Performance Assessments of Joint Linear and Nonlinear Pre-Equalization Schemes in Next Generation IM/DD PON

Nonlinear distortions in passive optical network (PON) systems degrade signal transmission performance. Joint linear and nonlinear pre-equalizers based on the Volterra series, memory polynomials and artificial neural networks (ANNs) are utilized to mitigate linear and nonlinear impairments. Centralized pre-equalization at the transmitter side of the optical line terminal (OLT) keeps the receiver side digital signal processing (DSP) at the optical network unit (ONU) simple. In this study, performance assessments of these three different pre-equalization schemes in intensity modulation and direct detection (IM/DD)-based 25-GBaud PON systems were evaluated. The joint linear and nonlinear pre-equalization schemes were based on the weight coefficients and the model obtained by channel estimation on the receiver side. We studied and compared the performance of three pre-equalization methods under various implementation conditions, such as received optical power (ROP), number of equalizer taps, and semiconductor optical amplifier (SOA) current. Besides, we studied the equalization performance under different modulation orders, which were applied for data rate optimization to support different power budgets, being the potential solutions for future PON systems with various fiber-reach and split-ratio distributions. As a proof-of-concept, the equalization performance of the 25-Gbaud signals modulated with non-return-to-zero (NRZ), four-level pulse amplitude modulation (PAM-4) and PAM-8 formats, corresponding to 25-, 50-, and 75-Gbps data rates, respectively, were tested. Both numerical simulations and experimental demonstrations were conducted to assess the performance of different pre-equalization schemes. With ANN pre-equalization, more than 4dB improvement in power budget was achieved for 25-GBaud PAM4 and PAM8 signals after 20 km fiber transmission. ANN-based pre-equalizer performed better than ANN-based post-equalizer even when the received signals were in low SNR. The power budgets of the 25-Gbaud NRZ, PAM4 and PAM8 signals after 20 km fiber transmission were 36.2 dB, 29.5 dB and 18 dB, respectively.