Research On Routing And Localization Techniques In Three-Dimensional Underwater Sensor Networks

With the depletion of land resources, people turn their eyes on ocean. Various marine activities have been developed. The field of underwater wireless sensor networks(UWSNs) attracts more and more researchers, since it can support people’s marine activities. The research about UWSNs mainly includes acoustic communication, data routing and sensor node localization. However, the underwater environment is complex. Low bandwidth, high latency, limited energy and node float mobility are the basic challenges for underwater sensor network research, which hinder the applications of underwater sensor networks.This paper focuses on the localization and routing issues for dynamic underwater sensor networks. The routing of UWSNs mainly concerns packet deliver ratio, average end-to-end delay and energy consumption, and these metrics are mutually restricted during the design of actual networks. Vector based forwarding protocol is a directional flooding protocol, which is robust and can satisfy the routing of UWSNs well. Compared with flooding protocol, vector based forwarding protocol can save energy by limiting the number of nodes participating in the data routing. It requires the location information of nodes. Previous research assumes that the location information of nodes is known, which is not practical for many underwater sensor networks. Thus, the localization of nodes is unavoidable.Time of arrival(TOA) is a common method for node localization in UWSNs, which is used in the measurement of distances between nodes. Since the transmission speed of acoustic waves in water is five orders of magnitude lower than the transmission speed of electromagnetic waves in air, the requirement for synchronization between underwater nodes is less strict. In this paper, we also adopt TOA based ranging method in the node localization process. For the case of beacon nodes deployed on the surface of the sea, to achieve the localiz ation of nodes in the deep sea, a spreading localization is essential. This method can realize the successfully localization of almost all the nodes. However, as network localization processes, the localization error diffusion problem becomes serious, i.e., nodes in deeper water are located with larger localization errors.By analyzing the architecture of UWSNs, this paper proposes a localization based VBF protocol for three-dimensional dynamic underwater sensor networks. It can update the location information of nodes periodically and adapt to the dynamically changing topology. Besides, to save energy consumption further, we adopt a controlled delay and greedy forwarding during data routing process as well as maintaining a relatively high packet deliver ratio.Uniqueness of localization is a basic issue of network localization. The underwater environment is complex, and the signal transmission is susceptible to multipath effect. The nodes in UWSNs usually can communicate with each other with longer transmission range than the terrestrial case and can move randomly. All the factors will result in larger localization error than terrestrial wireless sensor networks. Furthermore, uniqueness of localization cannot be guaranteed. Flip ambiguity is a common ambiguous problem of network localization, which can cause large localization error. Moreover, partial or entire network can be flipped during the localization process.This paper studies the flip ambiguity in network localization. Firstly, for the two-dimensional network structure, we analyze and formulate the range of actual distance and present a flip ambiguity detection algorithm based on ranging error threshold. Then, for three-dimensional network case, with the help of sensors providing depth information of unknown nodes, the three-dimensional flip ambiguity problem is transformed into the plane which is at the same depth with unknown nodes. Using a similar analysis with two-dimensional case, the flip ambiguity detection algorithm is extended to three-dimensional underwater sensor networks. Simulation results show the presented flip ambiguity dete ction algorithm can effectively avoid flip ambiguities in network localization, and improve the robustness of underwater sensor node localization.