demodulation algorithm,an error correction technology with confidence calibration as a priori information is designed,which improves the probability of signal detection and reduces the signal-to-noise ratio threshold for correct demodulation of messages.Finally,the above algorithm is simulated and analyzed.When the signal-to-noise ratio and frequency offset are different,good results are achieved for the signals of various modulation methods in the VDES system;(3)Taking the GMSK modulation method in the VDES system as an example,the hardware implementation of the anti-large frequency deviation and high sensitivity receiving scheme proposed in this article is carried out,and a test platform is built using KC705 and AD9361.The vector signal source is used to generate the signal in the test.The results show that within the frequency offset range,the sensitivity of a single-slot message reaches-123.5 dBm,and the sensitivity of a dual-slot message reaches-122.4dBm.