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本文基于高阶正交幅度调制(QAM)和大线宽的相干光正交频分复用(CO-OFDM)系统,提出了一种基于时域和频域卡尔曼滤波联合估计的相位噪声补偿算法。该算法在发射端时域插入训练符号和导频序列,首先在接收端基于训练符号进行频域卡尔曼滤波得到信道估计。其次进行时域扩展卡尔曼滤波得到相位噪声粗略估计值。因此时相位估计值为复数值,为获得较好的补偿效果,根据导频序列数目,将每个OFDM符号分割为若干个亚符号。同时在导频序列处进行时域扩展卡尔曼滤波,通过在每个亚符号最后一个导频处进行线性插值得到每个采样点的相位噪声粗略估值。将粗略相位噪声补偿后的频域数据进行预判决后,最后进行时域相位噪声精细补偿。基于50Gb/sCO-OFDM系统传输100km进行了仿真验证,该算法较其他方法相比取得了较好的补偿效果。在激光器线宽为1MHz且16QAM以及线宽为800KHz且32QAM两种情况下,误码率性能可达前向纠错(FEC)上限。该算法能极大促进CO-OFDM系统在长距离接入网和城域网中的应用。
In this paper, based on coherent optical Orthogonal Frequency Division Multiplexing (CO-OFDM) system with high order Quadrature Amplitude Modulation (QAM) and wide linewidth, a phase noise compensation based on joint estimation of Kalman filter in time domain and frequency domain algorithm. The algorithm inserts the training symbols and pilot sequences in the time domain of the transmitting end. First, the channel estimation is obtained by performing Kalman filtering in the frequency domain based on the training symbols at the receiving end. Second, time-domain extended Kalman filter to get a rough estimate of phase noise. Therefore, the phase estimation value is a complex value. In order to obtain a better compensation effect, each OFDM symbol is divided into several sub-symbols according to the number of pilot sequences. At the same time, time-domain extended Kalman filtering is carried out at the pilot sequence, and a rough estimation of the phase noise at each sampling point is obtained by linear interpolation at the last pilot of each sub-symbol. After the coarse-phase noise-compensated frequency-domain data is pre-judged, finally the fine-time phase noise compensation is performed. The simulation is carried out based on 50km transmission in a 50Gb / s CO-OFDM system. Compared with other methods, the proposed algorithm achieves a better compensation effect. With a laser linewidth of 1MHz and 16QAM and a linewidth of 800KHz and 32QAM, the bit error rate performance can reach the upper limit of forward error correction (FEC). This algorithm can greatly promote the application of CO-OFDM system in long-distance access network and metropolitan area network.