提出了一种两阶段载波相位估计改进算法。该算法为降低计算的复杂度，选取差分相移键控(QPSK)分区法处理16阶正交振幅调制(16QAM)星座图中第一圈和第三圈的点；利用最大似然法对星座图中所有的点进行精确的处理。通过搭建640 km的224 Gb/s、三信道波分复用偏振复用16阶正交振幅调制(WDM-PM-16QAM)相干检测实验系统，对本文提出的改进算法和几种已有算法的误码率(BER)和幅度矢量误差(EVM)性能进行分析。实验结果显示，改进算法的误码率性能略优于传统的两阶段算法,矢量幅度误差性能几乎保持不变，但是计算复杂度降低为传统两阶段算法的一半。

An improved two-step algorithm for carrier phase estimation is proposed. In order to reduce the computational complexity, only the points in s1 and s3 circles of the 16-ray quadrature amplitude modulation (16QAM) constellation are processed with the quadrature phase shift keying (QPSK) partitioning algorithm. Then all the points in the constellation are processed accurately with the maximum likelihood algorithm. A 640 km experimental system for 224 Gb/s, three channels wavelength division multiplexing polarization-multiplexing 16QAM (WDM-PM-16QAM) with coherent detection is set up to test the bit error rate (BER) and error vector magnitude (EVM) of the improved algorithm and several existing algorithms. The results show that comparing to the traditional two-step algorithms, the modified algorithm has a better BER and keeps a good EVM. Furthermore, the computational complexity reduces to half of the traditional algorithms complexity.