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.

大多数 PAM4 接收机通过均衡消除线性和非线性失真，并使用独立符号进行决策。在这项工作中，为了去除均衡后相邻符号之间的残余相关性，基于多维基扩展的增强硬判决（HD）在基于 10 GHz 直接调制激光器（DML）的 50 Gb 上进行了理论研究和实验证明/s PAM4 系统。30km标准单模光纤（SSMF）传输后，与传统HD相比，具有二维基扩展（2D-HD）的HD可以通过线性馈送实现低于HD前向纠错（FEC）阈值的可靠传输- 前向均衡器 (FFE)。通过3维基扩展（3D-HD）和5维基扩展（5D-HD）的HD可以进一步获得1dB和2.2dB的灵敏度增益，分别。而且，即使经过最优的Volterra非线性均衡器（VNLE），5D-HD仍然可以将接收光功率（ROP）降低0.6dB。