# An Energy Balanced and Lifetime Extended Routing Protocol for Underwater Sensor Networks
## 2 Related Work
The relay nodes in the network can cancel duplicated transmissions by checking whether the received packet is an innovative one or not. Only independent packets can be encoded and forwarded to the sink. This scheme outperforms other flooding-based routing protocols in terms of energy consumption. In [5], a channel-aware routing protocol called CARP was proposed to select robust links with link quality information. CARP uses a PING-PONG strategy to obtain the adjacent nodes information. Channel quality, residual energy and buffer space are also considered for selecting best relays. E-CARP [23] improves CARP by considering the reusability of previously collected sensory data. The sensory data are required to be routed to a relay node, only when the difference between the current data and the previous one is obvious enough. Besides, the PING-PONG strategy is simplified and the route table remains unchanged if the network topology is relatively steady. Recently, a group of balanced energy consumption routing protocols for UWSNs were proposed to balance the energy consumption in the network and then prolong the network life. [PERSON] et al. presented a probability and sub-optimal distance-based lifetime prolonging strategy (PAS) [24] for UWSNs. Energy consumption is balanced by carefully choosing transmission modes between single-hop direct transmission (DT) and multi-hop transmission (MT). The network is divided into many circular slices. Then a probability finding algorithm (PFA) can find a set of probabilities for each slice to decide the nodes' transmission mode. A similar balance transmission mechanism called BTM [17] was proposed by [PERSON] et al. BTM divides the transmission process into two phases. Nodes operate an efficient routing algorithm in the routing set-up phase. In the stable data transmission phase, nodes determine one-hop or multi-hop data transmissions based on the adjacent nodes energy level. If the adjacent node energy level is lower than the energy level of the forwarding node, one-hop direct data transmission to the sink is performed. Or else, data can be forwarded to the sink via multi-hop transmissions. This scheme can balance the energy consumption between two adjacent nodes. However, each node only has one relay node and unbalanced transmissions still exist if nodes and traffic load are distributed unevenly. [PERSON] et al. proposed a balanced energy consumption based adaptive routing [16] (BEAR) scheme for IoT enabling underwater WSNs. In BEAR, the network field is divided into a series of cones, the traffic load can be evenly distributed by using zone-to-zone communications. Moreover, BEAR generally chooses two alternative relay nodes. One-hop direct transmissions are only carried out when the residual energy of the two alternative relay nodes are both lower than the average residual energy of the network. Results show that BEAR can prolong the network lifetime significantly. [PERSON] et al. presented two energy balanced strategies [25] to maximize the lifetime of networks. The first is an energy balanced hybrid data propagation algorithm (EBH) which changes the transmission mode between multi-hop and one-hop transmissions according to the residual energy grade. The second is differential initial battery assignment strategy which tries to pre-assign differential initial battery power according to workloads in different nodes. Although the above balance transmission mechanism can balance the energy consumption between different traffic load nodes and prolong the network lifetime to some extent, a great amount of energy is wasted due to the long range one-hop transmissions that balance transmission mechanism brings. Moreover, some UWSNs applications cannot bear such long range transmissions because of the heavy packet collisions and high requirements of the power amplifier. Therefore, their applications are limited in practical use.