KEYWORDS: Light emitting diodes, Signal detection, Receivers, Photons, Signal attenuation, Optical design, Wireless communications, Transmitters, Optical communications, Free space optical communications, Indoor communication systems, Communication theory
The shortage on radio spectrum forced to a high sophistication in spectrum efficiency. Optical wireless communication (OWC), rather than RF communication, may be a game changer, as the available optical spectrum is sheer unlimited. In addition, light can more easily be directed to the desired user (only). Narrowing the emitted light beams allows denser reuse, even within one room, and enables an increase in throughput. The authors report their experience from creating indoor OWC systems and verify these insights against throughput models. The trade-off between high throughput in only a narrow beam versus offering a wide coverage area is discussed. LEDs and free-form optics allow simple ways to direct a beam, which is more attractive than a phased-array as used in RF. The suitability of a Lambertian radiation patterns is challenged and compared to an optical system that is designed to provide constant irradiance. An example of a sectorized system comprising four segments with free-form optics is presented and its performance and characteristics are discussed, for a Lambertian and a directional detector
This paper builds a model for the benchmarking and the selection of a suitable LED for wireless optical communication, in particular for indoor LiFi Infrared or visible light communication. It reviews LED measurements and theoretical models for such trade-off and applies these into communication bit-rate throughput expressions. While illumination LEDs are chosen for a large quantum efficiency, for communications also a large 3 dB bandwidth is preferred. In the LED, electron hole pairs recombine radiatively (thereby emitting a photon) or non-radiatively (causing a leakage current and reducing EQE). Non-radiative recombination also contributes to the response speed of the LED and increases its 3 dB bandwidth. On the other hand, a reduction in effective optical power may counterproductively lead to an inadequate signal-to-noise ratio. A trade-off is postulated empirically, in the form of a rule of thumb: “transmit power raised to the power alpha times bandwidth raised to the power one minus alpha” appears to be an LED constant. This semi-empirical model gives straight lines on a log-log scale. This paper searches for a theoretical justification for such a model, where current density acts as a parameter to make the trade-off. According to communication theory, the achievable bit rate grows approximately linearly with an increasing bandwidth but approximately logarithmically with the received energy per bit. However, this needs to be reviewed for a gentle low pass roll-off of the LED response, as it allows modulation far beyond the 3 dB bandwidth. These lead to a perspective on how to operate the LED: a system design faces the challenge to trade-off power versus bandwidth according to the physics LED properties, to optimize a communication throughput target.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.