# Cooperative Communication Based Protocols for Underwater Wireless Sensors Networks: A Review
## 1 Introduction
### Motivation and Contribution  
The paradigm of nodes cooperation was firstly introduced in terrestrial RF systems, defining that communication scenario where multiple nodes share their own transmit antenna to realize a virtual distributed multiple-antenna transmitter and benefit from spatial diversity to achieve improved performance in terms of data rate and reliability. Hence, nodes cooperation was only considered to improve the received signal quality. In RF communications the challenges introduced by channel propagation represent a well known and largely investigated subject. So, the procedures ongoing at the link and network layers are handled quite independently of what happens at the physical layer. Differently, the UWSNs work in a time-varying and hardly predictable scenario, where the channel impairments have a straight impact not only on pure signal propagation, but also on other aspects of network management. Therefore, researches have explored the potential of a cooperative approach to address many different issues. In fact, at the physical layer, the presence of cooperative nodes may help to improve the link reliability, mitigating multipath, attenuation and Doppler effects through adaptive schemes and signal processing. Another crucial challenge in UWAC is the long propagation delay, which has a significant impact when dealing bi-directional signaling between nodes. This is typically the case of error control, handled at the link layer by means of Automatic Repeat reQuest (ARQ) protocols. In this regard, involving cooperative nodes in the mechanism for ensuring a reliable data transmission seems to be promising for the achievement of a more efficient channel occupation as well. Lastly, nodes cooperation is also explored in the context of data routing at the network layer, aimed to not only optimize the data traffic, but also to minimize the nodes energy consumption and increase the network lifetime.  
Based on these considerations, in this work we discuss how the paradigm of cooperative communications can be fruitfully exploited in the context of UWSNs, reviewing the state of the art of related strategies and protocols.  
By investigating the most relevant issues about the physical, link and network layers of the protocol stack, herein we review:  
* The signal combining techniques based on network nodes cooperation to mitigate the channel impairments;
* The ARQ protocols and medium access control (MAC) strategies recast in the context of cooperative UWSNs and employed for error control and channel resources management;
* The clustering and routing protocols tailored to cooperative UWSNs and aimed to traffic optimization and energy saving.  
As a graphical support to the reading of the paper, Figure 2 reports at glance the classification of the cooperative techniques investigated in this work. A detailed analysis of the mentioned techniques is provided in the next sections. Some numerical results about the performance of the considered mechanisms and strategies are also provided. Finally, the recent trends in cooperative UWSNs management supported by machine learning are presented.