# Self-Organized Proactive Routing Protocol for Non-Uniformly Deployed Underwater Networks
## 1 Introduction  
Underwater wireless sensor networks (UWSNs) have many applications related to environmental monitoring, disaster alerts, and military surveillance. UWSN may be deployed in either shallow or deep waters. When a sensor network is deployed in shallow waters and sensors are attached to the bottom of the sea, a 2D deployment of the network is considered. In deep waters, the nodes are suspended by a rope or a chain, which is attached to the surface buoys or the anchors at the bottom ofthe sea (Figure 1). The suspended sensor nodes in 3D networks keep moving in all directions. This movement is limited by the length of the connecting rope or chain. The continuous random movement of the nodes makes the protocols design more challenging in 3D networks. In a cooperative network, the nodes farther away from the sink node, also known as the gateway (GW) node, send their data to the GW with the help of intermediate nodes known as relay nodes. To select a relay node, the position of its candidate nodes should be known. For example, nodes A, B, C, D, E, F, and G in Figure 2 are at one hop distance from node S. All of these nodes will receive the data transmitted by node S, but only the nodes closer to the GW should retransmit the data. Therefore, before node S can select one of these nodes, it needs to know the position of nodes A, B, C, D, E, F, and G.  
In WSNs, determining the location of a sensor node is trivial. The sensor nodes can directly determine their position with the help of Global Navigation Satellite Systems (GNSS). However, this process in UWSNs is a challenging task because the GNSS signal is not present. One way to determine the position of the underwater nodes is that the GW gets its position with help of GNSS and the rest of the nodes determine their position with respect to the GW. However, in a 3D network, it is very difficult to determine their position by this method because of the continuous limited movement of the nodes. In UWSNs, the position of the nodes can also be determined by Time Difference of Arrival (TDoA) [1] and Time of Arrival (ToA) [1]. TDoA is based on the two different transmission-media,  
Figure 1: 3D underwater wireless sensor networks (UWSN) architecture. Figure 2: Selection of relay nodes. like radio frequency and acoustic wave. The distance is estimated by different arrival times due to the dissimilar velocities of the radio wave and acoustic wave [2]. However, EM waves are not available and TDoA cannot be used in underwater environments. The time of Arrival (ToA) technique is an alternative approach; it is based on the travel time of the acoustic wave from the source to the destination. The sender stamps the time of sending the packet and the receiver calculates the travel time by comparison to its local time and estimates the distance. This method requires time synchronization between the sender and the receiver nodes. Achieving time synchronization among the nodes in a UWSN is very difficult because of variance in the acoustic waves' propagation speed and limited mobility of the sensor nodes caused by the water currents. As an alternative method, the position of the nodes can be determined by measuring their distance from the water surface by means of depth sensors. This method is used by many routing protocols, like DBR [3], LB-AGR [4], and VBF [5]. However, depth sensors have their own disadvantages, like measurement errors, increase in power consumption due to depth sensing and computation, and the extra cost [6]. Because of the constraints and disadvantages mentioned above, the routing protocols which are based on neither the location of the sensor nodes or use the depth sensors are more practical and beneficial. In this paper, we use the distance between the sender node and the candidate nodes as a metric of selection for the relay node, estimated by the received signal strength (RSS). After a review of the state of the art of underwater routing protocols in next section, the three metrics used in the new protocol proposed are explained in detail (Section 3).