Optical Wireless Communication (OWC) systems are limited in bandwidth by their electro-optical components. We study the receiving photo diode, its surface area and the associated capacitance. A large–area detector improves the signal–to–noise ratio, but a small size allows a high bandwidth. An optimum detector size exists. It depends on photodiode properties, the trans-impedance amplifier noise performance and the received signal strength. Results are relevant to the design of indoor LiFi systems with wide coverage as well as for tracking acquisition in long–range free space optical applications.
In an optical wireless communication system, the LEDs require high bandwidth to achieve data rates compare to RF communications. Therefore, it is preferred to use high current densities to drive the LED, even beyond its maximum efficiency point. Nevertheless, the LED is a self-heating device where part of the electrical power is converted to optical power and the rest into heat. This increment of temperature in the LED will reduce its optical power and efficiency, causing a degradation of the optical system SNR. Firstly, we start by deriving the relationship between the LED voltage with its junction temperature using the Shockley equation. Then, we measure the forward voltage at different temperatures to prove a linear relation between them. The optical power, LED bandwidth, and efficiency can be calculated using the ABC parameters from the rate equation. Therefore, we propose a method to obtain these parameters by measuring the emitted optical power and its rise time when varying the driving current. Finally, the ABC parameters are calculated by solving an MMSE problem with the measurements previously done. The throughput of an optical wireless link depends on the efficiency and the LED bandwidth, thus it is temperature dependent. We calculate the degradation of the rate caused by the temperature increment.
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.
LEDs can be modulated at relatively high speeds to support wireless optical data communication (OWC). Yet, particularly LEDs optimized for illumination act as a non-linear low-pass communication channel. It has become clear in recent literature that their non-linearity and low-pass behavior cannot be seen as two separable, cascaded mechanisms. Although standard nonlinear equalizer schemes, e.g. based on Volterra Series, have been proposed and tested before, our recent research results show that a more dedicated approach in which we specifically analyze the hole-electron recombination mechanisms, yield a very effective and computationally-efficient compensation approach. In this manuscript, we will review the non-linear differential equations for photon emissions, its electrical equivalent circuit and a discrete-time variant with delays and non-linearities. This can be inverted, in the sense that we can actively eliminate or mitigate the non-linear dynamic LED distortion by adequate signal processing. We propose an aggressive simplification of the compensation circuit that allows us to use a relatively simple structure with only a couple of parameters.
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