# An Energy-Efficient and Obstacle-Avoiding Routing Protocol for Underwater Acoustic Sensor Networks
## 1 Introduction  
Underwater Acoustic Sensor Networks (UASNs) have proven their potential in a variety of aquatic applications, such as marine environmental monitoring, resource exploration, marine animal studies and assisted navigation [1, 2]. However, due to the complex underwater environment, realizing effective communication among sensor nodes in the UASN is much more difficult than those in traditional terrestrial wireless sensor networks (TWSNs) [3]. The reason is that radio waves employed in the terrestrial sensor networks are not feasible in underwater communications because of the rapid attenuation of the electromagnetic radio waves and the sharply decrease of the communication range [4]. Currently, the only feasiblemethod for long range communication in underwater environments is acoustic signals, which can cover long-time and large-scale underwater communications. However, the speed of sound is approximately 1500 m/s underwater, five-orders slower than the speed of radio waves, which causes a 0.67 s/km propagation delay in UASNs [5]. In order to overcome the above limitations and make aquatic applications viable, efficient communication protocols, especially the routing protocols, are essential for successful communications. Geographic routing can work together with opportunistic routing (geo-opportunistic routing) [6] to improve packet delivery ratio. Using the geo-opportunistic routing paradigm, every sensor node in the UASN is aware of its own location. A data packet is broadcast by its source node to the candidate forwarding relay set composed of neighbors of this node first. Then the nodes in candidate forwarding relay set are ordered according to some metrics which define their priorities. In such a way, the candidate forwarding relays broadcast the received packet one by one with a small time-gap in order of priorities. If a lower priority forward relay overhears the same packet forwarded by a higher priority forwarding relay before it starts to broadcast this packet, this relay will discard the packet and will not broadcast it anymore. This procedure is repeated until the packet finally reach the destination node. Such kind of multi-node participation in forwarding can guarantee the packet delivery ratio. However, if multiple nodes participate in the data forwarding procedure, the network energy consumption will increase sharply because of a large amount of information transmission and reception. Therefore, the energy consumption issue has a major impact on designing the routing protocol. Besides, the lifetime of UASNs is restricted by the limited energy due to the difficulty to replace the batteries of the underwater sensor nodes. How to balance the energy of the sensor nodes in order to extend the network lifetime has become a hot issue in UASN research [7]. In [8], a location-free reliable and energy efficient pressure-based routing protocol (RE-PBR) is proposed, which defines the route cost based on link quality and residual energy of the sensor nodes. The forwarding relay selection of RE-PBR takes the residual energy of sensor nodes into account, so that the lifetime of the network can be extended. However, sensor nodes are deployed in UASNs sparsely, and most of the nodes are moving because of water currents or any other underwater action, which causes the problem of void area. The routing protocols like RE-PBR cannot solve the problem of avoiding routing void, leading to the need for an adaptive routing protocol that is able to handle the void area problem. In recent years, to handle the void area problem, many routing protocols [9; 10; 11] have been proposed. In [9], convex void handling and concave void handling based on VHGOR are proposed to bypass the void area. This method can adaptively determine the reconstruction way of routing path according to whether the void encountered by the node is convex or concave and effectively address the void crisis.