# Cooperative Communication Based Protocols for Underwater Wireless Sensors Networks: A Review
## 2 Physical Layer Cooperative Mechanisms  
Providing a reliable underwater acoustic link is crucial to realize efficient UWSNs and, in general, an effective UWAC. In this regard, several studies in the literature have considered the paradigm of relay nodes cooperation in order to mitigate the channel impairments such as multipath and attenuation. The reference scenario is that one depicted in Figure 3, considering a first direct path between source and destination and a second one passing through a relay node. Figure 2: Classification of cooperative communication based protocols for UWSNs. Since the signal emitted by the source can be received by the relay as well, this latter can act as support to the destination by forwarding its own copy of the transmitted signal, exploitable for detection especially when the direct link has poor quality. The simplest mechanism adoptable by the relay is amplify and forward (AF), so it only amplifies the physical signal received from the source and forwards it to the destination. The authors in [16] present a cooperative AF scheme to mitigate the channel impairments. Specifically, the cooperation of the relay is exploited to turn the multipath into favorable conditions allowing a more reliable signal detection by the destination node. In [17], a more complex scenario with multiple relay nodes is considered, with AF combined with distributed space-time cooperative block coding (STCBC) to mitigate intersymbol interference (ISI) rising on the direct link and improve the communication reliability. Still related to a multi-relay configuration, in [18] relay selection and power loading for an orthogonal frequency division multiplexing (OFDM) based communication are investigated, with the goal to maximize the system capacity. Overall, AF is simple and computationally efficient since concerning only the received signal amplification. However, by doing so, together with the useful signal component, noise is amplified too. So, in the case of poor source-relay link quality, noise amplification may lead to the forwarding of a poor quality signal and a bad detection at the destination. As alternative, decode and forward (DF) overcomes the problem of noise amplification since the relay nodes performs detection and re-encoding before forwarding. Of course this mechanism improves the reliability of the forwarded signal, but at the expense of a higher computational and energy cost with respect to AF. Cooperative DF is considered in [19], where the authors investigate the theoretical performance and constraints related to a single-carrier communication suffering from ISI. In [20], a cooperative DF combined with OFDM is discussed, proposing a capacity criterion-based power allocation mechanism for performance improvement. A particular implementation referred as decode, interleave and forward (DIF) is proposed in [21], where turbo equalization, multiuser detection, and combining techniques are also realized to improve the link reliability while reducing the end-to-end delay. A deep performance analysis related to cooperative AF and DF OFDM based communication is given in [22; 23], with authors demonstrating how relay node cooperation can be fruitfully exploited to improve the link performance from different points of view, including reliability, achievable data rate and outage probability. Finally, the authors in [24] propose a cooperative hybrid mechanism where, based on the channel quality, the relay node decides to perform either AF or DF. Table 1 summarizes the state of the art about underwater cooperative communication, providing a general overview of the mechanisms employable at the physical layer. The nature of the underwater acoustic channel poses several challenges affecting the link  
Figure 3: Reference Scenario for Cooperative UWSNs. performance in terms of reliability. The presence of a cooperative relay node allows the destination to benefit from the presence of an additional communication path where to receive a reliable signal when the direct link has poor quality.