# Research on Localization Algorithms Based on Acoustic Communication for Underwater Sensor Networks
## 3 Localization Algorithm
### The State of the Nodes
#### 3.4.1 Stationary  
We first consider the active messaging properties based on the stationary state and collect several papers to describe it. In [49], an Asymmetrical Round Trip based Localization (ARTL) algorithm is presented. The algorithm with excellent scalability does not require time synchronization. In this scheme, there exists only one pair of message exchanges and the others passively listen to the beacons. The authors in [50] consider the direction of the sensor node and the beam width and develop a Loop Assisted Synchronization-free localization Algorithm (LASA) to achieve a synchronization-free localization algorithm. This algorithm obtains the distance measure through the two-way range TOA. The link status detection and the symmetric links ranging is completed with the help of a link detection stage. Then, the first part of ordinary nodes is located through the anchor nodes, while the other ordinary nodes obtain the positioning information via an iterative method. Finally, the loop-assist method is employed to locate those nodes whose positioning information is still unknown.  
A reactive localization scheme proposed in [51] can be described as three phases, namely, finding the anchor nodes, reactive localization of sensor nodes, and delivery of information. The first step is finding a subset of anchor nodes to ensure that every sensor node is covered by four non-coplanar anchor nodes. In the second phase, a sensor node detects an event and broadcasts a message. After the anchor nodes receive the messages, they send their positioning information as a response. In the third step, the node transmits messages that contain its position and the sensed information to the sink.  
As for the silent messaging properties based on the stationary state, we still introduce some papers to show it. In [52], an Underwater Sensor Positioning (USP) scheme is proposed. Once two distance information from two anchor/reference nodes is obtained, the to-be-localized sensor node computes its position. If two distances are not obtained, its position can be iteratively resolved.  
A cubescan-based 3D multi-hop localization algorithm presented in [53] is suitable for large-scale UWSNs. The scheme is described as three steps. Firstly, the authors restrain the location of the unknown sensor node in a feasible set. Secondly, the authors construct a weighted constrained multi-hop localization model by studying the factor that may influence distance estimation of the multi-hop localization. Thirdly, dividing the corresponding geometrical intersection into a number of equal size cubes search a certain center as the final results.  
In [54], a time synchronization-free localization algorithm is proposed. This algorithm contains two steps, namely, distance measurement and position estimation. In the first step, the two messages received from the mobile beacon at a different time are used to measure the distance from the sensor node to that mobile beacon. Then, three distances of the sensor node from three different mobile beacons can be obtained. In the second phase, forming a nonlinear equation system by using the three distances can obtain the coordinates of the sensor nodes.