Optical Wireless Sensor Network System Using Corner Cube Retroreflectors
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Optical Wireless Sensor Network System Using Corner Cube Retroreflectors Shota Teramoto Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
Tomoaki Ohtsuki Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Email: [email protected] Received 18 March 2004; Revised 16 September 2004 We analyze an optical wireless sensor network system that uses corner cube retroreflectors (CCRs). A CCR consists of three flat mirrors in a concave configuration. When a light beam enters the CCR, it bounces off each of the three mirrors, and is reflected back parallel to the direction it entered. A CCR can send information to the base station by modulating the reflected beam by vibrating the CCR or interrupting the light path; the most suitable transmission format is on-off keying (OOK). The CCR is attractive in many optical communication applications because it is small, easy to operate, and has low power consumption. This paper examines two signal decision schemes for use at the base station: collective decision and majority decision. In collective decision, all optical signals detected by the sensors are received by one photodetector (PD), and its output is subjected to hard decision. In majority decision, the outputs of the PDs associated with the sensors are subjected to hard detection, and the final data is decided by majority decision. We show that increasing the number of sensors improves the bit error rate (BER). We also show that when the transmitted optical power is sufficiently large, BER depends on sensor accuracy. We confirm that collective decision yields lower BERs than majority decision. Keywords and phrases: corner cube retroreflector, optical wireless sensor network, collective decision, majority decision.
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INTRODUCTION
Recently, sensor networks consisting of small sensors that have the abilities of detection, data processing, and communication have attracted much attention owing to the development of wireless communications and electric devices [1, 2]. Since wireless sensor networks have several advantages, such as autonomous distributed control, network extensibility, and simple setup, their use to realize surveillance and security in various places, such as hospitals, dangerous areas, and polluted areas, is expected. However, since the electric power, memory, and throughput of the sensor itself are restricted, we need to improve its power efficiency. Therefore, the use of passive transmitters such as the corner cube retroreflector (CCR), which do not have a light source in the sensor itself, is attractive for improving the power efficiency of the sensor. An ideal CCR consists of three mutually orthogonal mirrors that form a concave corner. A CCR, as a This is an open-access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
micro machine, has attracted much
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