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

In this paper, a novel hierarchical pre-distorted layered asymmetrically clipped optical orthogonal time frequency space (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 the maximum achievable SE. Meanwhile, 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. To further reduce the PAPR, a DFT spread technology is also introduced. In a preliminary experiment, compared to the DCO-OTFS, 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 increases from 1 to 6, respectively. Besides, the BER performance of the proposed HPD-LACO-OTFS significantly outperforms that of DCO-OFDM and HPD-LACO-OFDM. Through the comparison among the schemes with 4, 5, and 6 layers, it is found that the scheme with higher layers can achieve a higher data rate under a high received optical power (ROP) condition. Yet, the scheme with lower layers is robust at a low ROP or a high data rate. 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 ROPs of −43.00 dBm and −46.00 dBm, respectively, by a time-frequency domain (TFD) equalizer. By implementing the DFT spread, the receiver sensitivity of the scheme with 4 layers is improved by 1.00 dB at a data rate of 147.06 Mbps. Finally, the proposed UWOC system successfully achieves a data rate of 162.7 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. With the help of DFT spread, the maximum data rate is further improved to 168.0 Mbps. To the best of our knowledge, this is the first time to employ 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 deployments.