LDPC-Coded Probabilistic Shaping PAM4 Based on Many-to-One Mapping in WDM-PON

In this article, a 5 x 25-GBaud wavelength division multiplexing passive optical network (WDM-PON) is reported and experimentally demonstrated with each channel carrying a low-density parity check- (LDPC-) coded probabilistic shaping 4-ary pulse amplitude modulated signal (PS-PAM4). In such a splitter-based WDM-PON system, 8-point 3-bits/symbol PAM8 is mapped into 4-point PAM4 by adoption of a many-to-one mapping scheme, which enables improved receiver power sensitivity and increased spectral efficiency, compared with the uniformly-distributed PAM4. Different from the typical constant composition distribution matching method for PS, no shaping codes or PS overheads are needed in our PS-PAM4signal which is to be encoded and decoded in a LDPC-based bit-interleaved coded modulation iterative decoding (BICM-ID) system for the compatibility with the forward error correction (FEC) applied in most of the optical transponders currently. In the BICM-ID system, the extrinsic messages are delivered circularly between the PS de-mapper and the LDPC decoder to deal with PS many-to-one mapping ambiguity. The experimental results strive to make the WDM-PON system better spectrum efficiency, as well as superior receiver power sensitivity and fiber nonlinear effect tolerance, thus maximizing the input power at the optical line terminal and economic efficiency of the system due to ampler loss budget provided for the legacy optical distribution networks.