# Cooperative Position Aware Mobility Pattern of AUVs for Avoiding Void Zones in Underwater WSNs
## 3 Proposed Schemes
### Position-Aware Mobility Pattern
Therefore, it is assumed that transmission power is adjusted on the basis of the n-confidence region, which is (power adjustment) computed according to Equation (3). Estimation of the confidence regions is performed in order to adjust the transmission power precisely and to minimize the packet error rate \((PER=(1-BER)^{n})\), which results in a high packet drop ratio. \(n\) denotes the number of bits transmitted in a packet. The BER is calculated according to Equation (4). \[BER=\frac{1}{2}erfc(\sqrt{S}NR) \tag{4}\]  
where \(erfc()\) denotes the complementary error function, and the SNR is computed by Equation (5). \[SNR=\frac{\Delta PA(\Delta R,f)}{N(f)\Delta f} \tag{5}\]  
\(\Delta P\) denotes the adjustment made in transmission power \((P_{n+1})\) according to the distance between the source and the destination; \(A(\Delta R,f)\) represents the attenuation on the acoustic channel over a range \(\Delta R\)\((\Delta R=(R-r))\) for a signal of frequency \(f\); and the total attenuation of the transmitted signal is computed on the basis of spreading factor [28] and Thorp model [29] given in Equation (6). \[10 log(\alpha(f))=\left\{ \begin{array}{l}\frac{0.11f^{2}}{(1+f^{2})}+\frac{4 4f^{2}}{(4100+f)}+0.000275f^{2},\\ f\geq 0.4\\ \\ 0.002+0.11\left(\frac{f}{(1+f)}\right)+0.011f,\\ f\leq 0.4\end{array} \right. \tag{6}\]  
where \(\alpha\) is absorption loss in dB/km and \(f\) denotes operating frequency in Hz. Absorption loss \(\alpha(f)\) is calculated according to Equation (7). \[\alpha=\frac{10^{\alpha(f)}}{10} \tag{7}\]  
If \(\alpha(l,f)\) is the total attenuation of the transmitted signal over path \(l\) due to spreading loss and absorption loss, then:  
\[10 log(A(l,f))=k10 log(l)+l10 log(\alpha(f)) \tag{8}\]  
where \(k\) is the spreading coefficient. \(N(f)\) is the ambient noise computed by considering four noise components (turbulent \(N_{t}\), wind \(N_{w}\), shipping \(N_{s}\) and thermal \(N_{th}\)), as given in Equation (9). \[N(f)=N_{t}(f)+N_{w}(f)+N_{s}(f)+N_{th}(f) \tag{9}\]  
where:  
\[10 log(N_{t}(f))=17-30 log(f) \tag{10}\]  
\[10 log(N_{w}(f))=50+7.5w^{1/2}+20 log(f) \tag{11}\]  
\[10 log(N_{s}(f))=40+20(s-0.5)+26 log(f)-60 log(f+0.03) \tag{12}\]  
\[10 log(N_{th}(f))=-15+20 log(f) \tag{13}\]The s-confidence region estimation solely depends on the present state of a glider irrespective of the channel state and the distance between neighbors; whereas, underwater, the channel state has a major impact on ongoing communication. Sojourn positions in the network decrease the probability of uncertainty in the estimation of the n-confidence region; however, the volume of a region will always be less than an s-confidence region. The neighboring glider estimates the confidence region by considering the link quality (\(lq\) between the source and the destination) and the distance between the receiver and the transmitter. Due to this reason, the n-confidence region is less than the s-confidence region, and the transmission power is adjusted according to the n-confidence region; where \(lq\)[30] is computed as given in Equation (14). \[lq_{d}^{s}=\beta S_{d}^{s}+(1-\beta)lq_{d}^{s-1} \tag{14}\]  
where \(\beta\) denotes the confidence factor of receiving the data successfully and \(S_{d}^{s}\) denotes the success ratio between the sender and the receiver that depends on the \(lq_{d}^{s-1}\) success ratio of the transmissions between the source and the destination after \(t-1\) transmissions. When gliders move to their next respective locations, the following possibilities may occur:  
1. Neighboring gliders are too close
2. Gliders head towards each other
* Neighboring gliders are too close: Figure 3 shows that Gliders 1, 2 and 3 are too close to each other; as a result, they create an excess zone. Basically, the 'excess zone' is an area of the network in which an extra number of sensing nodes or gliders is present as compared to the appropriate number of nodes to fully cover that area. Let us suppose that Gliders 1, 2 and 3 are at sojourn positions.