# A Mobility-Assisted Localization Algorithm for Three-Dimensional Large-Scale UWSNs
## 1 Introduction
However, due to underwater acoustic communication, long propagation delay and the speed of sound variation, time synchronization is difficult to achieve underwater. In general, the idea of TDOA-based underwater localization algorithm is to simultaneously transmit acoustic and electromagnetic signals by using anchor nodes. Then, the unlocated node uses the time difference of the received signal to estimate the distance between the sensor node and the anchor node. Although this type of localization algorithm does not require time synchronization. However, its multipath effect is relatively serious. Furthermore, since electromagnetic signals cannot be transmitted over long distances underwater, this method is not suitable for large-scale UWSNs. Therefore, in this paper, we consider and implement time-synchronization-free distance measurement of sensor nodes, which is suitable for large-scale UWSNs. To obtain more accurate positioning of the sensor node, most underwater localization schemes estimate the location of sensor nodes by using anchor nodes, detachable elevator  
Figure 1: Range-based localization algorithms: time of arrival (TOA), time difference of arrival (TDOA), angle of arrival (AOA), direction of arrival (DOA), and received signal strength indicator (RSSI). and unmanned underwater vehicle (UUV) [13] assistance. In addition, a UUV can be a remotely operated vehicle (ROV) or autonomous underwater vehicle (AUV). Compared with other auxiliary equipment, the cost of mobile anchor nodes is relatively low. In a three-dimensional (3D) UWSN, four non-coplanar anchor nodes are required to estimate the location of sensor nodes. In a two-dimensional (2D) UWSN, the sensor node can be located by using only three non-collinear anchor nodes. In this study, we assumed that all sensor nodes are equipped with underwater pressure sensors (UPS) to obtain the depth information in real time. Then, the projection algorithm was used to convert the 3D sensor node localization to 2D planar positioning. Moreover, in a realistic underwater environment, the flow of water causes the sensor node's (including anchor nodes) overall migration. Therefore, the motion of sensor nodes is considered and analyzed in this paper. In this paper, we propose a mobility-assisted localization scheme that is time-synchronization-free (MALS-TSF) for 3D large-scale UWSNs. In this localization scheme, we consider the drift state of the sensor nodes in UWSNs and the deployment distribution of anchor nodes. Moreover, the localization scheme can be divided into two phases. In the first phase, anchor nodes are distributed in the monitoring area, which reduces the monitoring cost. Then, we address a time-synchronization-free localization scheme to obtain the coordinates of the unknown sensor nodes. In the second phase, we use the method of two-way TOA to locate the remaining sensor nodes. Therefore, the localization scheme does not require time synchronization. Furthermore, compared with other localization schemes, the superiority of the localization scheme is expounded. The simulation results show that MALS-TSF can achieve a relatively high localization ratio. The main contributions of our paper are as follows:  
1. A novel localization scheme called MALA-TSF is proposed in this paper. The localization scheme does not require time synchronization. 2. The deployment distribution scheme of anchor nodes in the monitoring area is designed. 3. This scheme is suitable for the localization of sensor nodes in large-scale UWSNs. 4. Numerical simulation of the proposed scheme is presented, and simulation results and comparisons with other methods are analyzed. The rest of the paper is organized as follows: In Section 2, we survey localization algorithms according to their different natures. The related system model is described in Section 3. In Section 4, a novel mobility-assisted localization scheme called MALS-TSF is proposed. Then, simulation results and comparative analysis are presented in Section 5. Finally, Section 6 concludes this paper.