低信噪比大频偏空间光信号的频率捕获研究

卫星相干光通信系统信号解调过程中，卫星相对运动会导致接收的信号光产生GHz量级的多普勒频移，信号光的远距离传输导致光信噪比极低，传统方法无法在低信噪比下补偿大范围多普勒频移，严重影响通信系统的能力。针对上述问题，本文提出了一种两段式频率捕获算法，该算法包含自动扫频和锁频控制两个阶段。自动扫频阶段通过本振光自动频率扫描将频差缩小至100 MHz量级；锁频控制阶段通过高精度的本振光频率控制与FFT变换，在低信噪比下继续缩小频差至MHz量级。仿真验证结果表明：该算法可在2 dB信噪比下补偿大动态范围的10 GHz范围多普勒频差，满足卫星相干光通信解调需求。

Research on Frequency Acquisition of Low Signal-to-Noise Ratio and Large Frequency Offset Space Optical Signal

In the signal demodulation process of the satellite coherent optical communication system, the relative motion of the satellite will cause the received signal light to produce a Doppler frequency shift of GHz magnitude. The long-distance transmission of the signal light leads to an extremely low optical signal-to-noise ratio. The traditional method cannot compensate for the large-scale Doppler frequency shift at a low signal-to-noise ratio, which seriously affects the ability of the communication system. In view of the above problems, this paper proposes a two-stage frequency acquisition algorithm, which includes two stages: automatic frequency sweeping and frequency locking control. In the automatic frequency scanning stage, the frequency difference is reduced to the order of 100 MHz by the local oscillator''s automatic frequency scanning. In the frequency locking control stage, the frequency difference is further reduced to the MHz level at a low signal-to-noise ratio through high-precision local oscillator frequency control and FFT transformation. The simulation results show that the algorithm can compensate for the Doppler frequency difference in the 10 GHz range of the large dynamic range under the 2 dB signal-to-noise ratio, and meet the demodulation requirements of the satellite cohe-rent optical communication.