Joint estimation method of timing frequency offset of satellite borne automatic identification system (AIS) signals and implementation system thereof

Abstract

A joint estimation method of timing frequency offset of satellite borne automatic identification system (AIS) signals comprises that an antenna receives radio frequency signals sent by a ground ship AIS and inputs the radio frequency signals to a receiver; the receiver demodulates the received radio frequency signals into baseband signals and inputs the baseband signals to a field-programmable gate array (FPGA) data collection module; the FPGA data collection module carries out analog-digital conversion of the received baseband signals and obtains digital baseband signals and transmits the digital baseband signals to a signal processing module; the signal processing module processes the received digital baseband signals and obtains accurate AIS ship information and transmits the accurate ship information to a data storage module; and the data storage module stores the received AIS ship information. The joint estimation method of the timing frequency offset of the satellite borne AIS signals has the advantages of eliminating the problem of poor estimation precision in an easy timing estimation working mode by introducing an autocorrelation operation and a weighting multiplication operation, confirming sampling sites by utilizing a timing estimated value, and achieving the frequency offset estimation of being largest in estimation range and smallest in estimation variance on the satellite borne AIS signals by using the autocorrelation operation, a maximum likelihood operation and signal drop sampling.

Description

(1) Technical field: [0001] The invention belongs to the technical field of ship automatic identification system (Automatic Identification System, AIS) communication, and in particular relates to a joint estimation method of timing and frequency offset of spaceborne AIS signal and its realization system. (2) Background technology: [0002] In the low-orbit satellite-borne AIS receiver, the speed of the satellite is 7.5 kilometers per second, and the Doppler frequency shift caused by the AIS signal in the VHF band is close to 4KHz, exceeding 15% of the bandwidth. It is difficult to receive and detect correctly. AIS is a time division multiple access system, and synchronization is the most basic requirement. However, the distance of signal transmission is different, and the time to reach the receiver is different. The time slot guard interval designed by the general AIS receiver can meet the transmission distance of 120 nautical miles. In this range, the signal delay caused b...

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Problems solved by technology

At present, the most cited and most classic is the timing-frequency offset joint estimation technology based on autocorrelation (see literature [1] M.Morelli, U.Mengali "Joint Frequency and Timing Recovery for MSK-Type Modulation", IEEE Communications Letters[ J].1999,2(8):938-946), the accuracy of this working method is not high, and the large frequency offset contained in the spaceborne AIS signal cannot be accurately corrected, which greatly affects the detection performance of the receiver

Document [2] (Hua Peng, Jing Li, "Non-Data-Aided Carrier Frequency Offset Estimation of GMSK Signals In Burst Mode Transmission", IEEE Communications Letters[J].2003,8(2):576-579) records A frequency offset estimation method based on fast Fourier transform. This method uses fast Fourier transform to determine the frequency corresponding to the peak of the signal spectrum to obtain an estimated frequency offset. The accuracy of this working method is higher than that of the previous working method. However, this working method is only suitable for signals with small frequency deviation, so it is not suitable for spaceborne AIS receivers

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