Research Of Detection Key Technology For Satellite-based AIS Signals

The satellite-based Automatic Identification System(AIS),as a new type of navigation and security ensuring system on the sea,can realize ship monitoring all over the world and attracts more and more attention from different countries.The satellite-based AIS signals have low signal-noise ratio,big frequency offset and signal collisions.But the existing detection algorithms didn't consider these factors well in their detection.In the thesis,the detection algorithms in the presence of frequency offsets and interfering signals are discussed.Frequency offsets are compensated and interfering signals are suppressed respectively in the process of decoding,which simplifies the complexity of the receiver.(1)A detection method based on frequency compensation for satellite-based AIS signals is proposed.With the symbols that are definitive in the sequence of AIS signals,the sample signal can be constructed.The correlation coefficients of the sample signal and the received AIS signal under different time delay and frequency offset can be calculated.Then the time delay and the frequency offset can be estimated through comparing these coefficients.Base on the estimates above,the signal sequence can be decoded with two methods.One is that estimating the symbols of the received AIS signal based on waveform matching method.This method is easy to be realized and has good performance than 2 bit differential detection.The other is detecting the signal sequence base on differential detection algorithm with frequency compensation.This method can realize the same performance in the presence of large frequency offset as the differential detection without frequency offset.(2)A multiple-symbol combined differential Viterbi decoding algorithm,which is insensitive to frequency offset,is proposed.According to the theories of multiple-symbol differential detection and maximum-likelihood detection,we combine the multiple-order differential information with the Viterbi algorithm.The phase shift caused by the frequency offset is estimated and compensated from the above information in the process of decoding.The simulation results show that this algorithm has good performance and is insensitive to constant phase shift and frequency offset.(3)An interference suppression detection method for satellite-based AIS signals is proposed.The optimum judgment formula of information sequence in the presence of interference signals is obtained according to the maximum likelihood criterion.Then the amplitudes and phases of total interference signals in the optimum judgment formula were estimated in the process of Viterbi decoding,which suppressed the influence of interference signals.The simulation results show that this algorithm performs well under low signal-to-interference power ratio.