Asynchronous Low Duty-Cycling for Reduced Event-Reporting Delay in Underwater Acoustic Sensor Networks

Event-driven Underwater Acoustic Sensor Networks (UASNs) face the critical challenge of minimizing event-reporting delay while maintaining energy efficiency in resource-constrained underwater environments. This paper presents a novel asynchronous duty-cycling communication protocol integrating anycast forwarding and collision avoidance mechanisms to address this challenge without requiring energy-intensive synchronization overhead. The proposed protocol enables nodes to perform independent sleep-wake scheduling while opportunistically leveraging multiple forwarding candidates to minimize transmission latency. To handle packet collisions inherent in asynchronous networks, we developed a specialized MAC-layer collision avoidance mechanism optimized for underwater acoustic communication characteristics, specifically accounting for long propagation delays and effectively mitigating collisions when multiple nodes detect events in close temporal proximity or when multiple forwarding candidates wake up concurrently. The protocol employs a sender-initiated approach that allows nodes to remain silent until events occur, thereby reducing acoustic emissions and making the network less susceptible to detection. We validate the protocol through a comprehensive evaluation comprising hardware implementation, hardware-in-the-loop simulations, and sea trials in the Adriatic Sea. Comparative analysis against a state-of-the-art benchmark demonstrates significantly lower event-reporting delay and reduced per-event energy consumption under equivalent duty-cycle conditions. Experimental results further confirm efficient power management and reliable data delivery even in highly challenging underwater acoustic environments, making the protocol particularly suitable for long-term covert underwater monitoring applications where both performance and stealth are essential.