# An Energy-Efficient and Obstacle-Avoiding Routing Protocol for Underwater Acoustic Sensor Networks
## 2 Related Work  
In UASNs, the oceanographic data collection is often completed by communication among nodes. An efficient communication is inseparable from the support of routing protocols. In recent years, the design of acoustic network routing protocol has gradually become a hot topic in underwater acoustic network research [14; 15], and has been committed to solve the unique characteristic of UASNs such as high energy consumption, high-latency and low packet delivery ratio. In this section, we provide a review on research works on this topic. The Vector-Based Forwarding (VBF) protocol [16] is a classical underwater acoustic routing protocol for three-dimensional underwater sensor networks. The protocol establishes a cylindrical routing pipeline between the source node and the destination node. Only the nodes in the pipeline can participate in packet forwarding. This protocol can select the optimal nodes to forward data in a network with many sensor nodes, thereby saving energy consumption. In [17], hop-by-hop vector-based forwarding routing protocol (HHVBF) is proposed. In HHVBF, each forwarding relay that performs forwarding establishes a virtual routing pipe between the itself and the destination node. Thus, HHVBF divides the long pipe in the VBF into several short pipes to forward data. Besides, HHVBF and VBF protocols both set a waiting time for each forwarding relay to avoid the packet collision, thereby further saving energy consumption. In [18], the authors proposed a depth-based routing (DBR) protocol for underwater networks. DBR uses the depth sensors to perceive depth information for simple greedy forwarding. The forwarding relay with lower depth is selected to forward the packet, which increases the packet delivery ratio but also consumes much more energy. Many proposed routing protocols, like DBR, have high energy consumption. However, the power of the sensor nodes is limited, so the energy problem has gradually become a key challenge of the underwater acoustic routing protocol. In [13], a power-efficient routing protocol with the aid of the fuzzy logic system and decision tree, which determines the appropriate forwarding relay to forward the packets to the destination node,is proposed. This routing protocol uses a tree trimming mechanism to prevent the growth of the packet forwarding tree, thereby effectively reducing the energy consumption of the sensor nodes. Besides, recharging the battery for a sensor node deployed under water for communication is a very laborious task [19], so the limited energy restricts the network lifetime of UASNs, which makes it important to find routing protocols that extend the network lifetime. Energy Balanced and Lifetime Extended Routing (EBLE) protocol is proposed in [20]. This protocol, which selects the forwarding relays with high residual energy and relatively low-cost paths based on the cost function and residual energy level information, not only balances the traffic load with the remaining energy but also optimizes the data transmission by selecting the routing path with low cost. In [21], Q-learning-based delay-aware routing (QDAR) algorithm utilizes a Q-learning algorithm to extend the network lifetime for UWSNs. QDAR algorithm defines an action-utility function in which propagation delay ratio and residual energy are both considered for acoustic routing decisions, and thus can prolong the network lifetime by distributing the residual energy. Moreover, sensor nodes are usually sparsely deployed in UASNs, and movement of the sensor nodes caused by water currents is inevitable, which cause the void problem. In order to reduce the probability of encountering void holes in the sparse networks, the protocols proposed in [22; 23] can reduce the void hole problem during forwarding path by selecting forwarding relays based on the information on the number of two-hop potential neighbors. In [24], the authors proposed weighting depth and forwarding area division DBR routing protocol (WDFAD-DBR).