## 2 Underwater Acoustic Sensor Networks
In [18], an automatic repeat scheme based on a particular request for communication in an underwater scenario is proposed, where they used cooperative device nodes for source-destination reliable path identification. It defines and provides alternative paths to certain connections of source-destination, which rely on the inter-node available information and the distance between source-destination and all other possible cooperative nodes available along the path. Upon receiving of any kind of packets that are erroneous (lost or dropped), the receiver would request the retransmission of that packet from the closest cooperative neighbor node. However, this has a high delay since at every time the node needs to buffer the packet for retransmission [26]. Thus, both the Automatic Retransmission reQuest (ARQ) and Stop and Wait (S & W) protocols suffer from long underwater propagation delays, therefore, they are not suitable for ensuring reliability in underwater cooperative environments that would result in better achieved performance. A cooperative technique which uses virtualization of sensor array from source-destination by adopting the benefits of Multi-input Multi-output (MIMO) systems in diversity space is proposed in [27]. This study adopts two protocols called decode and forward (DF) and amplify and forward (AF), which are cooperative protocols. When the relay node receives the signals, it decodes and forwards them to the destination node. As the source node sends the signal to the relay, it would decode them first and then forwards the decoded signal to the final destination in the DF protocol. Similarly, in the  
Figure 2: Cooperative-based architecture of UASNs. AF protocol, the relay node performs amplifying actions on the received signal and then forwards the amplified signal to the destination node. They provide two methods of transmission, first the direct transmission which is between source-destination and the second is signal transmission which includes the relay node [27]. A study of underwater AF and underwater DF that provides an improvement in the performance of BER in underwater for different configuration channels is presented in [25]. In this research, the focus is on providing reliable communication in which the authors study different underwater applications in three dimensions with cooperative transmission. Moreover, they introduce a hybrid method of forwarding in which the relay nodes have some flexibility and responsibilities of selecting the best scheme for transmission between underwater AF and DF. In order to forward the packet, a source node may look for and locate the relay node that would help the node by forwarding to a certain depth. Upon reception of the packet by the relay node, it would automatically adopt certain forwarding scheme based on the proper conditions of the underwater channels. Thus, these schemes do not provide a mechanism to select the appropriate relay node in a 3D underwater environment. Moreover, it does not take into consideration the energy consumption and the problem of having more than one relay node in the source-destination path. In [28] the authors introduced a cooperative authority method in order to maximize their energy underwater. It is demonstrated by different authorities that cooperation in forwarding the packet of other authorities achieves a maximum lifetime. Thus, the authorities that are selfish exploit the behavior of other cooperative authority. This method of cooperation was investigated by the concept of game theory, i.e., evolutionary game, in which they perform some simple analysis to show an achievement in cooperation even without an incentive mechanism. However, the major drawback of this work is not taking into consideration the mobility of nodes, which results in a dynamic topology and multipath. A study of network topology for multi-hop communication through diversity of cooperative transmission is introduced in [29], which demonstrates a better multi-hop performance as compared with its direct transmission counterpart.