# An Energy Balanced and Lifetime Extended Routing Protocol for Underwater Sensor Networks
## 2 Related Work  
Energy limitation, unstable links, long end-to-end delay are key characteristics of UWSNs. Therefore, numerous UWSNs routing protocols have been devoted to solving these problems. In this section, we discuss some of the existing UWSNs routing protocols and analyze their limitations in dealing with underwater reliable transmissions. [PERSON] et al. [13] proposed a vector-based forwarding protocol (VBF) for underwater sensor networks. VBF is a position-based routing approach and a routing \"pipe\" path is constructed to guide data transmissions, only nodes close to the vector from the source to the destination are chosen to forward the message. In this way, only a small fraction of the nodes are involved in routing. VBF also adopts a self-adaptation algorithm which allows nodes to weigh the benefit of forwarding packets. The data forwarding process in low priority nodes can be suppressed to avoid too many redundant transmissions. Packet delivery ratio and average delay performance are improved at the cost of energy consumption. HHVBF [18] was later proposed to further improve the packet delivery ratio performance and robustness of VBF. HHVBF uses hop-by-hop routing vectors and is less sensitive to \"routing pipe\" radius threshold. Results show that HHVBF can improve data delivery  
Figure 1: Explanations on the basic idea of energy balanced and lifetime extended routing protocol (EBLE). ratio performance for sparse networks but consumes more energy. DFR [12] exploits a packet flooding technique to increase the reliability. The number of forwarding packets nodes is controlled with the information of node position and link quality. DFR also performs well in data delivery ratio at the cost of large and unbalanced energy consumption. In [11], a depth-based routing (DBR) protocol was proposed to forward data from the seafloor to the sea surface. A key advantage of DBR is that DBR does not require full-dimensional location information of sensor nodes and only needs nodes depth information which can be easily obtained with an inexpensive depth sensor. DBR can achieve high packet delivery ratios (at least 95%) for dense networks but multiple sinks are required and redundant transmissions are induced. EEDBR [19] is an enhanced version of DBR. Different from DBR, the residual energy of the sensor nodes in EEDBR is also taken into account to improve the network life-time and energy consumption is balanced to some extent. An energy-efficient and void avoidance depth based routing (EVA-DBR) [20] protocol was proposed which can exclude trapped and void nodes from the routing paths using a passive participation approach. The number of participated nodes can be adjusted by changing transmission range settings and then redundant transmissions can be controlled. [PERSON] et al. [15] proposed a void-aware pressure routing protocol (VAPR) which uses periodic beacons to set up next-hop directions and to build a directional trail to the closest sonobuoy. The hydraulic pressure based anycast routing [21] (HydroCast) also utilizes periodic beacons to obtain the neighbor nodes and prioritize forwarding nodes with Expected Packet Advance (EPA). VAPR and HydroCast can solve the void region problem but node residual energy is not considered and some nodes may deplete their energy due to the unbalanced heavy traffic load. Flooding-based protocols perform well in terms of packet delivery ratio in underwater sensor networks but are usually not energy efficient. To solve the high energy consumption problem, [PERSON] et al. [22] proposed an advanced flooding-based routing protocol for UWSNs. The number of the replicas can be reduced by considering the relative positions to reduce the number of relays. The selection of relays is based on the relative distance between relay and source/destination. A node is only chosen as a candidate forward when it is located inside the communication range of the source and is enough close to the destination. Moreover, network coding is introduced into transmissions.