Using Volterra Nonlinear Equalizer and Probabilistic Shaping in an IM/DD System

IM/DD (Intensity Modulation and Direct Detection) systems are widely used in short-haul optical transmission for its low complexity and low cost. IM/DD systems with PAM (Pulse Amplitude Modulation) can be applied to intra- and inter-DC (data center) which has an exponentially growing transmission rates demand. Meanwhile, PS (Probabilistic Shaping), as a channel coding method, can shape the probability of symbols in different amplitude in order to maximize the transmission rate to be close to the Shannon limit. Nonlinear compensation based on DSP (Digital Signal Processing) plays an important role in compensating the degradation of the channel. In this paper, a method which combines with PS and 2nd-order VNLE (Volterra Nonlinear Equalizer), is proposed to reduce signal degradation while VNLE is used for nonlinear compensation at the receiver. The compensation performances by (1) VNLE and FFE (Feedforward Equalizer) with PS, (2) VNLE with PS and without PS are compared in a DML (Directly Modulated Laser)-based IM/DD systems. PS is implemented with CCDM (Constant Composition Distribution Matching) which is used to shape the symbols. In a 10 km PS based IM/DD systems, the SER (Symbol Error Rate) of the transmitted symbol obviously decreases compared to that without PS. The introduction of VNLE method gets a lower SER than that using FFE method. In summary, the proposed method is proved to have a better performance and is supposed to be implemented in the future IM/DD systems.