## 4 Designed Schemes
### Neighbors Recognition  
As the network is initialized, all nodes communicate with the sink nodes to know their position coordinates. Then an SRN transmits a CP. All the nodes that exist inside the communication range with respect to the SRN, receive the CP. The CP format is depicted in Figure 2, which consists of the information of maximum residual energy, shortest distance, and lowest BER. The received CP lets the neighbors of SRN to know its residual energy and distance from one of the sink nodes. initially, the  
Figure 1: Network Model. BER is computed by detecting the variations of the known bits pattern in the CP Once data routing takes place among the nodes, the BER is computed again for the exchanged data packets. The upper four sink nodes located at water surface are equipped with GPS. These sink nodes provide help for determining the coordinates of the undersea nodes. By taking one sink node as a reference node in \(x\) direction, second sink node in \(y\) direction and third in \(z\) direction, the distance is calculated by using the time of arrival and the speed of the acoustic waves [26; 27]. Moreover, the coordinates of the targeted nodes are estimated by using the relative distance from the surface sinks. Suppose a node has one hop from the sink node specified for \(x\) direction coordinates then it means the node has coordinate 1 in the \(x\) direction. Likewise, the second and third sink nodes will check the number of hops in \(y\) and \(z\) order, indicating the corresponding coordinates of the nodes. This process continues and nodes with known coordinates are used as references nodes by other nodes to specify their own coordinates. The process of coordinates' determination by nodes takes place early at the start when network initialization takes place and before the recognition of neighbors. The CP is also used by nodes to become aware of one another coordinates. After coordinates of the nodes are known, they exchange the information of coordinates. This makes the distance calculation easy among nodes. The CP is broadcasted periodically for the sake of keeping the routing table updated [28]. When a node gets the CP, it inserts its energy and distance information and retransmits it in the direction of the SRN. The SRN collects all the neighbor nodes information and creates a routing table. The unique ID of each sensor node differentiates each node position. The creation of the routing table is essential for the selection of DSN among the neighbors. The process is continued until all the neighbor nodes swap their information with each other. The BER of neighbors is then computed by the node that has received the CP by observing the change in the predefined bit patterns. For BER calculation, the SRN forwards the CP among the neighbors to check the link quality. By using the Binary Phase-Shift-Keying (BPSK) modulation for Raleigh fading channel, the average BER in data packets can be found as [29],  
\[BER=\left(1-\sqrt{\frac{SNR}{2+SNR}}\right) \tag{1}\]  
\[SNR=\frac{P_{t}}{A(d,f)N(f)} \tag{2}\]  
In the above SNR formula, \(P_{t}\) is considered to be the transmission power, which is fixed for all the nodes, while \(A(d,f)\) is the attenuation, \(d\) shows the separation of nodes that exchange data, \(f\) is the acoustic frequency and \(N(f)\) shows the noise level. The parameters involved in the computation of the SNR are computed according to their models described in [30; 31; 32]. These parameters can easily be computed keeping in view the distance and position coordinates of the nodes obtained using the CP and by specifying the acoustic frequency and nature of the acoustic link. The CP transmission among the neighbor nodes helps to select the favorable link for packet transmission, which has the minimum BER. When the destination node receives the CP in the form of bits, it checks the change in predefined bits pattern, which determines the BER. The bits pattern in the CP for BER calculation is specific and the same for all nodes' CP and is known to all the nodes.