22-05-2010, 04:10 PM
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INTRODUCTION
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.
Underwater Sensor networks using optical wireless communication.
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 increasing need for versatile and compact sensing systems is a further stimulus for research in water-quality monitoring solutions.
1.1. What is an underwater sensing network [USN]?
A sensor network is a group of specialized transducers with a communications infrastructure intended to monitor and record conditions at diverse locations.
Commonly monitored parameters are temperature, humidity, pressure, wind direction and speed, illumination intensity, vibration intensity, sound intensity, power-line voltage, chemical concentrations, pollutant levels and vital body functions.
A sensor network consists of multiple detection stations called sensor nodes, each of which is small, lightweight and portable. Every sensor node is equipped with a transducer, microcomputer, transceiver and power source. The transducer generates electrical signals based on sensed physical effects and phenomena. The microcomputer processes and stores the sensor output. The transceiver, which can be hard-wired or wireless, receives commands from a central computer and transmits data to that computer. The power for each sensor node is derived from the electric utility or from a battery.
2. Optical wireless communication [OWC].
Modern optical wireless communication (OWC) was an offshoot of the development of laser technologies in the 1960s. At that time, it was primarily financed and promoted by the Defense Advanced Research Project Agency (DARPA) and by other government agencies such as the National Aeronautics and Space Administration (NASA) and the Ballistic Missile Defense Agency (BMDO).
The main aim then was to develop communication between satellites and submarines beneath the surface of the sea. During the last 40 years, OWC has expanded to include deep space mission and terrestrial networks connected to nodes with a distance separation of 100 or 200 m at rates up to 10 Gbps.
Underwater owc
The main advantages of OWC are:
1) There are no licensing requirements or tariffs for its utilization;
2) There are no radiofrequency (RF) radiation hazards;
3) There is no need to dig up roads, etc.;
4) It has a large bandwidth, which enables very high data rates;
5) It is small, light, and compact; and
6) It has low power consumption.
2.1. LIDAR
Light Detection and Ranging is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target.
The prevalent method to determine distance to an object or surface is to use laser pulses.
Like the similar radar technology, which uses radio waves, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal.
LIDAR technology has application in Geomatics, archaeology, geography, geology, geomorphology, seismology, remote sensing and atmospheric physics.[1]
Applications of LIDAR include ALSM (Airborne Laser Swath Mapping), laser altimetry or LIDAR Contour Mapping.
The acronym LADAR (Laser Detection and Ranging) is often used in military contexts.
The term laser radar is also in use but is misleading because it uses laser light and not the radio waves that are the basis of conventional radar.
3. Underwater Sensing networks.
3.1 SONAR based underwater sensing networks
It is well established that sound waves, compared to electromagnetic waves, propagate long distances in the ocean.
Hence, in the ocean as opposed to air or a vacuum, one uses sound navigation and ranging (SONAR) instead of radar, acoustic communication instead of radio, and acoustic imaging and tomography instead of microwave or optical imaging or X-ray tomography.
Sonar senor system
Underwater acoustics is the science of sound in water (most commonly in the ocean) and encompasses not only the study of sound propagation, but also the masking of sound signals by interfering phenomenon and signal processing for extracting these signals from interference.
3.2 OWC based underwater sensing networks
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.
4. How OWC can be used for underwater sensing networks?
Optical sensing and wireless communications for underwater sensor networks may prove possible using a spectral diversity scheme
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 multi-access interference problems.1, 2 we now pose the question, can the same principle be applied for underwater contaminant? Detection and monitoring?
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. 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.
4.1 Adaptive MAC Protocol and Acoustic Modem for Underwater Sensor Networks
Implementation.
This section describes the implementation of an underwater modem and P-MAC for UW-ASN. Figure 1 shows devices and a water-proof housing case for UWASN.
For the convenience and efficiency of our research and implementation, we developed separate boards for PHY layer and MAC layer respectively. For the physical interface between boards, SPI (Serial Peripheral Interface is used. A software interface suitable for underwater environment is specified by referring to IEEE 802.15.4 standard [3].
Table 1 shows the detailed description of the base board which contains P-MAC. We plan to integrate each board into one board later on.
Modem
Figure 2 shows the block diagram and the implemented hardware of an underwater modem. At MCU, a PPDU is generated based on the PSDU from MAC layer. After D/A conversion and amplification, ultrasonic waves are emitted to a water medium. For data reception, acoustic signal is captured at the transducer, and then amplified, filtered and detected. Table2 describes the detailed specification of the modem.
The developed modem is tested in a water tank, a pond and the ocean. According to the experiments, the modem supports the maximum data rate of 5 kbps and the working range of 30 m. Table 3 shows the environment for the pond *This research was supported by the MKE(The Ministry of Knowledge Economy), Korea, under the ITRC(Information Technology Research Center) support program supervised by the NIPA(National IT Industry
Promotion Agency (NIPA-2009-C1090-0902-0044)
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