Abstract: The shipborne automatic identification system (AIS) signals received by satellite based receiver have the advantages of low complex structure low signal to noise ratio (SNR). Moreover, the Gaussian pulse shaping filter in Gaussian filtered minimum shift keying (GMSK) modulation can cause inter-symbol interference (ISI)，and the non-coherent GMSK detector is sensitive to noise. To deal with these issues, a modified decision feedback algorithm is presented. It is to narrow the bandwidth of postdetection filter to sufficiently suppress the noise in the output of frequency discriminator. Meanwhile, we adjust the feedback level according to the bandwidth of the postdetection filter in order to compensate for the resulting distortion as well as ISI caused by Gaussian filter. We also design a method to calculate the actual feedback level for the proposed modification in practical system by utilizing the synchronization preamble and starting flag in AIS frame. Simulation results show that a bit error ratio (BER) of 10-5 can be obtained as the SNR is 4.5dB by using the proposed method. Its simple structure and effectiveness enable its promising application in other GMSK mobile systems.

Key words: Gaussian filtered minimum shift keying (GMSK), non-coherent detection, feedback decision, postdetection filter, inter-symbol interference (ISI), automatic identification system (AIS)