Mode-Dependent Loss and Gain Estimation in SDM Transmission Based on MMSE Equalizers

The capacity in space division multiplexing (SDM) systems with coupled channels is fundamentally limited by mode-dependent loss (MDL) and gain (MDG) generated components amplifiers. In these systems, MDL/MDG must be accurately estimated for performance analysis troubleshooting. Most recent demonstrations of SDM perform estimation digital signal processing (DSP) techniques based on the coefficients multiple-input multiple-output (MIMO) adaptive equalizers. Although methods provide a valid indication order magnitude accumulated over link, MIMO equalizers are usually updated according to minimum mean square error (MMSE) criterion, which known depend channel signal-to-noise ratio (SNR). Therefore, filter also impaired noise. this paper, we model analytically influence SNR DSP-based estimation, show that technique prone errors. Based transfer function MMSE equalizers, assuming SNR, calculate correction factor improves process moderate levels SNR. validated simulation 6-mode long-haul transmission experimentally using 3-mode link. results confirm limitations standard method scenarios high additive noise MDL/MDG, indicate as possible solution practical scenarios.