# Self-Adjustment Energy Efficient Redeployment Protocol for Underwater Sensor Networks
## 8 Performance Evaluation
### Energy Efficiency Evaluation  
To evaluate the energy consumed during the deployment phase, we measured the distance traveled by sensor nodes to reach their final position in the network area. Our target is to reduce the sensor nodes' displacements as much as possible during the deployment without affecting the coverage rate.  
Figure 8 shows the total distance traveled by sensor nodes with and against water currents during the deployment phase. We notice that the total distance traveled by nodes against the currents is much higher than the distance traveled by nodes with the current. Hence, a large amount of energy of the sensor nodes is dissipated in these displacements. Our target is to reduce this energy (and hence the distance traveled by nodes) while maintaining a satisfactory coverage rate. Our solution, SA-DVFA, as explained in Section 6.2, consists of reducing the displacement of nodes if the work is negative because the node will consume more energy to reach its destination.  
In the following, we study the effect of delaying some node displacements if the work is negative on the network performance in terms of coverage rate, the time needed to reach this rate, and the distance traveled by nodes during the deployment. \(k\) represents the number of consecutive times a node must wait if the work is negative. In the following simulation series, we varied \(k\) from 0 to 5. \(k=0\) means the node should move immediately  
Figure 8: Total distance traveled by the underwater sensor nodes against and with the water currents.  
Figure 7: Comparison of the coverage rate between DVFA and SA-DVFA.  
to the new position and should not wait, even if the work is negative. \(k=i\) means the node can wait maximum \(i\) iteration(s) if the work is negative.  
Figures 9-12 present the coverage rate, total distance traveled by the underwater sensor nodes against the water currents, average distance traveled by the underwater sensor nodes against the water currents, and average distance traveled by the underwater sensor nodes with the water currents, respectively, while varying \(k\) from 0 to 5.  
Figure 10: Total distance traveled by the underwater sensor nodes against the water currents with different values of \(k\).  
Figure 9: Coverage rate with different values of \(k\).  
We remark that the distance traveled by nodes decreases when the value of \(k\) increases. However, the performance of the network in terms of coverage rate also decreases with the value of \(k\). Hence, the target is to obtain a trade-off between the energy consumed and the minimum coverage rate tolerated by the application. For example, if \(k=3\), the coverage rate is reduced by 1%, and this rate is obtained after 2000 s from the beginning of the simulation. However, the distance traveled against the currents and hence the total distance is considerably reduced by 46% and 41%, respectively. The loss of 1% of the coverage can be accepted by the application considering the energy that will be saved. For \(k=5\), the distance traveled by nodes is reduced by 51%. However, the coverage rate decreased from 98% to 92% and this rate was reached at the end of the simulation time. To summarize, the SA-DVFA protocol maintains almost the same good performance as the 3D-DVFA protocol while taking into account the water currents. Moreover, SA-DVFA profits from these water currents to save the underwater sensor nodes by reducing the nodes' displacements during the deployment phase.  
Figure 11: Average distance traveled by the underwater sensor nodes against the water currents with different values of \(k\).  
Figure 12: Average distance traveled by the underwater sensor nodes with the water currents with different values of \(k\).  
Based on the simulation results, we recommend the value 3 for \(k\) which means nodes can wait until 3 times if the displacement is against the water currents. With this value, we obtained an accepted coverage rate while decreasing considerably the energy dissipated by the nodes' displacements during the network deployment phase.