# Research on Localization Algorithms Based on Acoustic Communication for Underwater Sensor Networks
## 3 Localization Algorithm
### Range Measurement
#### 3.3.2 Range-Based Scheme  
The range-based scheme is discussed in two aspects, namely, synchronization and asynchronization. Firstly, we present some papers to introduce the synchronization scheme. In [42], the localization in UWSNs is classified into four steps, namely, node discovery, range measurement, data filter, and location calculation. In the node discovery step, the active sensor node measures the range from the neighboring nodes. In the second phase, the nodes send data to the gateway buoy after all nodes have finished the distance measurement. Then, all data are sorted into a stack by the gateway buoy. In the third step, the statistical theories are employed to help the gateway buoy filter all measured data. After that, the results are a 2D matrix. Finally, the algorithm first calculates the position of the unknown node. After all unknown nodes are finished, the iterative process starts. The algorithm has some similar point with the greedy algorithm. In [43], a Dual Hydrophone Localization (DHL) method converts the localization problem into half-plane intersection issues. In this scheme, the network consists of an acoustic source (S), some surface buoys and some dual-hydrophone nodes (shown in Figure 9). The dual-hydrophone nodes connect with the surface buoy by a cable. The buoy sends the sensed data that is received by the nodes to the central station, and then the station calculates the coordinates of the acoustic source by analyzing the data. The beep emitted by the acoustic source propagates along the cable. The node within the sensing range will receive the signal, and then the Generalized Cross Correlation (GCC) algorithm is employed to compute the TDOA from S to its dual-hydrophone. For the ith node,  
Figure 8: System architecture contains sensor nodes and a mobile AUV equipped with four directional beams [39; 40]. we draw a perpendicular plane to classify the whole localization area into two regions. Then, the node judges the source location to the right or left of the perpendicular plane and marks with binary code 0 or 1. A station gathers and analyzes the binary information from the sensor nodes and draws the feasible (polyhedron) region of S. We take the polyhedron center as the estimated location of the acoustic source. In [44], a Sequential Time Synchronization and Localization (STSL) algorithm in UWSNs is proposed. The packet exchanges between the anchor and non-localized nodes are employed in this scheme. Hence, to get accurate short-term motion estimation, the scheme takes advantage of the directional navigation systems used in nodes. The STSL algorithm consists of two steps, namely, time-synchronization step and the localization step. The first step is to provide estimations of the propagation delays that are accomplished by two-way packet exchange. In the localization, the location of the non-localized nodes can be obtained. Sequentially, we introduce some papers that are asynchronous based on a range-free scheme. The issue of asynchronous clocks being located in UWSNs is considered and an On-demand Asynchronous Localization (ODAL) scheme is proposed in [45]. The authors use the sequential transmission protocol and the broadcast nature of the acoustic signal to locate the network with a small overhead. The localization protocol is executed in the initiator node by broadcasting to initiate a transmission sequence with the anchor node. In the localization broadcast procedure, anchor nodes collect and forward information. Passive nodes silently listen to the broadcast messages and localize themselves. In [46], an Underwater signal Reflection-enabled Acoustic-based Localization (UREAL) scheme is proposed. It makes use of the multimodal directional underwater piezoelectric transducers that are used to generate either omnidirectional or directional beams. Therefore, the proposed scheme can utilize both Line-of-sight (LOS) and surface-reflected Non-line-of-sight (NLOS) links to realize the localization of nodes.