# A Novel Cooperative Opportunistic Routing Scheme for Underwater Sensor Networks
## 1 Introduction  
Nowadays, resource discovery in the underwater environment has become one of the important goals to reduce dependency on land resources. However, it is a difficult and costly task to monitor and discover the underwater environment. Underwater sensor networks (UWSNs) have recently attracted much attention due to their significant ability in ocean monitoring and resource discovery. Due to restrictions on the use of radio waves, acoustic transmission is most commonly used in the underwater environment. Required data are collected by the underwater sensors and directed towards the sink on the surface. Afterwards, the sink can transmit collected information to the monitoring centre via satellite for further analysis [1; 2; 3; 4], as shown in Figure 1.  
Some unique features of UWSNs make data forwarding in this environment a challenging task. This includes node movement, low available bandwidth, slow propagation speed, high deployment cost and a lossy environment [5; 6; 7]. It also should be mentioned that the Global Positioning System (GPS) cannot be used in an underwater environment as a localisation system because of the quick attenuation of its waves in water [5]. Furthermore, nodes cannot be aware of their positions by pre-configuration, because they are not stationary due to the water current. Nevertheless, the depth of each node in the water can be estimated through an embedded pressure gauge [8]. Then, depth information can be used during the data forwarding procedure. The presence of void areas, a high bit error rate and energy conservation are perhaps the most challenging issues from the perspective of routing protocols in UWSNs. A void communication area is a three-dimensional region between underwater nodes that lacks any nodes inside (similar to holes). The void area can prevent communication between some of the nodes in the network [9]. There are various reasons for the presence of void areas, such as sparse topology, temporary obstacles, unreliable nodes or links, _etc._[10]. In most cases, the lack of employing enough sensor nodes, due to their high cost, while covering a large monitoring area might lead to sparse deployment of the sensors and, consequently, the creation of some void area. Moreover, the relocation of underwater sensor nodes by the water current can potentially create a void area [11; 12]. On the other hand, the adverse characteristics of the underwater channel can cause a high bit error, resulting from high attenuation, channel fading, noise, Doppler spread, _etc._ The communication channel quality varies at different ocean depths under varying pressure, temperature and salinity. The limited bandwidth of acoustic transmission also reduces the efficiency of communication between underwater nodes [13; 14]. Generally, nodes are considered connected to each other if the transferred signal between them can be decoded without any error. In terms of energy consumption, there are also some restrictions due to the difficulties of replacing or recharging batteries, which are the main energy supply for the nodes, in the adverse and often deep underwater environment. In addition, underwater sensors consume more energy than terrestrial sensors because they use acoustic communication [15; 16; 17]. Thus, employing an efficient routing protocol is quite essential to prolong the whole network lifetime. Opportunistic routing is a promising scheme in sensor networks because of its remarkable ability to increase transmission reliability and network throughput. In this way, packet forwarding is enhanced by taking advantage of simultaneous packet reception of neighbouring nodes of a forwarding node and their collaboration to forward the packet [18; 19; 20]. However, applying a terrestrial opportunistic routing protocol in UWSNs without considering its specific features is not possible in most cases.