# Adaptive Power-Controlled Depth-Based Routing Protocol for Underwater Wireless Sensor Networks
## 1 Introduction  
While land serves as the primary habitat and activity space for human beings, it only accounts for approximately 20% of the Earth's surface area. In contrast, approximately 80% of the Earth's surface is covered by water bodies. Exploring the oceans, much like exploring space, presents both indispensable opportunities and challenges for humanity. In the deep sea, the task of manually exploring the underwater environment is nearly impossible due to factors such as low visibility, high water pressure, and unpredictable underwater events. However, the emergence of underwater wireless sensor networks (UWSNs), consisting of numerous autonomous and self-organizing sensor nodes, has made it possible to explore natural resources on the seabed and collect data during collaborative monitoring tasks [1, 2]. UWSN routing is crucial for enabling underwater applications as it facilitates the transmission of information and data among underwater nodes. However, UWSN routing faces significant differences and poses additional challenges compared to terrestrial wireless sensor networks [3, 4, 5]. On one hand, underwater networks require a higher level of energy balance among nodes, as underwater nodes cannot replenish their energy and have limited energy storage. If a node is excessively utilized, it may become ineffective due to prematureenergy depletion. The premature demise of such nodes can lead to network fragmentation and disrupt normal network operation. Therefore, when designing routing algorithms, it is important to ensure that the energy of all nodes decreases synchronously. On the other hand, underwater network structures exhibit a certain level of dynamism. Due to water currents, nodes may experience movement within a certain range, resulting in dynamic changes in the network's structure. This necessitates routing protocols that can adapt to changes in the network's topology. These characteristics impose higher requirements on routing protocols, demanding the design of protocols that are more suitable for underwater networks. In this paper, we propose an adaptive power-controlled depth-based routing protocol (APCDBRP) that achieves efficient packet forwarding at each hop. In APCDBRP, both data packets and control packets contain information such as the sending node's transmission power, remaining energy, and depth. During the route establishment phase, we employ a sender-based routing algorithm. The sink node broadcasts beacon packets at maximum transmission power, and when a relay node receives a beacon packet from a particular node, it measures the received signal strength to determine the required power level for the previous hop node. Based on the information and power level, the relay node calculates the forwarding factor for that node and stores it in the routing table. Finally, the node updates the information in the beacon packet and broadcasts it at maximum power. To avoid redundant beacon packet transmissions, each node only sends a beacon packet once within a certain time period. To address the issues of node mobility and the low channel utilization and high energy consumption caused by frequent beacon packet transmissions, we employ an implicit routing update mechanism. In this mechanism, nodes receive all the data packets and control packets within their communication range. They then update and maintain the routing based on the signal strength and information contained in these packets. The main contributions of this paper are as follows:  
1. It has proposed a new routing protocol that utilizes the implicit routing update mechanism, enabling the network to update routing information with fewer control packets. 2. It has introduced a novel forwarding factor that considers both energy efficiency and remaining energy, allowing the sending node to select the optimal next-hop node, and significantly improving network efficiency. 3.