An On-site-based Opportunistic Routing Protocol for Scalable and Energy-Efficient Underwater Acoustic Sensor Networks

With the advancements of wireless sensor networks and the Internet of Underwater Things (IoUT), underwater acoustic sensor networks (UASNs) have attracted much attention, which also has been widely used in marine engineering exploration and disaster prevention. However, UASNs still face many challenges, including high propagation latency, limited bandwidth, and energy consumption, influencing the reliability of data transmission. In this paper, we propose a routing protocol based on the on-site architecture (SROA) for UASNs to improve network scalability, energy efficiency, and transmission reliability. The on-site architecture adopted by SROA is different from the most architectures in that the data center is deployed underwater which makes the sink nodes closer to the data source. The SROA is a cluster-based protocol which adapts to the changes of network scale and avoids single-point failure through the decentralized and distributed sensor networks. To seek optimal routing policies, the factors of energy, delay, and link quality are considered jointly in the reward function of Q-Learning. Moreover, the reduction of packet retransmissions and collisions is advocated using a waiting mechanism developed from opportunistic routing (OR). The SROA realizes opportunistic routing to choose candidate nodes and coordinate packet forwarding among candidate nodes. Scalability of the proposed routing protocols is also analyzed by varying the network size and transmission range. According to the evaluation results, with the network scale ranging from 100 to 500, the SROA outperforms the existing routing protocols, extensively increasing packet delivery ratio and decreasing end-to-end delay.