###### Abstract  
Underwater Wireless Sensor Networks (UWSNs) are subjected to a multitude of real-life challenges. Maintaining adequate power consumption is one of the critical ones, for obvious reasons. This includes proper energy consumption due to nodes close to and far from the sink node (gateway), which affect the overall energy efficiency of the system. These wireless sensors gather and route the data to the onshore base station through the gateway at the sea surface. However, finding an optimum and efficient path from the source node to the gateway is a challenging task. The common reasons for the loss of energy in existing routing protocols for underwater are (1) a node shut down due to battery drainage, (2) packet loss or packet collision which causes re-transmission and hence affects the performance of the system, and (3) inappropriate selection of sensor node for forwarding data. To address these issues, an energy efficient packet forwarding scheme using fuzzy logic is proposed in this work. The proposed protocol uses three metrics: number of hops to reach the gateway node, number of neighbors (in the transmission range of a node) and the distance (or its equivalent received signal strength indicator, RSSI) in a 3D UWSN architecture. In addition, the performance of the system is also tested with adaptive and non-adaptive transmission ranges and scalable number of nodes to see the impact on energy consumption and number of hops. Simulation results show that the proposed protocol performs better than other existing techniques or in terms of parameters used in this scheme. underwater sensor networks; packet forwarding probability; fuzzy logic +
Footnote †: journal: Sensors  
0000-0001-9980-0001-9980-0001-9980-0001-9980-0001-9980-0001-9980-0001-9980-0001-9980-0001-9980-0001-9980-0001-9980-0001-998001-9980-0001-9980-0001-9980  
sensor nodes which are deployed at locations that may have different depths in the sea volume. These sensor nodes collect the data at their locations and send the data to the sink node (sometimes called gateway node or simply gateway) at the sea surface which sends the data to the on-shore data gathering station. On the contrary, a TWSN uses radio frequency as communication medium and employs traditional protocols like Transmission Control Protocol (TCP), which are not suitable in the underwater environment due to many reasons including attenuation and shorter range. The common classification of sensor nodes is related to their mobility: mobiles, statics, and hybrids. In the mobile category, the sensor nodes are free to move or Mobile Ad hoc Network (MANET) (they are not anchored to the seafloor), which might cause changes in the network topology. It is the case of Autonomous Underwater Vehicles (AUVs) or sea gliders. The static nodes mean that nodes cannot move freely. In a 2D setup architecture in UWSN the static sensors are generally placed near the sea surface and also communicate with the gateway. In a 3D architecture, the nodes are be deployed at different depths using the appropriate gadgets anchored to the seabed. When the gadgets are wire based, the nodes will move around a central position due to the sea currents, which makes the design of a routing protocol more challenging than for a static 3D network. The hybrid setup, which employs a combination of the mobile and static sensors, is used to accomplish specific tasks [2]. Underwater networks vastly suffer from major factors that can affect their overall performance. These factors include channel utilization, localization, routing issues, choice of optimal packet size, environmental effects, MAC issues, etc. The behavior of the underwater acoustic communication channel (UAC) is affected by multiple factors like multipath fading and the Doppler effect, which is due to movement of both sender and receiver nodes, low and variable acoustic propagation velocity, and noise. To cope with all such issues, routing protocols for UWSNs play a vital role and, thus, routing is a primary concern.