Multidimensional Bit-Wise Sparse Decoding for Residual IC-XT Mitigation in Optical Long-Haul CC-MCF Links

Coupled-core multicore fiber (CC-MCF) transmissions rely on sufficient taps of multiple-input and multiple-output (MIMO) equalizers which should cover the channel temporal pulse response raised by the elevated spatial mode dispersion of fibers. Otherwise, the residual inter-core crosstalk (IC-XT) after MIMO will impede the signaling quality when employing finite taps of MIMO equalizers. In this study, we experimentally examine how residual IC-XT evolves by evaluating the covariance among MIMO-equalized spatial tributaries in multiple-span CC-MCF transmission links as the MIMO equalizers' tap length decreases. As indicated by results, increasing covariance values happen along with the stronger correlations among spatial modes after MIMO equalization with using shorter-tap equalizers. Multidimensional bit-wise decoding (MD-BWD) can process the spatial domain-constructed multidimensional symbols, offering adaptive decoding capability for the strongly-correlated spatial modes but introducing significant DSP computational complexity. To address this, we propose a sparse representation of log-likelihood ratio calculation in MD-BWD to minimize the reference labels needed for soft information acquisition during decoding procedure. Experimental results for optical 20-Gbaud DP-QPSK transmissions over 1500-km CC-4CFs show that the proposed multidimensional sparse decoding method reduces complexity by 96.5<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> compared to traditional MD-BWD, while maintaining Q factors below the 20<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> overhead FEC threshold using short-tap MIMO equalizers. This advancement in reducing DSP complexity could facilitate the practical implementation of MD-SD signaling to mitigate residual IC-XT issues.