## 2 Research Background
### Localization Protocols Proposed for UWSNs
In LDB, the AUV uses a directional acoustic transceiver. It is a range-free scheme and an extended version of UDB [27] while it assumed node deployment in 2D and LDB works for three-dimensional underwater sensor networks. Sensor nodes listen to the broadcasting messages from the AUV silently and estimate their locations based on the location of the AUV at the time of entry and exit from the communication range of the sensors. LDB is not accurate enough because its accuracy suffers from both vertical and horizontal errors. Moreover, its accuracy depends on the frequency of AUV messages [10], although it is an energy-efficient localization method. In [28], a multi-stage localization scheme is proposed for three dimensional underwater networks. 3 DUL is divided into two different phases: a Ranging phase and a Projection and Dynamic Trilateration phase. It is a synchronization-free method, but involves high message exchange. Moreover, 3 DUL has high localization delay that affects the localization accuracy in mobile underwater acoustic sensor networks [10]. Scalable Localization with Mobility Prediction (SLMP) is proposed in [8] by [PERSON] _et al._ It is a prediction-based localization method and it is assumed that every sensor node needs to determine its location periodically. Localization is performed in the same hierarchical architecture of LSL [29], with buoys, anchors and ordinary sensors. The localization process comprises two sub-processes: anchor node localization and ordinary node localization. First, the time is divided into multiple prediction windows with length set to \(T_{w}\). Window length \(T_{w}\) should be an integer multiple of the localization period \(T_{1}\), which is the period every node needs to get its location. In every localization period, anchor nodes can easily communicate with the surface buoys, but the authors omit how the anchors localize themselves [2]. They could also predict their future mobility patterns based on their past measurements. At the same time, an anchor will calculate its estimated location and check the validity of the predicted location. If the error between the estimated location and the measured location is smaller than the stipulated threshold \(S_{t}\), then the anchor mobility model is valid. Otherwise, anchors should determine the new mobility pattern and broadcast it to ordinary sensor nodes. While in SLMP, anchor node localization is a challenge, itself. For ordinary node localization, each node maintains a reference list to record all its known reference nodes (nodes with known locations and confidence value higher than the confidence threshold). In a localization period, if a nonlocalized ordinary node does not receive any localization message, it will update its current location estimation using its previous location estimation and its predicted speed vector. If it receives a localization message from a reference node, it will update its reference list and perform new location estimation [8]. Communication overhead and energy consumption in prediction-based schemes such as SLMP depend on the mobility pattern [2]. Moreover, the length of localization period \(T_{1}\) has a reverse influence on the communication overhead. Other drawback of SLMP is its high computational complexity [30]. Related to different localization schemes and their performances, various literatures and surveys are available [4, 9, 10]. Overall, in localization of underwater wireless sensor networks, range-based schemes are better choice because range measurements using acoustics are much more accurate than when utilizing radio [31]. Range free methods have high communication overhead and energy consumption [10]. TOA-based ranging techniques are generally the preferred mode of range-based schemes [4], but require synchronization. If the UWSN is used for a long term mission, an additional synchronization protocol needs to be executed before localization [2]. Most of underwater applicationsdemand distributed localization while they establish localization process with periodic beacon signals.