Low-Complexity Soft-Decision Viterbi Algorithm for IM/DD 56-Gb/s PAM-4 System

In this letter, a soft-decision Viterbi algorithm (SOVA) combined with low-density parity check is investigated to achieve high chromatic dispersion tolerance and alleviate the high optical signal-to-noise ratio (OSNR) requirement in pulse amplitude modulation 4-level (PAM-4) systems. To reduce the complexity increase over a maximum likelihood sequence equalizer, we propose a non-binary SOVA algorithm combining the conventional SOVA and the hard decision VA. To achieve low complexity, only one competing path in a state and time index is used to update reliability. After the updating process, reliabilities that remain un-updated are set to a fixed value according to the hard decided VA output. Also, the proposed algorithm generates reliability scalars instead of a reliability vector. The proposed algorithm was verified experimentally in a 56-Gb/s PAM-4 system with the transmission distance of 20 km. The results demonstrate that the proposed algorithm has an OSNR gain of 0.3 dB in comparison to the conventional SOVA with lower complexity at the BER of 10
^-6
.