Four-dimensional modulation employing hybrid concatenated multi-level coding in short-reach coherent optical...

A hybrid concatenated multi-level coded (MLC) four-dimensional (4-D) modulation scheme is proposed and experimentally demonstrated in a short-reach coherent optical transmission system. The inner polar-based MLC with optimal channel capacity is used for single-polarization protection in a 128-level set-partitioning quadrature amplitude modulation (128-SP-QAM) structure. The outer concatenated Bose-Chaudhuri-Hocquenghem (BCH) encoder is utilized to establish polarization interdependence by bundling all significant bits on 4-D modulation. The unification of 4-D decoding is realized through parity bit assisted set segmentation demapping with enlarged Euclidean distance, thus providing effective resistance to polarization-dependent loss (PDL) and in-phase and quadrature (IQ) skew impairments. The experimental results reveal that, when the 60 GBaud 128-SP-QAM signals are transmitted over standard single mode fiber, the proposed scheme achieves improved optical signal-to-noise ratio sensitivity by 0.93 dB, as well as enhanced robustness to PDL and IQ skew impairments, compared with the conventional polarization multiplexing 16QAM.