## 4 Next Forwarding Node Selection in UWSNs
### Physical Distance
Furthermore, the use of physical distance along with residual energy and link quality helps balance the energy between nodes and further ensures data delivery. However,  
Figure 13: Physical distance calculation in ERP\({}^{2}\)R.  
R-ERP\({}^{2}\)R has some disadvantages. Similar to ERP\({}^{2}\)R, the first phase should be updated with a short interval of time, which leads to high network overhead. Moreover, this protocol does not solve the communication void problem. Furthermore, the use of link quality with energy efficiency ensures better results than ERP\({}^{2}\)R, while the use of only ETX as a link quality metric is not sufficient due to only packet delivery ratio have been tested. In order to reduce the energy consumption and the end-to-end delay, a hybrid beacon-pressure-based routing protocol named energy-efficient fitness-based (EEF) routing protocol is presented in which each node knows its location and sink location [53]. To choose the next forwarding nodes, each node in EEF calculates a fitness value using the distance to sink, the distance to neighboring nodes, depth information, and residual energy based on the following equation:  
\[f\left(n_{f}\right)=\frac{E_{r}^{f}\times depth_{diff}\times d_{sf}}{d_{fd}}, \tag{2}\]  
where \(d_{fd}\) is the forwarder-sink distance, \(d_{sf}\) is the sender-forwarder distance, \(depth_{diff}\) is the sender-forwarder depth difference, and \(E_{r}^{f}\) is the residual energy. The node then embeds the sender ID, fitness value, sender's location information, and destination's location information with the data packet and broadcasts it to its neighbors. If the fitness value embedded in with data packet is less than the receiver's fitness, it can be a candidate for forwarding the data packets. Otherwise, it directly discards the packets. Moreover, the best forwarding node is the node with the highest fitness, i.e., the highest priority with lowest holding time. The main advantage of EEF is that it uses a multi-sink structure, which helps improve the performance of the protocol. Moreover, it utilizes residual energy in calculating the fitness function. However, the main drawbacks of EEF can be highlighted as follows. First, the fitness value has been computed using additional full location information that is finding this information is too costly in an underwater environment. Second, it uses residual energy along with location information without considering any link quality metrics; this leads to unreliable data delivery and insufficient fitness. Third, it uses the same weight function to select the shortest path and suppress unnecessary forwarding, which is too costly in an underwater environment due to the use of full position information. Another physical distance location-free routing protocol, named Distance-based Reliable and Energy-Efficient (DREE) routing protocol for UWSNs, has been proposed as an extension of R-ERP\({}^{2}\)R [54]. DREE aims at improving the reliability between nodes by selecting the next forwarding nodes based on link quality. Different from R-ERP\({}^{2}\)R, it utilizes a fuzzy-logic-based link Quality Estimator (F-LQE) [55] which has not been put to the test before in underwater networks along with physical distance. To this end, sensor nodes compute F-LQE and physical distance. Then, each node broadcasts this information to its neighbors. At the end of this step, each node will have information about its neighbors regarding physical distance and link quality. Next, the sender node selects the next forwarding nodes that are closer to the sink node (physical distance) with good link quality. The node with minimum cost is the best node to forward the data packets; the cost is calculated based on F-LQE and physical distance using this equation:  
\[Cost\ (i,j)=\left(1-\frac{\text{FLQE}(i,j)}{\text{FLQE}max}\right)+\left(1- \frac{\text{DIST}(i)-DIST(j)}{\text{DIST}max}\right). \tag{3}\]  
DREE has some advantages. It does not require localization information for identifying the node's position.