Low-Latency and Low-Complexity MLSE for Short-Reach Optical Interconnects

To meet the high-speed, low-latency, and low-complexity demand for optical interconnects, simplified layered 2-step maximum likelihood sequence estimation (L2S-MLSE) is proposed in this paper. Simplified L2S-MLSE combines computational simplification and reduced state in L2S-MLSE. L2S-MLSE with a parallel sliding block architecture reduces latency from linear order to logarithmic order. Computational simplification reduces the number of multipliers from exponential order to linear order. Incorporating the reduced state with computational simplification further decreases the number of adders and comparators. The simplified L2S-MLSE is evaluated in a 112-Gbit/s PAM4 transmission over 2-km standard single-mode fiber. Experimental results show that the simplified L2S-MLSE significantly outperforms the FFE-only case in bit error ratio (BER) performance. Compared with simplified 1-step MLSE, the latency of simplified L2S-MLSE is reduced from 34 delay units in linear order to 7 delay units in logarithmic order. The simplified scheme in L2S-MLSE reduces the number of variable multipliers from 512 in exponential order to 33 in linear order without BER performance deterioration, while reducing the number of adders and comparators to 37.2% and 8.4%, respectively, with nearly identical BER performance.