Triple-Convex Probabilistic Constellation Shaping PAM8 in IM/DD System

A novel triple-convex 8-ary pulse amplitude modulation (TC-PAM8) probabilistic constellation shaping (PCS) scheme with bipolar symmetric alphabet is proposed in the intensity modulation/direct detection (IM/DD) system. Maintaining the basic implementation structure of the probabilistic amplitude shaping (PAS), the target entropy of TC-PAM8 is adjusted by setting the one-degree-of-freedom shaping factor accordingly. The performance of the proposed scheme is verified by an O-band 20-GBaud experimental system over 20-km standard single-mode fiber (SSMF) transmission, where the entropy is set to 2.2, 2.5, and 2.8 bit/symbol. The experimental results show that the proposed TC-PAM8 exhibits a better transmission performance compared with Maxwell-Boltzmann PAM8 (MB- PAM8) and Reverse-Maxwell-Boltzmann PAM8 (Rev-MB-PAM8) shaping scheme. Specifically, TC-PAM8 achieves a maximum gain of up to 1 dB and 2.2 dB in receiver sensitivity compared with the MB-PAM8 and Rev-MB-PAM8, respectively when the entropy is 2.2 bit/symbol.