A Power-and Spectrum-Efficient Underwater Wireless Optical Communication System based on a DFT-Spread Hierarchical Pre-Distorted LACO-OTFS Scheme

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In this paper, a novel DFT-spread hierarchical predistorted layered asymmetrically clipped optical orthogonal time frequency space (DFT-s-HPD-LACO-OTFS) scheme is proposed to enhance the power efficiency (PE) and spectral efficiency (SE) of the underwater wireless optical communication (UWOC) system. Leveraging the benefits of layered ACO-OFDM (LACO-OFDM), the proposed scheme improves PE and achieves a maximum achievable SE. Furthermore, the challenges posed by the LACO-OFDM, such as inter-layer interference (ILI) and error propagation (EP), are addressed by incorporating the hierarchical pre-distorted (HPD) operation. Additionally, the optical orthogonal time frequency space (O-OTFS) modulation provides a diversity gain to further enhance its performance. In a preliminary experiment, the PEs of the proposed scheme are improved by 21.14%, 16.99%, 14.89%, 12.10%, 9.17%, and 4.39% as the layer number increasing from 1 to 6, respectively, compared to the DCO-OTFS. Moreover, under the hard decision forward error correction (HD-FEC) threshold, the HPD-LACO-OTFS with 4 layers achieves the maximum data rates of 186.2 Mbps and 154.5 Mbps at received optical powers (ROPs) of-43.00 dBm and-46.00 dBm, respectively. Finally, the proposed UWOC system successfully achieves a data rate of 168.0 Mbps in a 50-m anechoic water tank with an underwater transmission distance of 25 m and a total link attenuation of 90.37 dB. To the best of our knowledge, this is the first time to employ DFT-s-HPD-LACO-OTFS for a power-efficient and spectrum-efficient UWOC transmission, and it can be highly beneficial to the power-sensitive and cost-sensitive systems in future deployment.