## 2 Research Background
### Localization Protocols Proposed for UWSNs  
In this section, we briefly overview on investigated localization schemes for UWSNs. There are a couple of studies on localization for underwater acoustic networks. For example, underwater \"GPS\" systems [16] and PARADIGM [17] have been proposed based on surface buoys and one-hop communication [8]. [PERSON] _et al._[18] proposed an Area-based Localization Scheme (ALS). In this method anchor nodes periodically send out beacon signals at varying levels. A two-dimensional region is partitioned into non-overlapping subareas by the ranges of the different levels of anchor nodes. Each non-localized underwater sensor node keeps a list of anchors and corresponding lowest power level and forwards this information with the sensor data to the sink. Sink is able to estimate the node's location. This scheme provides an estimation of a sensor's location within a certain area, rather than the exact location (range-free localization technique). This method is a centralized method because the sink is responsible for estimating the node location where the ordinary sensor node resides [10,18]. ALS involves a high number of message exchanges and high energy consumption due to the sending of localization information to the sink [10], while it is able to estimate its location, already. Moreover, it is a stationary architecture network, so ordinary sensor nodes which are closer to the sink have lower lifetime rather than those further away from the sink because all messages must pass through them to reach the sink. Centralized location computational mechanism in static networks with periodic beacon signals is less efficient compared to distributed ones, while non-localized sensors achieve localization information via periodic beacon signals, so they are able to estimate their location, already before sending information to the sink, but this computational mechanism saves sensor energy in terms of calculating the position. A Dive and Rise Localization (DNRL) protocol is proposed by [PERSON] _et al._[19] for a mobile underwater wireless sensor network which utilizes mobile DNR beacons for localization. As GPS signals cannot propagate far through the water, the Dive 'N' Rise beacons carry GPS receivers and gain their coordinates when they rise to the surface and broadcast their positions, periodically in deep water to help underwater node's localization when they dive. Ordinary sensor nodes use TOA range measurement technique to calculate their distance to the DNR beacons. The main idea behind TOA method is based on the simple known followed equation:  
\[\mathrm{x}=\mathrm{v}.\mathrm{t} \tag{1}\]  
where x is defined as distance between two sensor nodes and will be measured by using time propagated signal and signal velocity. With this technique, measured distance between two nodes i and j is derived by [11]:  
\[\mathrm{dist(i,j)}=(\mathrm{t2}-\mathrm{t1}).\mathrm{v} \tag{2}\]  
where t1 and t2 are send and receive time, respectively. The distance estimates and the coordinates of anchors are used in lateration. Lateration can be used to estimate \(n\) coordinates if there are \(n\) + \(1\) or more beacon messages [2]. DNRL is a 3-D distributed method with high energy efficiency and provides accurate estimates because the beacons update their locations, periodically and consider mobility of water currents, but this method requires a large number of DNR beacons for high localization success and they are more expensive beacons [10,19]. In [20], Multi-Stage Localization (MSL) is proposed to solve the coverage problem of DNRL by adding an iterative localization phase. In fact, it is the extended version of DNRL which dives to a predetermined depth. It is a multistage method which means the successfully localized nodes act as new anchor nodes and help others to estimate their locations, but the major drawback of this method is its high overhead due to iterative localization, so it is less energy efficient than DNRL.