Probabilistically Shaped PAM-8 Transmission With Simplified Volterra Equalizer

Abstract

Intensity-modulation direct-detection (IM/DD) probabilistically shaped (PS) 8-level pulse-amplitude modulation (PAM) signal is promising for high-speed intra-data-center networks. However, the challenges of a PS-PAM-8 transmission system are complicated distribution matcher (DM) and nonlinear equalizer. In this work, we propose a simple PS-PAM-8 transmission system, where the probabilistic amplitude shaping (PAS) encoder consists of an energy-level-assigned (ELA)-based DM and a low-density parity-check (LDPC) encoder. In the ELA-based DM, a simple lookup table is designed to realize an approximate bilateral Maxwell-Boltzmann distribution by assigning energy levels to various symbols. Consequently, the DM encoded bits are used as the amplitude bits, and DM label bits and LDPC check bits are designated as sign bits to implement PAS encoding. In addition, a simplified Volterra equalizer (SVE) is introduced to eliminate the inter-symbol interference and improve the receiver sensitivity of PS-PAM-8 signal, where the SVE only includes the 2-order kernel to reduce its complexity. We experimentally demonstrated the proposed PS-PAM-8 scheme and verified its outperformance over uniform PAM-8 signal in the back-to-back and 2-km standard single mode fiber (SSMF) transmission. Compared to the conventional linear equalizer, the SVE contributes approximate 1-dB improvement of the receiver sensitivity in the PS-PAM-8 2-km SSMF transmission, for the error-free performance of post forward error correction bit error ratio.