# On Reliable and Efficient Data Gathering Based Routing in Underwater Wireless Sensor Networks
## 7 Relays Selection  
Relays selection is another basic concern of our proposed protocol since proper relays selection immensely contributes in network's enhanced performance. Multiple protocols have been suggested up-till now that incorporates relay selection method to achieve specified parameters considering different factors e.g., number of alive neighbors, node's energy, node's depth and others. However, as our main objective is the network's energy conservation and data reliability, therefore, considering these parameters our protocol selects relays by keeping in view the channel characteristics and depth information of nodes. Furthermore, in order to ensure successful packet acceptance at the destination, all nodes are enabled to demand for two retransmissions of a single received packet. This is done via transmitting NACK1 and NACK2 in consecutive phases. A new set \(B_{c}\) is generated by the following steps:  
Figure 4: Slicing of transmission range of source node. * Sender nodes broadcast the packet in their respective transmission ranges. * Nodes residing in the list of possible cooperation set takes part in cooperation process one by one. * If BER exceeds the threshold value, receiver sends NACK message towards the sender node as well as the source node. * Source node updates the history of wireless link quality based on the number of retransmission requests. * We assume a slow and flat Rayleigh faded environment. * \(B_{c}\) is updated occasionally depending upon the weight updates carried out for that specific path. * In this way multiple cooperating nodes are available to cooperate with the sender node. In our suggested technique, relay communication takes place in two stages as depicted in Figure 5. In the first stage, the source node broadcasts whereas cooperative and destination nodes receive that packet. Upon reception, the cooperative node uses AF to relay the received data. In comparison to DF, AF requires reduced E2E delay as the relay node operates time-slot by time-slot [43]. Moreover, AF demands lesser computational powers because of absence of decoding and quantizing at the relay node. In secondary stage, this processed data is re-forwarded towards the destination. Upon reception, the destination node applies the Maximal Ratio Combining technique (MRC) over the entire copies of received packets. MRC is adopted because it is the optimum combiner for independent AWGN channels. MRC functions by adding the signals such that different proportionality constraints are employed for each channel. Suppose \(\eta_{c}\) consists of two nodes (\(C_{1}\) and \(C_{2}\)), \(x\) is the original message signal being transmitted and AF is used as a cooperative relay model, then:  
\[S_{S,D}(t)=\alpha_{S,D}\;x(t)+n_{S,D}(t) \tag{13}\]  
\[S_{S,C_{i}}(t)=\alpha_{S,C_{i}}\;x(t)+n_{S,C_{i}}(t),\quad C_{i}\;\epsilon\; \eta_{C} \tag{14}\]  
\[S_{C_{i},D}(t)=\alpha_{C_{i},D}\;x(t)+n_{C_{i},D}(t),\quad C_{i}\;\epsilon\; \eta_{C} \tag{15}\]  
Considering the above equations, \(S_{S,D}(t)\) be the representation of a signal being sent from \(S\) and received at \(D\). Whereas, \(S_{S,C_{i}}\) and \(S_{C_{i},D}\) be the signals sent from \(S\) towards \(C_{i}\) and from \(C_{i}\) towards \(D\) respectively. Moreover, \(\alpha_{S,D}\) and \(n_{S,D}\) be the channel gain and channel noises existing on the link in  
Figure 5: Cooperative diversity system. between \(S\) and \(D\) whereas \(\alpha_{S,C_{i}}\) and \(n_{S,C_{i}}\) be the channel gain and channel noises in between \(S\) and \(C_{i}\) respectively. Similarly, \(\alpha_{C_{i},D}\) and \(n_{C_{i},D}\) be the channel gain and channel noises existing at the \(C_{i}\) and \(D\) link. In order to mitigate the impact of fading over the received signal, receive diversity is employed in our protocol. Thus, the probability that all the copies of that signal get faded is minimized. Initially, nodes broadcast control packets and maintain the neighboring list. Soon after, the destination and cooperative nodes are finalized (Algorithm 1).