# Cooperative Communication Based Protocols for Underwater Wireless Sensors Networks: A Review
## 5 Numerical Results
### Link Layer Cooperative ARQ Performance  
We pass now to detail the performance of link layer cooperative protocols, by specifically focusing on ARQ and HARQ. We still refer to the three-nodes scenario described above, with the relay located at the reference positions A, B and C. The implemented protocols for error control are cooperative SW-ARQ and cooperative SW-HARQ, respectively, that work as follows. The source transmits a single link layer packet to the destination. If detection fails, the destination asks for retransmission first to the a neighbor node, that is the relay, and finally, if necessary, to the source node. In order to rule the timing of feedback and retransmissions, we consider a timeout for each node sending a packet, so that if no feedback is received withing such time interval, the packet is interpreted as wrongly decoded and so automatically retransmitted. The relay node implements DF to perform packet retransmission. The performance are evaluated in terms of link layer throughput, that is function of the potential retransmissions and propagation delays. By referring to a single packet transmission, the throughput is calculated as:  
\[\mathcal{T}=\frac{L(1-\rho)}{T_{SD}+T_{RD}}=\frac{L(1-\rho)}{N_{s}(T_{e}+2\tau _{SD}+T_{f})+N_{r}(T_{e}+2\tau_{RD}+T_{f})}\]  
where \(L\) is the packet size expressed in bits, \(\rho\) is the percentage of packet bits employed for error detection and correction, \(T_{e}\) and \(T_{f}\) are the packet and feedback emission time, \(\tau_{SD}\) and \(\tau_{RD}\) are the signal propagation time from source to destination (and vice versa) and from relay to destination (and vice versa). Finally, \(N_{s}\) and \(N_{r}\) are the number of retransmissions operated by the source and by the relay. Note that \(N_{s}\geq 1\) since the source performs at least the very first transmission, while \(N_{r}\geq 0\). For simulations, we set \(L=100\) bits and \(\rho=0.05\) in SW-ARQ, while we consider \(L=110\) bits and \(\rho=0.13\) in SW-HARQ since in this latter mechanism the information block gathers also overhead for error correction. Furthermore, we considered \(T_{e}=100\) ms \(T_{e}=110\) ms in SW-ARQ and SW-HARQ, respectively, while \(T_{f}=1\) ms. Finally, \(\tau_{SD}\) and \(\tau_{RD}\) depend on the distance between the involved nodes. Even for this simulation case, throughput is also measured as a function of the SNR per bit \(E_{b}/N_{0}\) referred to the direct link between source and destination. In other words, it is measured for different node transmit power levels. Figure 7 reporting the performance of cooperative SW-ARQ shows that the presence of a relay node is beneficial for the management of retransmissions since reducing the propagation delay. Figure 6: BER performance of cooperative communication with MRC. In fact, non-cooperative SW-ARQ (so, involving only source and destination) shows poor performance. Furthermore, the highest throughput is achieved when the collaborative relay node is placed quite close to the destination, so that the link towards the destination suffers from lower attenuation and the probability of successful retransmission increases. Similar trends can be observed also in Figure 8 related to collaborative SW-HARQ. It is important to highlight that having low values of \(E_{b}/N_{0}\) means a low nodes a transmit power. As a consequence, due to channel attenuation and absorption, the quality of the link may be very poor and the occurrence of retransmission increases. In this case, HARQ outperforms ARQ since, despite the overhead carried within each packet increases the packet emission time, it can help to correct errors during detection and avoid retransmission requests. On the other hand, a high \(E_{b}/N_{0}\) means an increasing nodes transmit power, reflecting on a better link quality with the probability of erroneous packet detection lowering. So, in such condition, the performance of SW-ARQ approach that one in SW-HARQ.