Enhanced hard decision based on multi-dimensional basis expansion in PAM4 transmission

Abstract

Most of the PAM4 receivers eliminate the linear and nonlinear distortion by equalization and use independent symbols for decision. In this work, to remove the residual correlation between adjacent symbols after equalization, the enhanced hard decision (HD) based on multi-dimensional basis expansion is theoretically investigated and experimentally demonstrated for a 10-GHz directly modulated laser (DML) based 50-Gb/s PAM4 system. After 30-km standard single-mode fiber (SSMF) transmission, compared to the traditional HD, HD with 2-dimensional basis expansion (2D-HD) can achieve reliable transmission below the HD forward error correction (FEC) threshold with a linear feed-forward equalizer (FFE). Sensitivity gain of 1 dB and 2.2 dB can be further obtained by HD with 3-dimensional basis expansion (3D-HD) and 5-dimensional basis expansion (5D-HD), respectively. Moreover, even after the optimal Volterra nonlinear equalizer (VNLE), 5D-HD can still reduce the received optical power (ROP) by 0.6 dB.