# An Energy Balanced and Lifetime Extended Routing Protocol for Underwater Sensor Networks
## 5 Simulations
### Regular Node Distribution
When the sensor nodes in S5 are dead, the sensor nodes in \(S1\) still have over 90% battery power. At sending round 901, about 75% of sensor nodes are dead and cannot transmit data packets any more. In BEAR protocol (Figure 7d), the energy consumptions among sensor nodes in different circular rings are similar due to the use of energy balancing scheme. However, the overall energy consumption of BEAR is high as all sensor nodes are dead after 801 sending round. This is because in BEAR, each sensor node needs to broadcast control packets after each sending round to inform its neighbors of its residual energy. This operation wastes a significant amount of energy. In Figure 7a,c, the BTM protocol and our proposed EBLE show a similar improvement in terms of energy consumption when compared with BEAR and direct one-hop transmission. All sensor nodes are kept alive at the 901 sending round and nodes in different circular rings consume their battery power at a similar speed. Although EBLE use more candidate forwarding nodes than BTM, there are no significant gaps in the energy consumption performance. This is because the nodes in the network are distributed regularly, which means the optimal energy efficient paths for multi-hop transmission are the same for BTM and EBLE. When the next candidate forwarder's EL is lower than the sender, the sender cannot find other candidate sensor nodes which have more energy and are closer to the sink. So both BTM and EBLE change their transmission mode to direct one-hop transmission when their forwarders lack energy. Figure 6: Percentage of dead nodes at different sending rounds. Figure 8 illustrates the percentage of dead nodes in varying sending rounds. For direct transmissions, 20% of the sensor nodes are dead at the 38 th sending round and 70% of the sensor nodes are dead before the 270 th sending round. So it is clearly seen that the direct one-hop transmission protocol cannot use the battery power in a balanced way. For BEAR, the energy consumption among different nodes are balanced and almost all sensor nodes are dead at the same time. However, BEAR uses part of its battery power to broadcast its residual energy information and makes the network lifetime end at an early time. BTM and EBLE achieve better performance in the regular node distribution case as the sensor nodes can transmit more data packets before their battery power is depleted. Figure 9 shows the average end-to-end delay for the four protocols. The communication delay for direct one-hop transmissions is the lowest because data packets are forwarded to the sink directly without any relay. Here we assume that each node spends 0.1 s to process the data and state transition (from receiving state to transmitting state). So each relay induces at least 0.1s delay for each sending process. The delay also comes from possible longer routing paths. This part of the delay cannot be avoided for multi-hop transmissions. So EBLE/BTM is slightly inefficient in network throughput, but the network lifetime is prolonged significantly for EBLE and BTM. Figures 10 and 11 illustrate the \(FirstNodeDead\) and \(50\% Dead\) network lifetime performance, respectively. It can be seen that the time points when the first node is dead and 50% node are dead are the same for EBLE and BTM. However, parts of the sensor nodes in direct transmission mode deplete their battery power at an early time, which makes the network unable to sense some parts in the distribution area. So the network ends much earlier than EBLE. The lifetime performance of  
Figure 7: Percentage of residual energy: (**a**) EBLE, (**b**) Direct transmission, (**c**) BTM, (**d**) BEAR. BEAR protocol improves compared with direct one-hop transmission protocol, but it still performs significantly worse than EBLE and BTM. Figure 8: Percentage of dead nodes at different sending rounds. Figure 9: Average end-to-end delay. The network radius is 1500 m.  
Figure 12 shows the overall network consumed energy per sending round for different protocols.