Recombinant real-Björck sequences assisted frequency-domain MIMO scheme for multi-impairment compensation in self-homodyne MDM system

Self-homodyne mode division multiplexing (SH-MDM) is considered a promising candidate for achieving high capacity and low complexity in next-generation optical interconnects. However, practical implementation remains challenging with channel impairments caused by transmission mechanisms, particularly mode coupling (MC) and differential mode group delay (DMGD), as well as receiver hardware impairments (RHI) originating from imperfect coherent receivers, including IQ skew, IQ amplitude imbalance, and IQ phase imbalance. To address these challenges, this paper, for the first time, proposes a recombinant real-Björck sequences assisted frequency-domain multiple input multiple output (RRB-FD-MIMO) scheme to achieve the joint equalization of MC, DMGD and RHI . The enhanced RRB sequences with constant amplitude, ideal periodicity, and excellent autocorrelation properties are employed to achieve high-precision joint impairment matrix estimation of mode impairments and RHI. Simultaneous compensation of these effects is accomplished through frequency-domain equalization. Experimental validation using a 28 GBaud SH-MDM-16QAM signal transmission over strongly coupled few-mode fiber indicates that, by setting the MIMO with a fixed 21-symbol channel memory length, the scheme can effectively compensate for DMGD within the range of -705 to 603 ps, IQ skew of ±300 ps, IQ amplitude imbalance from -19 to 18 dB, and IQ phase imbalance from -70^∘ to 73^∘, by considering a Q-factor penalty within 1dB. For comparative study, a reference complex-Zadoff-Chu sequence assisted FD-MIMO (CZC-FD-MIMO) scheme shows significant sensitivity to various IQ impairments, with rapid performance deterioration occurring even under small amount of IQ skew, IQ amplitude imbalance, or IQ phase imbalance. The results confirmed the robustness and superiority of the RRB-FD-MIMO scheme and its potential as an efficient digital signal processing (DSP) solution for high-speed, low-cost optical interconnects in future networks.