4D modulation scheme employing concatenated multi-level coding for short-reach coherent optical communication systems

We propose and experimentally demonstrate a four-dimensional (4D) modulation scheme employing concatenated multi-level coded (MLC) for short-reach coherent optical communication systems. Different from conventional set partitioning (SP) quadrature amplitude modulation assisted with Exclusive OR parity bits, the polar-based eight-level MLC architecture is proposed to provide inner- and inter-polarization protection by encoding all significant bits across the 4D dual-polarization 16-ary quadrature amplitude modulation (4D-DP-16QAM), offering enhanced system bandwidth efficiency. An outer concatenated encoder is further introduced to strengthen polarization-domain interdependence, which effectively boosts cross-polarization correlation and mitigates the error floor. For 4D decoding, a multi-stage decoding (MSD) is utilized to facilitate efficient soft-information exchange between polarization branches, resulting in improved robustness against inter-polarization impairments. Experimental results demonstrate that under a 60 GBaud 4D-DP-16QAM communication system over standard single-mode fiber, the proposed scheme achieves a 0.77 dB improvement in optical signal-to-noise ratio (OSNR) sensitivity and exhibits superior robustness to both polarization-dependent loss (PDL) and in-phase and quadrature (IQ) skew, compared with a conventional coherent optical DP-16QAM communication system.