Technical Bulletin #02
by Majid Safari
How Li-Fi is different from RF wireless
Based on Maxwell’s theory of electromagnetics, the light and radio propagation follow the same rules of physics. However, the optical waves have significantly smaller wavelength compared to radio waves. This is particularly important when an approximation of Maxwell’s theory is required. For example, the geometric wave theory is only applicable when electromagnetic wave interacts with structures much larger than its wavelength. In effect, Geometrical or ray optics can greatly simplify the analysis of optical propagation although it fails to describe some important phenomena such as diffraction. The small optical wavelength is also responsible for higher scattering of light by particles like fog droplets. The so-called Mie scattering solution to Maxwell equations describes the scattering of the electromagnetic radiation by particles with comparable-size or larger than the wavelength. On the other hand, for particles much smaller than the wavelength, the scattering of electromagnetic waves can be explained by Rayleigh scattering.
Atmospheric turbulence caused by thermal variations in the air is another phenomenon that affects the propagation of light over the atmosphere. This phenomenon causes amplitude and phase distortions in the optical wavefront leading to fading in the received optical signal. The turbulence-induced fading is, however, inherently different from Rayleigh fading occurring in RF wireless channels. This needs to be carefully considered in the design spatial diversity and multiplexing techniques for optical wireless communication [1, 2].
Light further exhibits quantum behaviour such as photoelectric effect as a result of its small wavelength. Albert Einstein explained photoelectric effect by proposing that a beam of light is a collection of discrete wave packets (known as photons) with equal energies inversely related to the wavelength of light. As a result, Einstein was awarded the Nobel Prize in 1921 for “his discovery of the law of the photoelectric effect” rather than his more famous theory of relativity. The photoelectric effect leads to a so-called shot noise described by Poisson distribution, which is fundamentally different from the commonly considered Gaussian-distributed thermal noise.
Have you ever heard of “radio Lens” or “optical antenna”?
Efficient transmitter and receiver components are essential for any communication system. RF and optical wireless technologies enjoy matured but different engineering designs for transceiver components. In optical wireless communication, the light is emitted by a laser or light-emitting diode (LED) at the transmitter and is detected by a photodiode at the receiver. Furthermore, to collect, control, collimate, or direct optical beams, separate optical elements such as lenses or mirrors are commonly required. On the other hand, in RF wireless communication, a single antenna with a dimension in the order of radio wavelength can perform all tasks of transmission, reception and beamforming. Therefore, it is no surprise if you haven’t heard of “radio lens” or “optical antenna” although ongoing research has attempted to realize such components [3, 4].