# Cooperative Communication Based Protocols for Underwater Wireless Sensors Networks: A Review
## 3 Link Layer Protocols
The authors in [38] consider Time Division Multiple Access (TDMA) performance improved by resorting to relaying nodes that, through AF performed at physical layer, realize spatial diversity to achieve a more reliable signal detection by the destination node. The improvement of TDMA through nodes cooperation is also addressed in [39], where a novel a COoperative Protocol for PERvasive Underwater Acoustic Networks (COPER) is proposed. Specifically, during each channel access time slot, idle nodes support the active ones by relaying a copy of the transmitted signal towards the destination. The result is twofold, since spatial diversity at physical layer is realized to make signal detection more reliable at the receiver. Furthermore, in case of erroneous link layer packet detection, a cooperative SR-ARQ protocol is activated, with retransmissions being performed by the closest relay nodes and not by the source. By conveniently ruling the packet transmission timing, collision-free access is guaranteed with improved performance in terms of  
Figure 4: Reference scenario for cooperative ARQ. packet error rate, throughput and energy efficiency. In [40], cooperative HARQ is applied to support the reliability performance improvement in Code Division Multiple Access (CDMA). In the proposed mechanism, if the channel between source and destination has poor quality, the transmitting node performs piggybacking on the neighboring nodes with better channels available. This allows the number of packet retransmissions to be reduced, finally improving the system throughput while providing a robust communication. The use of cooperative approaches is also considered in non-orthogonal multiple access (NOMA), as for instance in [41]. Here, the presence of a relaying node performing DF is exploited to realize spatial diversity. So, the destination benefits from multiple copies of the received signal to perform the detection of simultaneously transmitted symbols by resorting to Successive Interference Cancellation (SIC). The reliability performance are investigated in the cases of perfect and imperfect channel knowledge, which plays a fundamental role in SIC operations. Finally, a mechanism for channel resources assignment is proposed in [42], where an underwater network composed of both cognitive and non-cognitive nodes is considered. The cooperation regards the capability of cognitive nodes to perform channel sensing and sharing the related information with non-cognitive nodes, in order to achieve an overall optimized channel usage. Overall, nodes cooperation for channel occupancy management and error control purposes allows to mitigate another main problem related to underwater acoustic propagation, that is the long propagation delay. However, nodes synchronization is a crucial aspect to deal with at the link layer as well. In fact, in a non-cooperative case, error control performed with ARQ or HARQ involves only source and destination, hence the timing of packet transmission and feedback is managed between a single pair of nodes. On the other hand, in a cooperative scenario with the presence multiple relay nodes, the forward and feedback signaling becomes more complex and, as a consequence, perfectly coordinated transmissions are required. Moreover, an efficient channel resources assignment among the different communication hops may help to further improve the efficiency of nodes cooperation.