Joint FDE and MLSD Algorithm for 56-Gbit/s Optical FTN-PAM4 System Using 10G-Class Optics

In this paper, we propose a joint frequency-domain equalizer (FDE) and maximum likelihood sequence detection (MLSD) algorithm to eliminate the serious inter-symbol interference for 56-Gbit/s optical faster-than-Nyquist 4-level pulse-amplitude modulation (FTN-PAM4) system using 10 G-class optics. The simplified FDE structure is first designed to replace the feed-forward equalization (FFE) and post filter structure in conventional joint FFE, post filter and MLSD algorithm. In the joint algorithms, the simplified FDE structure has approximately $13\%$ and $1\%$ computational complexity compared to FFE with least-mean-square (FFE-LMS) adaptive algorithm and FFE with recursive-least-squares (FFE-RLS) adaptive algorithm, respectively. Therefore, the proposed joint FDE and MLSD algorithm has much lower computational complexity than the conventional joint FFE, post filter and MLSD algorithm. Meanwhile, the experimental results show that the performance of joint FDE and MLSD algorithm is better than that of joint FFE-LMS, post filter and MLSD algorithm, and is similar with that of joint FFE-RLS, post filter and MLSD algorithm. In conclusion, the proposed joint FDE and MLSD algorithm has both low computational complexity and excellent performance for optical FTN-PAM4 systems, which shows its potential for applications in low-cost and low-power short-reach optical interconnects.