Advanced equalizer for 800-Gb/s DP-16QAM coherent transmission systems

In this paper, we propose an advanced equalization scheme by cascading a feed-forward equalizer (FFE)-based 2 × 2 MIMO with a simplified soft-output maximum likelihood sequence estimation (s-SMLSE) for high-speed coherent optical communication systems. The first-stage FFE mitigates the inter-symbol interference (ISI) resulted from bandwidth limitation, which can reduce the complexity of 2 × 2 MIMO equalizer. SMLSE is required to mitigate the residual ISI and achieve noise whitening, meantime achieving the decoding of a pre-FEC of 10^-2. With state compression, the complexity of SMLSE can be significantly reduced, however, undetermined or infinite log-likelihood ratio (LLR) values are generated resulting in soft-decision ineffective. We dynamically replace these undetermined LLRs with reliable LLRs calculated from the preceding FFE the output of cascaded equalization. to ensure high-fidelity soft information and eliminate the decoding penalty induced by state compression. We conduct experiments to transmit a 100-Gbaud DP-16QAM signal over an 80-km standard single-mode fiber to evaluate the proposed scheme. The results demonstrate that the proposed architecture exhibits robust tolerance against severe bandwidth limitations. Compared to the conventional multi-tap MIMO with full-state SMLSE, the advanced equalization scheme achieves almost the same SD-FEC decoding performance while contributing to an overall 28.4% reduction in computational complexity.