Study On Synchronization Algorithms In The Satellite-Based AIS Receiver

The satellite-based AIS(Automatic Identification System)is a novel security system of dynamic control and collision avoidance for maritime navigation,in which dozens of AIS receivers on the Low-Earth-Orbit satellites are used to receive and process the messages from the vessels.It results in expanding the AIS transmission radius.Not only the satellite-based AIS becomes a significant measure to grasp the international shipping industry for the government,but an important means to carried out macro adjustment and control of shipping and economic trade effectively for the administrators to promote the maritime economic booming.As a key technology in the satellite-based AIS receiver,the time and frequency synchronization has been a hot issue to the researchers for a long time.In this paper,the classic signal synchronization methods were simulated to verify their performance,and some improved methods were proposed on the bais of their disadvantages:(1)Addressing the problems of low synchronization accuracy and high complexity,a time delay estimation method based on phase response reconstruction was proposed.The phase response was reconstruct based on the least square principle.The value of time delay can be obtained from the frequency domain eventually.This method has high estimation accuracy,and it has better estimation performance than the four order auto-correlation method,especially at the low SNR.Additionally,the complexity is much less than the four order-correlation method.(2)A frequency estimation method based on FFT interpolation was presented for the intermediate frequency satellite-based AIS scheme.The method utilizes the pilot sequence was used to eliminate the modulated phase information.And the maximum spectral line and the neighbor spectral line of the signal were used to acquire the fine frequency offset by means of FFT interpolation.Analysis and simulation both show that the estimation accuracy is very high,which is 1.7dB higher than its MCRB,and its estimation range is pretty large.(3)As study the summation term of the two order auto-correlation function,an improved two-step frequency estimation method was proposed.The proposed algorithm utilized the character sequence to construct an auxiliary function to remove the information phase.Firstly,A coarsed frequency offsetwas obtained by using a small summation term to correct the signal.Then a fine frequency offset value was gotten with the help of a large summation term eventually.Simulation shows that the estimation range was highly increased.What’s more,it has high estimation accuracy,which is much closed to the MCRB.To verify the performance of the synchronization methods,the VDL of AIS is simulated,based on which the feasibility and reliability of the synchronization methods were tested.