Hierarchical coded modulation scheme employing symbol-class non-binary LDPC in short-reach coherent optical transmission

A hierarchical coded modulation scheme employing non-binary quadrature amplitude modulation (NB-HQAM) is proposed and experimentally demonstrated in a high-bandwidth-efficiency optical coherent interconnection system. The NB-16HQAM is generated from two sub-QPSKs coded by Galois Field (4) NB-LDPC, offering enlarged symbol Euclidean distance (ED), thus significantly improving the system performance. A priority hierarchy mechanism based on unequal error protection (UEP) for NB-LDPC codes is proposed to achieve further system reliability gain. Experimental validation on a 60 GBaud polarization division multiplexing (PDM-16QAM) optical coherent interconnection system over single mode fiber demonstrates that, the proposed scheme achieves an optical signal-to-noise ratio (OSNR) gain by more than 1.5 dB, compared with conventional symbol-class NB-16QAM and bit-class 16QAM considering severe inter-symbol interference (ISI) in ultra-high bandwidth efficiency implementation, due to the enhanced symbol correction ability for residual ISI and fiber channel impairments. Thanks to the reduced GF size by hierarchical modulation and UEP, the complexity of the NB-16HQAM is optimized by 75% compared with traditional NB-16QAM, offering a promising coded modulation solution for high-speed optical coherent interconnection.