针对卫星接收机接收的船载自动识别系统(AIS)信号信噪比低、星载设备要求复杂度低的特点，以及高斯滤波最小频移键控(GMSK)调制过程中高斯成型滤波产生码间干扰(ISI)和非相干解调对噪声敏感的问题，提出了一种改进的反馈判决方法.该方法通过减小后检测滤波器的带宽，来充分抑制鉴频噪声，并根据后检测滤波器的带宽来调整反馈判决电平，以补偿ISI和后检测滤波器对鉴频信号造成的失真；另外，还设计了一种根据AIS帧头的位同步序列和起始标志，来计算实际系统中的反馈电平的方法.仿真结果表明，提出的方法可以在信噪比为 4.5dB时，使误码率降至10-5，而未增加系统复杂度.该方法简单有效，也可以应用于卫星AIS接收机以及其他深空通信系统中.
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.
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