# An Energy Balanced and Lifetime Extended Routing Protocol for Underwater Sensor Networks
## 1 Introduction  
Recent advances in underwater sensor networks (UWSNs) have motivated the development of various applications for scientific, environmental, commercial and military purposes including environmental data collection, disasters prevention, assisted navigation, monitoring underwater equipments, offshore exploration, oil/gas spills monitoring and tactical surveillance [1; 2; 3; 4]. To avoid the high absorption rate of electromagnetic waves and scattering of optical waves in water, acoustic waves are preferred for long-distance underwater communication. However, adverse characteristics of UWSNs [5; 6; 7] such as dynamic structure, high energy consumption, limited available bandwidth, limited battery power, low transmission speed, severely attenuated channel, high latency, and high bit error rates pose great challenges to reliable underwater data transmissions. In order to prolong the network lifetime, some researchers focus on the energy harvesting technique [8; 9; 10] so that the sensor nodes can harvest energy from the environment and solve the energy limitation problem. However, the underwater sensors cannot use solar power chargers as it is hard for the sunlight to reach the deep sensors in an underwater environment. Besides, the underwater sensors are vulnerable to the seawater corrosion and marine animals' activities. The energy harvesting technique still needs to be improved in underwater environments. Multi-hop transmission is a promising technique for decreasing energy consumption and enhancing system stability in UWSNs and many underwater applications such as environmental data collection (temperature, conductivity, pH, dissolved oxygen, etc. ), imaging underwater life, supervising geological processes on the ocean floor, and monitoring underwater equipments exploiting multi-hop communication to collect sensed data and forward them to the sink nodes on the water surface. So the routing protocol, which aims at choosing a reliable and energy efficient path to forward data to the gateway nodes, is essential in underwater data transmissions. As it is hard to replace the battery in underwater nodes, energy limitation is a vital problem in underwater routing protocol design. Recently, many energy efficient routing protocols for UWSNs were proposed [11; 12; 13; 14; 15]. These energy efficient routing protocols may consume less energy in total, but the energy consumption may focus on a few hot spots and it makes these nodes deplete their energy at an earlier time while other nodes may still have a lot of energy left. This unhealthy energy distribution leads to the void region problem and should be avoided. To solve this problem, a energy balancing technique [16; 17] is proposed to balance the energy consumption in UWSNs. However, these balanced energy routing protocols induce frequent long range direct transmissions which result in high energy consumption and high probabilities of packet collision problem. In some large scale UWSNs, long range direct transmissions are unable to be realized due to the transmission power limitations of sensor nodes. In this paper, an energy balanced and lifetime extended protocol (EBLE) is proposed to prolong the underwater network lifetime. The routing process is divided into two phases: (1) candidate forwarding set selection phase and (2) data transmission phase. In candidate forwarding set selection phase, each node stores the position and residual energy information of its neighborhood nodes and obtains the cost values according to our proposed cost function. During the data transmission phase, high residual energy level and low cost nodes are given higher priorities to forward data and then long range direct transmissions are avoided whereas the network maintains energy balance. A brief explanation of the basic idea of EBLE is shown in Figure 1. A sink node is located on the surface and several sensor nodes are deployed in the semicircle around the sink. There are five paths P1\(\sim\)P5 (solid lines) to transmit sensed data to the sink via multi-hop communications.