This project develops and validates a novel technique for establishing acoustic communication links among nodes in an underwater setup. Basically, acoustic links have limited bandwidth and suffers from slow and multi-path signal propagation. These issues make underwater communication a challenge and motivate the use of directional antennas to establish line-of-sight (LOS) paths between communicating nodes. However, the node mobility while serving an application or due to water current will break LOS links and thus hinders effective communication among nodes. To overcome this challenge, the team develops a novel surface-based reflection (SBR) scheme that allows nodes to utilize reflected acoustic links from the water surface or bottom as non-line-of-sight (NLOS) communication links. A prototype modem is to be developed to utilize a multimodal directional transducer for validating the feasibility of establishing NLOS communication when using the SBR scheme. The prototype design could be leveraged for networking protocols such as node discovery, localization, medium access control and routing. The end result is an efficient directional acoustic communication platform that mitigates the effects of multipath propagation while fully utilizing the available spatial spectrum.
This project will boost the effectiveness of many civil and scientific applications. Examples of these applications include search-and-rescue, coastal patrol, oceanographic data collection, environmental monitoring, assisted navigation, and security surveillance. The project will also help enrich the undergraduate and graduate curricula in sensor networks, robotics, and embedded and distributed systems while providing outreach opportunities to demonstrate the prototype to local middle and high schools in order to stimulate interest in this technology among the next generations of scientists and engineers.
Recent years have witnessed an increase in applications of underwater acoustic sensor networks (UW-ASNs). Examples of these applications include search-and-rescue, coastal patrol, oceanographic data collection, environmental monitoring, assisted navigation, and security surveillance. Since radio waves quickly get absorbed in the water medium, acoustic channels are pursued for long range underwater communications. However, the limited bandwidth of acoustic links and the slow signal propagation make communication a challenge, especially when omni-directional antennas are being employed. Moreover, with omni-direction transmission a sender needs to transmit at high power and would thus inefficiently consume the node’s energy resource and reduce the network throughput due to spectrum underutilization. In addition, acoustic waves tend to take multiple paths in a shallow water environment due to reflections from the surface and the bottom. Because of these issues, directional transmission schemes are favored for underwater communications which require known transmitter and receiver positions to establish line-of-sight (LOS) links. However, the node’s position tends to change due to water current and the explicit node mobility in some applications. To address such a challenging networking problem, we have contributed a novel surface-based reflection (SBR) scheme which utilizes directional antennas to enable sensor nodes to establish both LOS and non-line-of-sight (NLOS) links. A NLOS link in this context will based transmissions that get reflected on the water surface to a particular receiver. Thus, SBR exploits the multipath nature of the acoustic channel by using reflections from the surface and bottom to establish NLOS links. The receiver only accepts signals that are reflected once (surface or bottom) by checking the received-signal-strength and comparing it to the calculated surface/bottom attenuation parameters. We opt to develop a SBR-based networking solution for underwater applications by devising protocols and design tools that leverage both LOS and NLOS links and take into consideration the underwater physical layer dynamics. The focus of this EAGER grant is to increase the fidelity of the underlying SBR communication model before tackling the networking issues. Throughout the funding period, major contribution has been made both on the research and educational fronts. The technical outcomes of the project include: (1) developing a prototype modem to utilize a multimodal directional transducer for LOS and NLOS communication; (2) building a test-bed to validate the SBR communication model using a custom-built acoustic node and a prototype water-tank-based environment; (3) validating the SBR-based communication using the water-tank based prototype and also the UMBC swimming pool; and (4) developing SBR-based node localization protocol to establish a relative coordinate system and locate drifted nodes. We believe that we have demonstrated the validity of the SBR model and its potential for providing a robust UW-ASN solution. The prototype will be leveraged in the development of networking protocols such as medium access control and routing. On the educational front, a module about the design of underwater communication and networking has been added to the wireless sensor networks course at UMBC. Since its inauguration in fall 2010, the course has been gaining popularity among the graduate computer science and engineering students. The course was offered in the fall 2013 semester and the capacity had to be expanded to accommodate demand. A Ph.D. graduate conducted his dissertation research on this project and multiple undergraduate students got trained in conducting research and gained hands-on experience while working on the prototype. Currently, a MS student is pursing a research project and is expected to graduate in December 2014. Multiple papers have been published to disseminate the research results.
