Underwater sensor network using optical wireless communication by broverya76


									                                                                                              SPIE Newsroom

Underwater sensor network
using optical wireless
Debbie Kedar

Optical sensing and wireless communications for underwater sensor
networks may prove possible using a spectral diversity scheme.

Potential applications for distributed networks of sensors are nu-
merous and varied. Concern for the environment is one of the
driving forces behind extensive research into all aspects of sen-
sor networks (communication protocols, energy harvesting, and
microelectronic device fabrication, to name a few), and motiva-
tion 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 car-
rier) with the transmission medium, such as scattering of light by
particles in the channel. However, this very scattering, consid-
ered 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 sen-
sor 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          Figure 1. The underwater probing scheme, in which the base station
degradation. Studying marine ecosystems, ensuring port secu-           with its three elements is separated from the sensor nodes by the aque-
rity, and monitoring oil pipelines and leaks of hazardous materi-      ous medium.
als in transit are some of the specific reasons for profiling the con-
centration and distribution of substances in the ocean. Consider-         The human body is composed of more than 70% water, and
able research has been conducted to develop and assess meth-           our daily survival depends on a constant supply of uncontam-
ods of sensing the ocean on spatial and temporal scales rang-          inated drinking water. The water distribution system, so essen-
ing from centimeters to hundreds of kilometers and from min-           tial for our well-being, is subject to strict scrutiny to ensure that
utes to years.3, 4 It would appear, however, that very small scale,    health risks are contained, and home security considerations
mobile, and low cost sensor networks could cater to a need for         have driven legislation to protect the population.5, 6 The increas-
fine-grained data acquisition systems operating at high resolu-
tion and over long periods of time.                                                                                 Continued on next page
                                                                                                            10.1117/2.1200701.0490 Page 2/2
                                                                                                                           SPIE Newsroom

ing need for versatile and compact sensing systems is a further         Our results lend credence to the possibility of developing an
stimulus for research in water-quality monitoring solutions.          underwater sensor network using OWC, and contending with
   In two recent conference papers we discussed our preliminary       MAI by means of a spectral diversity scheme. While theoretical
ideas regarding the feasibility of an underwater distributed sens-    feasibility is no assurance that a practical sensor network could
ing system for contaminant detection and monitoring based on          be implemented, we hope that our germinal idea may stimulate
lidar concepts and OWC.7, 8 We addressed three primary chal-          further activity to promote the well-being of our planet.
lenges to be met: sensing, data communication, and the in-
evitable multiaccess interference (MAI). In this ‘snapshot’ report
                                                                      Author Information
we summarize some of our work.
   The basic concept is illustrated in Figure 1, which shows a
                                                                      Debbie Kedar
cluster of sensor nodes separated from a base station by the
                                                                      Electrical and Computer Engineering Department
aqueous medium. The base station receiver consists of an en-
                                                                      Ben Gurion University of the Negev
trance aperture, a tunable spectral filter, and a matrix of detec-
                                                                      Beer Sheva, Israel
tors. The spectral filter enables consecutive reception of data sig-
nals at different wavelengths, and the detector matrix separates
                                                                      Debbie Kedar has presented papers at SPIE conferences for the
the signals from sensor nodes located in reduced field-of-view
                                                                      past five years and chaired at the Advanced Free-Space Optical
(FOV) cones.
                                                                      Communication Techniques and Applications III conference last
   The overriding challenges of underwater OWC derive from
                                                                      September in Stockholm.
the acute scattering and absorption encountered by light prop-
agating underwater. While this differs with water composition         References
and radiation wavelength, a pulse of light will be attenuated         1. D. Kedar and S. Arnon, Laser ‘firefly’ clustering: a new concept in atmospheric prob-
and distorted by its passage through the aqueous medium. This         ing, IEEE Phot. Tech. Lett. 15 (1), pp. 1672 – 1624, 2003.
                                                                      2. D. Kedar and S. Arnon, Second generation laser firefly clusters: an improved scheme
will severely limit the possible transmission range and data          for distributed sensing in the atmosphere, Appl. Opt. 44 (6), pp. 984 – 992, 2005.
rate. Current underwater wireless communication uses acoustic         3. T. D. Dickey, Emerging ocean observations for interdisciplinary data assimilation sys-
                                                                      tems, J. Marine Sys. 40–41, pp. 5–48, 2003.
waves that can propagate over long distances, but the modula-         4. E. Boss, D. Stramski, T. Bergmann, W. S. Pegau, and M. Lewis, Why should we
tion bandwidth is limited. In a sensor network, however, use-         measure the optical backscattering coefficient?, Oceanography 17 (2), pp. 44–49, 2004.
                                                                      5. R. Murray, J. Uber, and R. Janke, Model for estimating acute health impacts from con-
ful information can be gathered by a base station from numer-
                                                                      sumption of contaminated drinking water, J. Water Res. Plan. Manag. 132 (4), pp. 293–
ous sensors located nearby so that long transmission ranges are       299, 2006.
not required. The individual data can be fused to provide an en-      6. J. J. Danneels and R. E. Finley, Assessing the vulnerabilities of U.S. drinking water
                                                                      systems, Universities Council on Water Resources, J. Contemp. Water Res. Edu.
hanced collective picture of the sensed parameter. Hence, a large     (129), pp. 8–12, 2004.
number of densely distributed sensors can effectively convey a        7. D. Kedar and S. Arnon, The laser firefly: a distributed optical wireless sensor system
                                                                      for atmospheric and oceanic probing, 5th Intl. Workshop Inf. Opt., Toledo, 2006.
picture of the data of interest, and high redundancy can be ex-       8. D. Kedar and S. Arnon, A distributed sensor system for detection of contaminants
ploited to reduce error and increase robustness.                      in the ocean, Advanced Free-Space Opt. Commun. Tech. Applications III, Stock-
                                                                      holm, 2006.
   However, the dense dispersal of sensors will exacerbate the
problem of MAI, as signals from different sensors arrive simul-
taneously at the receiver. This is combatted partially by the ma-
trix detector, which reduces the effective FOV so that fewer sig-
nals would interfere at any single detector. The drawbacks of
underwater OWC, mentioned above, preclude the possibility of
using orthogonal coding to distinguish between interfering sig-
nals, as was applied in the atmospheric probing sensor concept,
and limit the possibilities of low-energy medium access proto-
cols. 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 sen-
sors, 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 key-
ing regime is employed and suggested two metrics for evaluat-
ing the efficacy of spectral diversity.                                             c 2007 SPIE—The International Society for Optical Engineering

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