Enabling gigabits-per-second underwater wireless optical communication with a Kramers–Kronig relation-based modulation scheme

In this work, we propose a novel, to our knowledge, modulation scheme based on the Kramers-Kronig relation for underwater wireless optical communication (UWOC) systems, utilizing 8-coupled green laser diodes (LDs). This scheme enables a data transmission rate of 1 Gb/s over a 100-m distance for the UWOC link. Similar to quadrature amplitude modulation (QAM) and carrierless amplitude and phase (CAP) modulation, the modulation scheme simultaneously conveys both amplitude and phase information through the construction of a minimum-phase signal at the transmitter by leveraging the well-established Kramers-Kronig relation. Notably, its implementation does not require a mixer but only a single Hilbert filter, significantly reducing computational complexity compared to both QAM and CAP. This reduction in complexity makes the proposed scheme a more efficient solution for UWOC. This scheme achieves a bit error rate (BER) of 2 × 10^-³ for the 100-m 1 Gb/s UWOC link, which is well below the hard decision forward error correction (HD-FEC) threshold. This performance is achieved by the application of a two-stage equalizer comprising a CAP equalizer and an artificial neural network (ANN) equalizer. Future work will focus on extending the transmission distance, with the aim of reporting further advancements in the near future.