# An Energy Balanced and Lifetime Extended Routing Protocol for Underwater Sensor Networks
## 1 Introduction
The power consumption increases rapidly with the increase of communication range (proved in Section 4.1), so the power consumption of P1 is much higher than P2 and P4, and data transmissions in P4 consume more energy power than P2. In order to save energy, P2 is preferred when all nodes have enough residual energy. However, as the traffic load of nodes nearby the sink (e.g., node 5) is much higher than that of nodes far away from the sink, a few nodes may deplete their energy while other nodes have abundant residual energy. This unbalanced energy consumption may result in void region problems in UWSNs and sensed data in some regions cannot be forwarded to the sink node efficiently. Some existing energy efficient protocols [11; 12; 13; 14; 15] only focus on finding energy efficient paths, which may easily lead to void region problems. Energy balancing protocols like BEAR [16] and BTM [17] change their transmission mode to direct one-hop transmissions if their optimal energy efficient paths are energy limited. This operation introduces extra energy consumption and cannot use low traffic load nodes efficiently. To prolong the network lifetime, some low traffic load nodes (e.g., node 10) should be distributed with more loads. These low traffic load nodes can communicate with some high residual energy nodes (perhaps with long communication ranges) and relax the burden of other low residual energy nodes. For example, when node 7 lacks energy, node 6 should select node 10 to relay data while current energy balancing protocols like BTM just allow node 6 to transmit data directly to the sink. Another aspect that should be taken into consideration is energy efficiency optimization. Too long distance communication can consume the energy power at a high speed and thus the network lifetime is reduced. In Figure 1, path P1 consumes more power than path P4, but consumes less energy than P5. So path P5 is only chosen when other paths are lacking residual energy obviously or there are no other paths. In our proposed protocol, the aim is to choose both energy balanced and energy efficient paths to forward data packets. The main contributions in this paper are summarized as follows:* An energy balanced and lifetime extended routing protocol, EBLE, is proposed. EBLE can choose several successor nodes according to the cost function and residual energy. When a possible forwarder's energy level is lower than the sender, another suboptimal forwarder can take the place of the current one and thus the energy consumption is more balanced. The network lifetime is extended through choosing both energy balanced and efficient routes. * Detailed analysis of optimal energy consumption for different transmission modes are given and an energy cost function is proposed to optimize data transmissions in UWSNs. * Extensive simulations are conducted to verify the effectiveness and validity of our proposed EBLE and the results show that EBLE outperforms other existing energy balancing routing protocols in terms of energy consumption and network lifetime. Using EBLE, more packets can be transmitted before a first node in the network is depleted of its battery power. The network lifetime is prolonged by 62.8% on average when compared with BTM on random node distribution case. The rest of the paper is organized as follows: Section 2 shows the recent hot research topics and the related research of UWSNs routing protocols. Section 3 describes the network and energy models in UWSNs and makes some assumptions about our proposed protocol. The detailed design of EBLE is illustrated in Section 4 and numerous simulations are given in Section 5. Section 6 concludes the paper at last.