# Cooperative Communication Based Protocols for Underwater Wireless Sensors Networks: A Review
## 3 Link Layer Protocols  
The link layer of the network stack implements many protocols to accomplish different tasks, with the most important being related to error control and channel access. Table 2 summarizes the most important studies performed in this field, focused on the paradigm of cooperative UWSNs. Since providing a reliable data transfer is essential, an efficient check of received information integrity is required. In the case where there is no feedback link between destination and source nodes, the link layer packets gather additional overhead to let the receiver perform forward error correction (FEC) whenever the information is detected as corrupted. On the other hand, when a bi-direction link is available between nodes, ARQ protocols are considered to guarantee the correct transmission of information. ARQ relies on the use of feedback signaling from the destination to the source to acknowledge whether data delivery was successful or not. In this latter case, the source retransmits the erroneous packet until its correct reception is achieved. There exist many ARQ implementations, including Stop and Wait (SW-ARQ), Go-Back-\(m\) (GBm-ARQ) and Selective Repeat (SR-ARQ) [28]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Authors** & **Year** & **Protocol/Strategy Type** \\ \hline [PERSON] et al. [16] & 2008 & Cooperative AF (single-relay) \\ \hline [PERSON] et al. [17] & 2008 & Cooperative AF-STCBC (multi-relay) \\ \hline [PERSON] et al. [18] & 2018 & Cooperative AF-based OFDM (multi-relay) \\ \hline [PERSON] et al. [19] & 2016 & Cooperative DF (single-carrier) \\ \hline [PERSON] et al. [20] & 2012 & Cooperative DF (OFDM) \\ \hline [PERSON] et al. [21] & 2019 & Cooperative DIF (single-carrier) \\ \hline [PERSON] [22] & 2013 & Cooperative AF and DF (OFDM) \\ \hline [PERSON] [23] & 2017 & Cooperative AF and DF (OFDM) \\ \hline [PERSON] et al. [24] & 2011 & Cooperative hybrid AF-DF \\ \hline [PERSON] et al. [25] & 2015 & Cooperative AF-based diversity combining \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of physical layer protocols and strategies presented in the literature. The major drawback of ARQ is that packet retransmission is time consuming, especially in the UWSN scenario where the low sound speed and the potential long distance between source and destination may lead to large propagation delay lowering the throughput. In this regard, the cooperation among nodes has been investigated to mitigate such problem. The authors in [29] implement a cooperative SW-ARQ scheme where, together with a source and destination nodes, several intermediate nodes are involved. Given the position of such relay nodes as known to the destination, a cooperative region is defined. So, when the destination node receives a corrupted packet transmitted in a broadcast fashion from the source, it asks first for retransmission from the relay nodes belonging to the cooperative region. A reference description of the cooperative network scenario is given in Figure 4. This strategy reduces the propagation delay related to retransmissions, since relay nodes are expected to be closer to destination than the source node. In [30], a further investigation of the same cooperative ARQ mechanism is provided, by discussing the handshaking procedure to let source and destination nodes identify the cooperative region and the corresponding relay nodes. Finally, a more detailed performance analysis related to throughput, latency and energy efficiency is proposed in [31]. Still related to cooperative SW-ARQ, the authors in [32] define a particular feedback signaling mechanism that allows the relay nodes retransmission scheduling to be managed through a time table. This makes cooperation exploitable without any knowledge about nodes position. Juggling Stop and Wait ARQ (JSW-ARQ) is a modified version of conventional SW-ARQ, that is further adapted to the context of cooperative networks in [33].