Optical wireless communication in distributed sensor networks

In a typical sensor network, a large number of miniature sensing and communicating nodes are deployed at the location of interest, where they establish a network and wirelessly communicate sensed data to one another or to a base station. The sensed parameter could be temperature, pressure, acceleration, chemical composition, changes in a magnetic field or any other variable for which a tiny sensor can be constructed. Temporal variations of the measured parameter can easily be recorded and, thanks to the large number of nodes, a spatial contour can be obtained. Measurement accuracy can be improved by fusing the data from many nodes, and sensor collaboration can increase energy efficiency. Processed data may be used to actuate control mechanisms, alert systems, or imaging devices, but it must be communicated in some way in order to be useful.

Radio frequency (RF) communication is a mature technology and has been applied with success in sensor network applications.^1–;3 However, OWC has clear advantages such as extremely small and lightweight hardware, and ultra-low power consumption. It can also support a highly-directional beam (a critical feature for covert sensing applications, as well as energy efficiency) and offers the possibility of Space Division Multiple Access (SDMA) through the use of matrix detectors.⁴

In our work, we have modeled and analyzed OWC in the context of networked sensors and evaluated its inherent drawbacks as well as its benefits. To be specific, we have examined the difficulties posed by the absence of guaranteed line-of-sight (LOS) links in the network and with signal degradation due to atmospheric effects such as scattering by aerosols.⁵ We have also proposed novel atmospheric probing and communication methods that take advantage of the aerosol scattering that is usually seen as a handicap to OWC system operation.^6,7

Atmospherically induced scattering of propagating light deflects optical power out of the beam and distorts the received signal when multiscattered light reaches the receiver via multiple paths. We have modeled scatter-induced signal degradation in the context of distributed sensor networks, where the proximity of transmitting sensor nodes can result in multiple access noise.

Paradoxically, scattered light has also been found to render a link operable in the absence of perfect LOS conditions.⁵ Taking this idea one step further, the backscattering of light by molecules and aerosols in the atmosphere can serve as a vehicle of communication much like the deployment of numerous tiny reflecting mirrors. We have treated this curious feature of light propagation in a study of a non-LOS sensor network operating in the solar-blind ultra-violet wavelength range, where receivers with very large fields of view can be used with impunity since solar radiation at these wavelengths is almost totally absorbed in the upper atmosphere.

A similar appreciation of the hidden merits of aerosol-induced scattering inspired a proposal for a novel method of atmospheric probing.^6,7 Thousands of tiny sensor nodes, which we dubbed laser fireflies, emit light to elicit a backscatter response from neighboring particulates. The resulting backscattered light contains a response signature that testifies to the presence of specific contaminants, or even weaponized spores of bacteria. The backscattered light signal is detected and processed on the firefly. Analysis of the received signals makes it possible to obtain a spatial map of the atmospheric constituents without having to perform mechanical scanning. We found that by orthogonally coding optical data-carrying signals to a ground station and using a matrix detector, many fireflies could simultaneously communicate with a ground station, and high altitude atmospheric layers could be probed. Bit-error-rates of 10^-8 and less were achieved for differently weighted optical orthogonal codes, cluster sizes, and detector-matrix dimensions.

Our research has addressed many of the issues hindering the extensive application of OWC in distributed sensor networks, and are optimistic that further theoretical and experimental research will accelerate the development and implementation of this important technology. We foresee that the unique features of OWC will extend the utility of distributed sensor networks to many applications and expand our capabilities in fields as varied as pollution monitoring, medical care, homeland security and biocomplexity research.