# Cooperative Communication Based Protocols for Underwater Wireless Sensors Networks: A Review
## 3 Link Layer Protocols
In the considered scenario, the source node transmits a sequence of packets to a relay node that, after a correct detection, forwards the packet the destination. By assuming an omnidirectional transmission, the packet forwarded to the destination is received back also by the source and interpreted as a positive reception feedback. In this way, the presence of the relay nodes facilitates not only the packet delivery to the destination, but reduces the typical propagation delay rising due to feedback signaling from the destination to the source. As a result, the transmission efficiency is improved. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Authors** & **Year** & **Protocol/Strategy Type** \\ \hline [PERSON] et al. [29] & 2010 & Cooperative SW-ARQ (relay location aware) \\ \hline [PERSON] et al. [30] & 2016 & Cooperative SW-ARQ (relay location aware) \\ \hline [PERSON] et al. [31] & 2018 & Cooperative SW-ARQ (relay location aware) \\ \hline [PERSON] [32] & 2019 & Cooperative SW-ARQ (relay location unaware) \\ \hline [PERSON] and [PERSON] [33] & 2012 & Cooperative JSW-ARQ \\ \hline [PERSON] et al. [34] & 2013 & Cooperative HARQ \\ \hline [PERSON] and [PERSON] [35] & 2021 & Adaptive Cooperative HARQ \\ \hline [PERSON] and [PERSON] [36] & 2023 & Cooperative HARQ \\ \hline [PERSON] et al. [37] & 2019 & Multi-hop DCC \\ \hline [PERSON] et al. [38] & 2019 & AF-based TDMA \\ \hline [PERSON] et al. [39] & 2018 & Cooperative SR-ARQ-based TDMA \\ \hline [PERSON] et al. [40] & 2019 & Cooperative HARQ-based CDMA \\ \hline [PERSON] and [PERSON] [41] & 2021 & DF-based NOMA \\ \hline [PERSON] [42] & 2022 & Cooperative-Cognitive channel resource management \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of link layer protocols and strategies presented in the literature. Sometimes, when the link layer packet size is too large, retransmitting an entire block of information may be costly and inefficient. So, Hybrid ARQ (HARQ) strategies are considered to merge the advantages brought by FEC and ARQ [43]. Several works have also investigated the reshaping of HARQ in cooperative UWSN scenarios to achieve performance enhancements. In [34], the authors propose a retransmission protocol where the principles of cooperative ARQ are merged with incremental redundancy HARQ. By resorting to rate-compatible punctured convolution codes, it is possible to achieve not only throughput improvement, but also a higher energy efficiency for both source and relay nodes. Cooperative HARQ is also considered in [35], where the destination closest nodes are involved in packet retransmission procedure. Furthermore, the authors propose an optimization algorithm aimed to adapt the modulation scheme and packet size based on the transmission distance between end nodes, thus maximizing nodes energy efficiency with respect to channel conditions. Further analysis of cooperative HARQ is performed by the same authors in [36], where Reed-Solomon coding and selective packet retransmission are considered to improve energy efficiency. Cooperative ARQ schemes are typically investigated in a single-hop scenario where the relay node is assumed to directly receive the data packet from the source and forward it to the destination. A broader study on the use and impact of retransmission mechanism is instead reported in [37], where Dynamic Coding Cooperation (DCC) is implemented to reduce the end-to-end delay effectively and achieve higher energy efficiency performance with respect to conventional cooperative ARQ and HARQ protocols. The effectiveness of physical layer techniques and error control based on ARQ can be evaluated also in terms of minimization of channel occupancy due to retransmissions, implicitly resulting as beneficial for channel resources management in a multi-node communication context. In general, channel access in handled by resorting to either orthogonal strategies exploiting time, frequency, code and spatial domains, or non-orthogonal schemes [44].