## 2 Research Background
### Message Exchange Mechanism  
A major constraint of underwater acoustic (UWA) networks is their limited energy supply [32]. Whereas the battery of a wireless modem can easily be recharge or replace on land-based systems, the replacement of an underwater modem battery involves ship time which is costly and time-consuming. Therefore, transmission energy is precious in underwater applications [33]. The energy consumption of acoustic modem is asymmetric, where the transmit power is often 100 times more expensive than that of the receiver mode [34], so acoustic transmissions are very power demanding. Moreover, the available bandwidth of underwater acoustic channels is severely limited. However, the receive mode in acoustic modems are inexpensive in terms of power consumption, but it has the same impact like transmit mode in limited bandwidth and increase the communication overhead. Consequently, in USN, localization protocols are expected to avoid excessive overhead and establish message exchange mechanism with the least exchange messages, both received and \begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Anchor** & **Sensor** \\ \hline
**D’N’R** & Send = I \(\times\) A & Send = 0 \\  & Receive = 0 & Receive = \(\sum_{i=1}^{I}\sum_{i=1}^{A}\mathbf{c_{a}}\) \\ \hline
**MSL** & Send = I \(\times\) A & Send = \(\sum_{i=1}^{I-1}L_{i}\) \\  & Receive = 0 & Receive = \(\sum_{i=1}^{I}\sum_{i=1}^{m}L_{i}\)S\({}_{i}\) \\ \hline
**UPS** & Send = A \(\times\) I & Send = 0 \\  & Receive = (2A - 2) \(\times\) I & Receive = A \(\times\) I \\ \hline
**AAL** & AUV-Send = 2I & Send = \(\sum_{i=1}^{i}\mathbf{c_{a}}\) \\  & AUV-Receive = \(\sum_{i=1}^{I}\mathbf{c_{a}}\) & Receive = \(2\sum_{i=1}^{I}\mathbf{c_{a}}\) \\ \hline
**LDB** & AUV-Send = 1 & Send = 0 \\  & AUV-Receive = 0 & Receive = (2r / \(\times\) T)\(\times\)**Ca** \\ \hline
**3 DUL** & Send = 2I \(\times\) A & Send = \(\sum_{i=1}^{I}\mathbf{c_{a}}\) \\  & Receive = \(\sum_{i=1}^{I}\sum_{i=1}^{I}\mathbf{c_{a}}\) & Receive = \(2\sum_{i=1}^{I}\mathbf{c_{a}}\) \\ \hline
**SLMP** & Send = \(\sum_{i=1}^{k}\mathbf{A_{i}}\)(_not valid_) & Send = \(\sum_{i=1}^{k}\mathbf{L_{i}}\) \\  & Receive = \(\sum_{i=1}^{k}\mathbf{A_{i}}\) & Receive = \(\sum_{i=1}^{k}\mathbf{c_{a}}\) (_not valid_) + \(\sum_{i=1}^{k}\mathbf{c_{c}}\) \\ \hline
**RLS** & Send = \(\sum_{i=1}^{e}\mathbf{C_{i}}\) & Send = \(\sum_{i=1}^{e}\mathbf{S_{i}}\) \\  & Receive = \(\sum_{i=1}^{e}\mathbf{C_{i}}\) & Receive = 0 \\ \hline \hline \end{tabular}  
**I:** The total number of intervals. It is calculated based on \(\frac{T}{T_{s}}\) where T is the localization period and \(T_{s}\) is the time slice. **I,** will be decreased with increasing \(T_{s}\); **K:** The number of localization period; \(\mathbf{S_{d}}\):** The number of detector sensor; _Ca:_ The number of sensor nodes located in anchor communication range. \(\mathbf{C_{a}}\) is increased by enlarging the anchor communication range; \(\mathbf{C_{a}}\)(_not valid_): The number of sensor nodes located in anchor communication range with invalid prediction pattern; \(\mathbf{L_{i}}\): The number of localized nodes in \(t^{th}\) interval; \(\mathbf{Cs}\): The number of nodes located in the communication range of the localized sensor nodes; \(\mathbf{A}\): The number of anchor nodes; \(\mathbf{A_{not\ valid}}\): The number of anchor nodes which their mobility prediction is not valid; \(\mathbf{r}\): Sensor nodes communication range; \(\mathbf{v}\): The speed of AUV movement; \(\mathbf{C}\): The number of anchors located in ordinary sensor nodes communication range; \(\mathbf{e}\): The number of events which occurred in T.  
\end{table} 
Table 2: Message Exchange Mechanism in Terms of Communication Cost. transmitted. In Table 2 we present a comparison between average communication cost in different highlighted and efficient existing underwater localization schemes which were described in Section 2.2. We illustrate the message exchange mechanisms of these methods based on receive and transmit messages from both anchors and ordinary sensor nodes.