Maximizing effective secrecy throughput in SLIPT-based underwater wireless optical communication systems with artificial noise

In this paper, we investigate the physical layer security of a vertical relay-assisted underwater wireless optical communication system with simultaneous lightwave information and power transfer and artificial noise to enhance both security and energy sustainability. The relay node is equipped with an energy harvesting device, using the harvested energy to transmit the confidential information and artificial noise to the user and eavesdropper. Taking into account path loss, turbulent fading, pointing error, link interruption, and solar noise, we derive the reliability outage probability, secrecy outage probability, and effective secrecy throughput (EST) and analyze the effects of water types, turbulence strength, pointing errors, and link interruption on secrecy performance. Based on this, to further improve the secrecy performance, we propose an EST optimization problem considering the power splitting ratio, including the ratio of energy harvesting and information decoding, as well as the ratio of artificial noise and confidential information. Considering the complexity and coupling of the objective function, we use an alternating optimization (AO) algorithm to achieve EST maximization. Monte Carlo simulations are conducted to verify our derived expressions, and simulation results demonstrate that the power splitting ratio obtained by AO can improve the EST effectively.