Deep-Depth Probability-Maintained Notch Sequences for Pulse Amplitude Modulation

Nonlinear distortion is critical for optical communication systems with a high baud rate and a high-order modulation format. Thus, a simple and accurate method to measure the nonlinear distortion is highly desired. Although simple notch, which directly removes the certain frequency components of nonlinear system input and then measures the re-growth components of nonlinear system output, is straightforward to measure nonlinear distortion, it is only applicable to the Gaussian signal. However, pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM), which are widely used in communication systems, are not Gaussian signals. In this study, we propose a deep-depth probability-maintained notch generation method for the PAM to accurately characterize the nonlinear distortion based on the signal spectrum measurement. Employing the proposed time order perturbation, the notch depth for PAM2/4/8 achieves 20/24/28 dB, which allows for a large dynamic range nonlinear distortion measurement for various modulation formats. The experiment demonstrates that the nonlinear noise power ratios are measured within a root mean square error of 0.6 dB for the uniform PAM2/4/8 as well as the probabilistic shaped PAM4/8. The proposed method is also applicable to square QAM formats because a square QAM is composed of two independent PAMs.

非线性失真是高波特率、高阶调制格式光通信系统中的关键问题。因此，业界亟需一种简单且准确的非线性失真测量方法。尽管简易陷波法可直接移除非线性系统输入的特定频率分量，并随后测量非线性系统输出的分量再生情况，其非线性失真测量操作直观，但该方法仅适用于高斯信号。然而，通信系统中广泛应用的脉冲幅度调制（PAM）与正交幅度调制（QAM）并非高斯信号。本研究提出一种基于信号频谱测量的保概率深度陷波生成方法，用于精准表征脉冲幅度调制信号的非线性失真。通过采用所提出的时序扰动手段，PAM2/4/8的陷波深度分别可达20 dB、24 dB、28 dB，这为各类调制格式下的宽动态范围非线性失真测量提供了可行方案。实验结果表明，对于均匀分布的PAM2/4/8以及概率成形的PAM4/8，非线性噪声功率比的测量均方根误差均控制在0.6 dB以内。由于方形正交幅度调制由两路独立的脉冲幅度调制信号构成，因此所提方法同样适用于方形QAM格式。