基于MMSE滤波器的SDM传输模式相关损耗和增益估计
Mode-dependent Loss and Gain Estimation in SDM Transmission Based on MMSE Equalizers
摘要：
具有耦合信道的空分复用（SDM）系统中的容量从根本上受到在组件和放大器中产生的模式相关损耗（MDL）和模式相关增益（MDG）的限制。在这些系统中，MDL/MDG必须准确估计的性能分析和故障排除。具有耦合信道的SDM的最新演示基于多输入多输出（MIMO）自适应均衡器的系数通过数字信号处理（DSP）技术执行MDL/MDG估计。尽管这些方法提供了链路上累积的MDL/MDG的数量级的有效指示，但是MIMO均衡器通常根据最小均方误差（MMSE）准则来更新，已知最小均方误差（MMSE）准则取决于信道信噪比（SNR）。因此，基于自适应滤波器系数的MDL/MDG估计技术也受到噪声的损害。在本文中，我们分析了SNR对基于DSP的MDL/MDG估计的影响，并表明该技术容易出错。基于MIMO MMSE均衡器的传递函数，并假设一个已知的SNR，我们计算出一个校正因子，提高了中等水平的MDL/MDG和SNR的估计过程。通过6模长距离传输链路的仿真和3模传输链路的实验验证了校正因子。结果证实了标准估计方法在高加性噪声和MDL/MDG的情况下的局限性，并指出校正因子作为实际SDM情况下的可能解决方案。
英文摘要：
The capacity in space division multiplexing (SDM) systems with coupled channels is fundamentally limited by mode-dependent loss (MDL) and mode-dependent gain (MDG) generated in components and amplifiers. In these systems, MDL/MDG must be accurately estimated for performance analysis and troubleshooting. Most recent demonstrations of SDM with coupled channels perform MDL/MDG estimation by digital signal processing (DSP) techniques based on the coefficients of multiple-input multiple-output (MIMO) adaptive equalizers. Although these methods provide a valid indication of the order of magnitude of the accumulated MDL/MDG over the link, MIMO equalizers are usually updated according to the minimum mean square error (MMSE) criterion, which is known to depend on the channel signal-to-noise ratio (SNR). Therefore, MDL/MDG estimation techniques based on the adaptive filter coefficients are also impaired by noise. In this paper, we model analytically the influence of the SNR on DSP-based MDL/MDG estimation, and show that the technique is prone to errors. Based on the transfer function of MIMO MMSE equalizers, and assuming a known SNR, we calculate a correction factor that improves the estimation process in moderate levels of MDL/MDG and SNR. The correction factor is validated by simulation of a 6-mode long-haul transmission link, and experimentally using a 3-mode transmission link. The results confirm the limitations of the standard estimation method in scenarios of high additive noise and MDL/MDG, and indicate the correction factor as a possible solution in practical SDM scenarios.
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