Energy-Efficient Covert Communications for Underwater Acoustic Backscatter Systems

In this work, we explore energy-efficient covert underwater acoustic backscatter communications (EC-UABCom) within the framework of the Internet of Underwater Things (IoUT). Specifically, a passive buoy node covertly transmits acoustic information passively to a maritime receiver by reflecting incident acoustic carrier signals from an autonomous underwater vehicle (AUV) transmitter. Simultaneously, the system exploits the uncertainty of underwater noise to mask the covert backscatter information, thereby evading detection by a submarine warden. To optimize the covert strategy, we first derive the warden's optimal power-detection threshold that minimizes the detection error probability, accounting for underwater noise uncertainty. In response to this optimal detection strategy, we propose an energy-efficient covert policy by jointly optimizing the AUV's transmit power and the buoy's reflection coefficient to meet the covertness constraint. We then conduct a performance analysis, providing a closed-form expression for the expected detection error probability at the warden and outage probability at the receiver, thereby revealing their inherent trade-off. Numerical simulations validate the effectiveness of our approach compared to the state-of-the-art methods, demonstrating that higher transmit power and reflection coefficient degrade covertness while improving covert rate performance.