Optimal Polarization Demultiplexing for Coherent Optical Communications Systems
Optimal Polarization Demultiplexing for Coherent Optical Communications Systems
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DOI：
10.1109/jlt.2009.2035526
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发表时间：
2010-04
影响因子：
4.7
通讯作者：
Ioannis Roudas;A. Vgenis;C. Petrou;D. Toumpakaris;Jason Hurley;Michael Sauer;John Downie;
中科院分区：
文献类型：
--
作者：
Ioannis Roudas;A. Vgenis;C. Petrou;D. Toumpakaris;Jason Hurley;Michael Sauer;John Downie;
频谱效率高的光通信系统采用偏振分复用(PDM)作为实际解决方案，以便将光纤链路的容量加倍。可以使用偏振分集相干光接收器以电子方式执行偏振解复用。本文的主要目标是在没有偏振模色散(PMD)的情况下，利用最大似然准则对偏振复用光信号的偏振分集相干光接收机进行优化设计。结果表明，在没有相位噪声和中频偏移的情况下，在两个接收偏振态上同时联合估计符号可以产生最佳的性能。相反，通常使用的迫零偏振解复用器，随后是偏振多路复用的支路的单独解调，表现出较差的性能，并且仅当信道传输矩阵是么正的时，例如在没有偏振相关损耗(PDL)的情况下，并且如果极化分集支路处的噪声分量具有相等的方差，才变得最优。在这种特殊情况下，迫零偏振解复用器可以由一个2？2格型自适应滤波器实现，该滤波器仅由两个独立的实参数控制。这些参数可以使用恒模算法(CMA)递归计算。通过仿真，对上述迫零偏振解复用器在采用正交相移键控(QPSK)信号的相干光通信系统中的性能进行了评估。我们表明，到目前为止，它在收敛精度和速度方面远远优于传统的基于CMA的偏振解复用器。最后，我们通过实验测试了所提出的约束CMA偏振解复用器对偏振分集相干光接收机实际缺陷的稳健性。提出的解复用器的PMD和PDL容限可以作为基准，以便比较更复杂的自适应电子PMD/PDL均衡器的性能。
Spectrally-efficient optical communications systems employ polarization division multiplexing (PDM) as a practical solution, in order to double the capacity of a fiber link. Polarization demultiplexing can be performed electronically, using polarization-diversity coherent optical receivers. The primary goal of this paper is the optimal design, using the maximum-likelihood criterion, of polarization-diversity coherent optical receivers for polarization-multiplexed optical signals, in the absence of polarization mode dispersion (PMD). It is shown that simultaneous joint estimation of the symbols, over the two received states of polarization, yields optimal performance, in the absence of phase noise and intermediate frequency offset. In contrast, the commonly used zero-forcing polarization demultiplexer, followed by individual demodulation of the polarization-multiplexed tributaries, exhibits inferior performance, and becomes optimal only if the channel transfer matrix is unitary, e.g., in the absence of polarization dependent loss (PDL), and if the noise components at the polarization diversity branches have equal variances. In this special case, the zero-forcing polarization demultiplexer can be implemented by a 2 ? 2 lattice adaptive filter, which is controlled by only two independent real parameters. These parameters can be computed recursively using the constant modulus algorithm (CMA). We evaluate, by simulation, the performance of the aforementioned zero-forcing polarization demultiplexer in coherent optical communication systems using PDM quadrature phase shift keying (QPSK) signals. We show that it is, by far, superior, in terms of convergence accuracy and speed, compared to conventional CMA-based polarization demultiplexers. Finally, we experimentally test the robustness of the proposed constrained CMA polarization demultiplexer to realistic imperfections of polarization-diversity coherent optical receivers. The PMD and PDL tolerance of the proposed demultiplexer can be used as a benchmark in order to compare the performance of more sophisticated adaptive electronic PMD/PDL equalizers.