The 'internet of things' will require very low power wireless communications, preferably using sensors that scavenge
power from their environment. Free space optics allows communications over long ranges, with simple transceivers at
each end, offering the possibility of low energy consumption. In addition there can be sufficient energy in the
communications beam to power simple terminals. In this paper we report experimental results from an architecture that
achieves this. A base station that tracks sensors in its coverage area and communicates with them using low divergence
optical beams is presented. Sensor nodes use modulated retro-reflectors to communicate with the base station, and the
nodes are powered by the illuminating beam. The paper presents design and implementation details, as well as future
directions for this work.
Optical wireless channels that use modulated retro-reflectors can provide low-data rate communications to self-powered
'smart-dust' motes. The retro-reflectors are illuminated using a base station that incorporates diffractive beamsteering to
direct radiation onto the motes, and these return the radiation to an imaging receiver within the base station. The motes
consist of a photodiode to provide power, a novel logarithmic receiver to receive data from the base station, and a
modulating retro-reflector to send information from the mote to the base station.
Several of the components elements of this system have been implemented and tested. In this paper we report a
logarithmic receiver that can be self powered by the source communicating with it, and a retro-reflecting LC modulator
component that operates at 30b/s when driven at 0.7V. In addition an overall system model, together with the challenges
for future work are presented.
A key challenge for wireless sensor networks is minimizing the energy required for network nodes to communicate with each other, and this becomes acute for self-powered devices such as 'smart dust'. Optical communications is a potentially attractive solution for such devices. The University of Oxford is currently involved in a project to build optical wireless links to smart dust. Retro-reflectors combined with liquid crystal modulators can be integrated with the micro-machine to create a low power transceiver. When illuminated from a base station a modulated beam is returned, transmitting data. Data from the base station can be transmitted using modulation of the illuminating beam and a receiver at the micro-machine. In this paper we outline the energy consumption and link budget considerations in the design of such micro-machines, and report preliminary experimental results.
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