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Optoelectronics & Communications

Underwater sensor network using optical wireless communication

Optical sensing and wireless communications for underwater sensor networks may prove possible using a spectral diversity scheme.
18 January 2007, SPIE Newsroom. DOI: 10.1117/2.1200701.0490

Potential applications for distributed networks of sensors are numerous and varied. Concern for the environment is one of the driving forces behind extensive research into all aspects of sensor networks (communication protocols, energy harvesting, and microelectronic device fabrication, to name a few), and motivation to explore this many-faceted world is high. Optical wireless communication (OWC) must contend with phenomena resulting from the interaction of the propagating light beam (the optic carrier) with the transmission medium, such as scattering of light by particles in the channel. However, this very scattering, considered an obstacle for achieving high-performance OWC, can also be exploited as a sensing mechanism, as is familiar from lidar (light detection and ranging) probing. We have proposed a sensor system for atmospheric investigation based on the principle of lidar and using orthogonally coded data signals to overcome multiaccess interference problems.1,2 We now pose the question, Can the same principle be applied for underwater contaminant detection and monitoring?

The ocean covers some 70% of our planet and is a rich fund of information on global health, climate change, and resource degradation. Studying marine ecosystems, ensuring port security, and monitoring oil pipelines and leaks of hazardous materials in transit are some of the specific reasons for profiling the concentration and distribution of substances in the ocean. Considerable research has been conducted to develop and assess methods of sensing the ocean on spatial and temporal scales ranging from centimeters to hundreds of kilometers and from minutes to years.3,4 It would appear, however, that very small scale, mobile, and low cost sensor networks could cater to a need for fine-grained data acquisition systems operating at high resolution and over long periods of time.

The human body is composed of more than 70% water, and our daily survival depends on a constant supply of uncontaminated drinking water. The water distribution system, so essential for our well-being, is subject to strict scrutiny to ensure that health risks are contained, and home security considerations have driven legislation to protect the population.5,6 The increasing need for versatile and compact sensing systems is a further stimulus for research in water-quality monitoring solutions.

In two recent conference papers we discussed our preliminary ideas regarding the feasibility of an underwater distributed sensing system for contaminant detection and monitoring based on lidar concepts and OWC.7,8 We addressed three primary challenges to be met: sensing, data communication, and the inevitable multiaccess interference (MAI). In this ‘snapshot’ report we summarize some of our work.

The basic concept is illustrated in Figure 1, which shows a cluster of sensor nodes separated from a base station by the aqueous medium. The base station receiver consists of an entrance aperture, a tunable spectral filter, and a matrix of detectors. The spectral filter enables consecutive reception of data signals at different wavelengths, and the detector matrix separates the signals from sensor nodes located in reduced field-of-view (FOV) cones.

Figure 1. The underwater probing scheme, in which the base station with its three elements is separated from the sensor nodes by the aqueous medium.

The overriding challenges of underwater OWC derive from the acute scattering and absorption encountered by light propagating underwater. While this differs with water composition and radiation wavelength, a pulse of light will be attenuated and distorted by its passage through the aqueous medium. This will severely limit the possible transmission range and data rate. Current underwater wireless communication uses acoustic waves that can propagate over long distances, but the modulation bandwidth is limited. In a sensor network, however, useful information can be gathered by a base station from numerous sensors located nearby so that long transmission ranges are not required. The individual data can be fused to provide an enhanced collective picture of the sensed parameter. Hence, a large number of densely distributed sensors can effectively convey a picture of the data of interest, and high redundancy can be exploited to reduce error and increase robustness.

However, the dense dispersal of sensors will exacerbate the problem of MAI, as signals from different sensors arrive simultaneously at the receiver. This is combatted partially by the matrix detector, which reduces the effective FOV so that fewer signals would interfere at any single detector. The drawbacks of underwater OWC, mentioned above, preclude the possibility of using orthogonal coding to distinguish between interfering signals, as was applied in the atmospheric probing sensor concept, and limit the possibilities of low-energy medium access protocols. Consequently, we have tried to investigate the potential of spectral diversity as a means of overcoming MAI. A number of spectral ranges would be raffled randomly among all the sensors, so that only two or more sensors transmitting at the same wavelength could interfere. We have proposed a probabilistic approach to analyzing and quantifying MAI when an on-off keying regime is employed and suggested two metrics for evaluating the efficacy of spectral diversity.

Our results lend credence to the possibility of developing an underwater sensor network using OWC, and contending with MAI by means of a spectral diversity scheme. While theoretical feasibility is no assurance that a practical sensor network could be implemented, we hope that our germinal idea may stimulate further activity to promote the well-being of our planet.

Debbie Kedar
Electrical and Computer Engineering Department, Ben Gurion University of the Negev
Beer Sheva, Israel

Debbie Kedar has presented papers at SPIE conferences for the past five years and chaired at the Advanced Free-Space Optical Communication Techniques and Applications III conference last September in Stockholm.