Effective Design of Underwater Networks

Recent years have witnessed growing interest in the use of underwater acoustic networks (UWANs) in many applications. Examples of these applications include environmental state monitoring, oceanic profile measurements, leak detection in oil fields, distributed surveillance, and navigation, etc. For these applications, a set of nodes are employed to collaboratively monitor an area of interest and track certain events or phenomena. In addition, it is common to find autonomous underwater vehicles (AUVs) acting as mobile sensor nodes that perform search-and-rescue missions, reconnaissance in combat zones and coastal patrol. These AUVs are to work cooperatively to achieve a desired goal and thus need to be able to, in an ad-hoc manner, establish and sustain communication links in order to ensure some desired level of quality of service (QoS). Therefore, each node is required to adapt to environmental changes and be able to overcome broken communication links caused by external noise affecting the communication channel and due to node mobility. In addition, since radio waves tend to get absorbed in the water, it is common for most underwater applications to rely on acoustic rather than radio channels for long range communications. However, acoustic channels pose multiple Illustrating the SBR-based localization, where node S0 uses intersection points {R1, R2, R3, R4} on the surface to solve for the position of S1.
 challenging issues, most notably the high transmission delay due to slow signal propagation and the limited channel bandwidth due to high frequency attenuation. Furthermore, the inhomogeneous property of the water medium affects the sound speed profile while the signal surface and bottom reflections leads to multipath effects. To address these networking challenges, our research focus is geared towards establishing networking protocols that take into consideration the underwater physical layer dynamics.
Ongoing Research
To overcome these issues, we have developed a novel surface-based reflection (SBR) scheme, which uses the reflections from the water surface, and bottom, to establish signal-reflected or non-line-of-sight (NLOS) communication links. SBR is used to incorporate both line-of-sight (LOS) and NLOS links by utilizing directional antennas, which will boost the signal-to-noise ratio (SNR) at the receiver while promoting NLOS usage. We employ a directional underwater acoustic antenna which is composed of an array of hydrophones that can be summed up at various phases and amplitudes resulting in a beam-former. With SBR, the receiver only accepts signals that are reflected at the surface (or bottom) by checking the Received Signal Strength (RSS) and comparing it to the calculated reflection coefficients. This is done by applying a homomorphic deconvolution process to the received signal to obtain the impulse response of the acoustic channel containing the RSS information. Our on-going research is exploiting this SBR communication model to develop an effective and practical design solution for UWANs. A suite of protocols are to being developed for:
  1. Node discovery and localization in order to establish a coordinate system relative to the water surface (see illustration). The process is fully a distributed in which each node will use the SBR-based range measurements to its neighbors to accurately determine their relative position.
  2. Medium access control to take advantage of the multipath reflections in the underwater environment and overcome the unavailability and limitations of line-of-sight (LOS) paths. This not only allows the network to fully utilize the spatial spectrum but also boosts the overall network throughput by increasing the simultaneity of the transmissions and reducing collisions and delays.
  3. Routing unicast and geocast in order to achieve low latency and robust dissemination of data among the nodes without the reliance on surface nodes or centralized control.
National Science Foundation
Recent Publication
  1. L. Emokpae and M. Younis, “Throughput Analysis for Shallow Water Communication Utilizing Directional Antennas”, IEEE Journal on Special Topics in Communications, Special Issue on Communications Challenges and Dynamics for Unmanned Autonomous Vehicles, Vol. 30, No. 5, pp.1006 – 1018, June 2012.
  2. L. Emokpae and M. Younis, "Reflection-enabled Directional MAC Protocol for Underwater Sensor Networks", IEEE/IFIP Wireless Days Conference (WD'11), Niagara Falls, Canada, October 2011.
  3. L. Emokpae and M. Younis, "Surface Based Anchor-free Localization Algorithm for Underwater Sensor Networks", IEEE International Conference on Communications (ICC'11), Kyoto, Japan, June 2011.
  4. L. Emokpae and M. Younis, "Surface Based Underwater Communications", IEEE Global Communications Conference(GLOBECOM'10), Miami, FL, December 2010.