[论文解读] Mutual Information as a Figure of Merit for Optical Fiber Systems

Tobias Fehenberger, Norbert Hanik|arXiv (Cornell University)|Apr 10, 2014

Optical Network Technologies参考文献 38被引用 7

一句话总结

本文提出将互信息（MI）作为光纤系统精确的性能度量指标，通过基于直方图的方法结合盲通道选择技术进行估计。对于6000 km标准光纤链路，16-QAM的MI为3.1 bit/symbol，需29%的前向纠错（FEC）开销；数字反向传播（DBP）使MI提升0.25 bit/symbol，从而在奈奎斯特信道间隔下实现5.7 bit/s/Hz的频谱效率。

ABSTRACT

Advanced channel decoders rely on soft-decision decoder inputs for which mutual information (MI) is the natural figure of merit. In this paper, we analyze an optical fiber system by evaluating MI as the maximum achievable rate of transmission of such a system. MI is estimated by means of histograms for which the correct bin number is determined in a blind way. The MI estimate obtained this way shows excellent accuracy in comparison with the true MI of 16-state quadrature amplitude modulation (QAM) over an additive white Gaussian noise channel with additional phase noise, which is a simplified model of a nonlinear optical fiber channel. We thereby justify to use the MI estimation method to accurately estimate the MI of an optical fiber system. In the second part of this work, a transoceanic fiber system with 6000 km of standard single-mode fiber is simulated and its MI determined. Among rectangular QAMs, 16-QAM is found to be the optimal modulation scheme for this link as to performance in terms of MI and requirements on components and digital signal processing. For the reported MI of 3.1 bits/symbol, a minimum coding overhead of 29% is required when the channel memory is not taken into account. By employing ideal single-channel digital back-propagation, an increase in MI by 0.25 bits/symbol and 0.28 bits/symbol is reported for 16-QAM and 64-QAM, respectively, lowering the required overhead to 19% and 16%. When the channel spacing is decreased to be close to the Nyquist rate, the dual-polarization spectral efficiency is 5.7 bits/s/Hz, an increase of more than 2 bits/symbol compared to a 50 GHz spacing.

研究动机与目标

• 将互信息（MI）确立为光纤系统可靠的信息论性能度量指标，尤其适用于软判决译码。
• 开发并验证一种无需对噪声分布做先验假设的基于直方图的MI估计方法，采用盲通道选择技术。
• 以MI为度量指标，评估光纤系统的可实现速率、频谱效率和编码开销。
• 量化数字反向传播和减小信道间隔对系统容量和频谱效率的影响。

提出的方法

• 通过接收信号分布的经验直方图估计MI，利用盲方法精确确定最优通道数，以最小化估计误差。
• 在简化QAM-AWGN信道（含相位噪声）上验证该方法，结果表明其与解析MI值高度一致，尤其适用于16-QAM。
• 在双Pol复用QAM格式下，对6000 km标准单模光纤链路进行仿真，计算不同发射功率和信道间隔下的MI值。
• 以单通道模式应用数字反向传播（DBP），评估非线性损伤的抑制效果并量化MI增益。
• 频谱效率（SE）按净SE计算，考虑理想FEC开销并假设误码率（BER）可趋近于零。
• 将MI和SE作为相对指标，用于比较调制格式、信道间隔和接收技术。

实验结果

研究问题

• RQ1 是否可以在不假设噪声为高斯分布的前提下，仅通过经验直方图准确估计光纤系统中的互信息？
• RQ2 对于6000 km标准光纤链路，从MI和系统复杂度角度出发，哪种调制格式（如16-QAM与64-QAM）为最优？
• RQ3 数字反向传播对长距离光纤传输中可实现的互信息和频谱效率有何影响？
• RQ4 将WDM信道间隔减小至奈奎斯特速率会对互信息和频谱效率产生何种影响？
• RQ5 互信息在多大程度上可作为光学系统中FEC后误码率（BER）和编码开销的可靠预测指标？

主要发现

• 基于直方图的MI估计方法具有高精度，其结果与16-QAM在含相位噪声的AWGN信道上的解析MI值高度吻合。
• 对于6000 km标准光纤链路，16-QAM在50 GHz信道间隔下达到最高的MI值3.1 bit/symbol，当忽略信道记忆时，所需FEC最小开销为29%。
• 数字反向传播使16-QAM的MI提升0.25 bit/symbol，64-QAM提升0.28 bit/symbol，相应地将所需FEC开销分别降低至19%和16%。
• 在30 GHz信道间隔下，净双Pol频谱效率达到5.7 bit/s/Hz，显著高于50 GHz间隔下的水平。
• 将信道间隔减小至27.5 GHz时，由于频谱重叠和干扰增加，MI值下降，尤其在非线性区域更为明显。
• MI指标能有效量化接收技术（如DBP）带来的性能增益，并可用于按可实现速率比较系统组件。