LDPC-coded 50-Gbaud IM/DD PAM-4 transmission employing probabilistic amplitude shaping based on prefix-free code distribution matching

Abstract

We propose and experimentally demonstrate the intensity-modulation direct-detection (IM/DD) transmission employing low-density parity-check (LDPC) coded probabilistic shaping 4-level pulse amplitude modulation (PS-PAM4), assist with pairwise-distributed probabilistic amplitude shaping (PAS) structure based on prefix-free code distribution matching (PCDM). Unlike traditional PAS, the probability of adjacent symbols in the pairwise-distributed PAS architecture is consistent, which facilitates the filling of check bits generated by LDPC in IM/DD systems. On the other hand, PCDM has attracted increasing attention in recent years because of its asymptotic optimality similar to constant composition distribution matching (CCDM) in block length, and its high parallelism in hardware implementation. This is the first time that PCDM is applied in PAM4 signal transmission of PAS architecture in IM/DD system together with LDPC. Experiments of different code rates of LDPC and PCDM are performed. The experimental results show that the receiver sensitivity can be improved by more than 1 dB compared with the uniform-distributed signal transmission.