# A Novel Cooperative Opportunistic Routing Scheme for Underwater Sensor Networks
## 2 Related Work  
In this section, we review some geographic routing protocols in the UWSN and how they take advantage of the opportunistic data forwarding to deal with the void and channel fading. It was mentioned that GPS does not work in the underwater environment; however, some studies still assume that underwater nodes can obtain their 3D geographic coordinates with the aid of the localisation service [23; 24; 25], which is reported to be a challenging issue in the underwater environment [26]. Some routing protocols, such as Vector-Based Forwarding (VBF), Hop-by-Hop Vector-Based Forwarding (HH-VBF) and Adaptive Hop-by-Hop Vector-Based Forwarding (AHH-VBF) [22; 27; 28] are location-based greedy routing in which forwarding nodes are selected within a virtual pipeline facing toward the destination. These protocols are receiver-based, which means that when a forwarding node broadcasts a packet, candidate nodes decide whether to collaborate in the packet forwarding [29]. These protocols also consider a desirableness factor to select forwarding candidates among the nodes inside the pipe. In order to reduce the latency, the nodes are selected in a way that a packet is forwarded using the longest possible hop from the transmitter while maintaining its closeness to the routing vector. However, in the underwater environment, the likelihood of bit error increases with increasing the traversed distance. They try to compensate this defect by increasing the radius of the pipe and involving more forwarding nodes, which, however, causes a higher probability of collisions and, hence, wastes energy. In addition, increasing the radius of the pipe does not help to resolve the problem of the void area, which mostly occurs in sparse networks. The transmitter cannot utilise the nodes outside the pipe to bypass a void area located in the pipe. Moreover, these protocols suffer from hidden terminal  
Figure 1: Underwater sensor network. nodes, because the neighbouring nodes of the sender can be out of range of each other (e.g., be placed in different directions of the pipe). Vector-Based Void Avoidance (VBVA) [11] is proposed to mitigate the negative impact of void communications on the vector-based routing protocols, such as VBF and HH-VBF [27; 28]. VBVA exploits two approaches, vector-shift and back-pressure, for dealing with the convex and concave voids, respectively. However, the recovery procedure of VBVA is too complicated to be performed in the real underwater environment. VBVA lets the packets be trapped in a concave hole, and then, it tries to recover them with a time-consuming procedure, leading to higher end-to-end delay. Relative Distance Based Forwarding (RDBF) [16] is another routing protocol that similarly relies on the use of location-based coordinates. In this protocol, packets are relayed through the nodes with the nearest geographical distance to the sink node. RDBF does not limit the forwarding nodes in a pipe or other geometric shape; however, it considers a fitness factor as a threshold to limit the number of forwarding nodes. RDBF also suffers from the high bit error rate, because it mostly relies on the nodes with the shortest path to the sink. In addition, RDBF does not represent a recovery mode to deal with the packets that are stuck in a local maxima node. A group of routing protocols, like GEographic and opportunistic routing with Depth Adjustment-based topology control for communication Recovery (GEDAR) [30], Depth-Controlled Routing (DCR) [31] and Greedy Routing with Distributed Topology Control (GR+DTC) [12], exploits a network topology control scheme, which enables them to deal with the communication void problem. In this way, all void nodes can move vertically to be connected to a non-void node. Network topology control improves the network connectivity and diminishes the impact of the void by utilising the vertical movement capability of the nodes. However, this technique consumes high energy for topology adjustment, which only can be justified in the long-term and non-time-critical applications.