LED Backhaul Project Engineer Blog

What's Li-Fi ? (1) Li-Fi and Lighting

Last Update: Aug 3rd, 2021

Introduction Why Li-Fi uses not visible white light but infrared?

Have you ever heard of Li-Fi, which many people think is similar to Wi-Fi? Li-Fi was proposed in 2011 by Professor Haas of the University of Edinburgh in the UK, but his TED talk was so well received that Li-Fi has become so well known in the industry that people now refer to Li-Fi as optical wireless communication. Li-Fi, as the name implies, is an optical wireless communication Wi-Fi that uses light, or "illumination". In the TED talk by Professor Haas, he mentioned that "if we can embed wireless communication in lighting, it will change the world," but it seems that most Li-Fi products today use infrared light, not white light. Of course, it was originally envisioned that the downstream communication from the lighting device to the child device would use illumination light, but the upstream communication from the child device to the lighting device would use invisible infrared light. However, in actual products, there are many devices that use infrared light not only for uplink but also for downlink. The Li-Fi MAX by French company OLEDCOM, which is handled by our company, also uses infrared light for both uplink and downlink communication. In this article, I would like to explain why many Li-Fi devices use infrared light. In this article, I would like to explain why most Li-Fi systems use infrared light.

fig.1 OLEDCOMM Li-Fi MAX(R)

Bottleneck in LED communication

As explained here (link), the speed bottleneck in optical wireless communication with LEDs is the slow speed at which the LEDs turn on and off. The LED in optical wireless communication is the same as the amplifier in radio communication. If the linear performance of the amplifier is poor, the signal waveform will be distorted, and there may be an upper limit to the amplification. Another phenomenon unique to LEDs is that they turn off slower than they turn on. This means that the light remains on for a short period of time, not long enough to be visible to the human eye, even after the voltage to the LED is stopped. The time it takes for the light of an LED to go from 10% to 90% is called the Optical Rise Time, and the opposite is called the Optical Fall Time. The time it takes for the LED light to go from 10% to 90% is called Optical Rise Time, and the opposite is called Optical Fall Time. The output of an LED is the amount of light it produces. The output of an LED is the amount of light it emits, so it is usually referred to as a slew rate. However, in a wireless amplifier, the output is a voltage, so it is referred to as a slew rate. The unit of the slew rate is usually (V/μs), which is the number of volts that can be changed in one microsecond. The formula for calculating the maximum frequency that will not be distorted at a given slew rate is "f = SR / (2πVpp)", so the frequency that will not be distorted by the LED Backhaul LED is approximately 19 (MHz). This number is just a pseudo-matching of the LED to the wireless amplifier, and there are many differences between the LED and the wireless amplifier, such as the fact that the LED responds much faster when the voltage is reduced (= poor linearity), and in addition, the LED Backhaul uses a technology to compensate for distortion. In addition, the LED Backhaul uses a technology to compensate for distortion, so this number does not necessarily determine the upper limit of a communication device, but it should still be understood that LEDs are not very fast as amplifiers used for high-speed wireless communication.

LED as lighting

For a long time, there were only red and yellow-green LEDs, including infrared ones. For a long time, only red and yellow-green LEDs, including infrared ones, existed, and LEDs were never used for lighting. in the 1990s, commuter trains were replaced with LED indicators instead of directional signs, but at that time, only red and green LEDs existed, so there were only three colors available: red, yellow-green, and orange. However, at that time, only red and green LEDs were available, so only three colors could be displayed: red, yellow-green, and orange, a mixture of red and yellow-green. Even today, there are still some old railroad cars and buses with orange LED displays, so many people, even young people, have seen them before. This situation changed in 1993. As we all know, Dr. Shuji Nakamura, who was working at the Nichia Chemical plant at the time, commercialized blue LEDs, and since then, LEDs have been used in lighting, traffic signals, and displays (especially large ones). In 2014, Dr. Nakamura, along with Drs. Akasaki and Amano, was awarded the Nobel Prize for the development of the blue LED, which was such a great invention.

White" on a computer means the maximum RGB color, and if you mix the light from red, green, and blue LEDs, it looks white**. However, if you only use those three colors, the light only looks white, and when you actually use it for lighting, it becomes quite uncomfortable. To put it concretely, when we look at something, there is a big difference between the color we see in daytime sunlight and the color we see under the three colors of red, green, and blue LED lighting. The reason why this is so can be seen by looking at a rainbow. As you know, rainbows are formed when sunlight is refracted by rain, and since the refractive index differs depending on the wavelength of the light, the frequency components of the light in the sunlight are broken down and appear as different colors, in other words, raindrops and water droplets act as prisms. In other words, raindrops and water droplets act as prisms. A rainbow is a visible expression of the components of sunlight. Rainbows contain a variety of colors, from dark red to purple, red, blue, green, yellow, and orange, but I don't think any one color looks particularly bright when you see a rainbow in the sky. Sunlight contains red, which has a long wavelength, and violet, which has a short wavelength, evenly and evenly.

Fig2 A rainbow containing all colors.

In contrast, the red-green-blue tri-color LED does not contain the full range of light elements. In particular, LEDs themselves contain only a few wavelengths, even if they look white, because each color contains only a few frequency components. This difference occurs not when you look at the light itself, but when you look at the reflection of the light on something. For example, something that looks clearly yellow in sunlight may look dark yellow under trichromatic LED lighting. This is because sunlight has a yellow component, while red, green, and blue LEDs do not (or do not contain enough of it). In fact, the issue of "fewer frequency components than sunlight" is not only an issue for LEDs, but also for lighting in general, including light bulbs and fluorescent lamps, and even before the birth of LED lighting, there was an index for evaluating how close lighting is to sunlight, called color rendering index. The higher the color rendering index, the better the lighting, but unfortunately, trichromatic LEDs have a low color rendering index, and in the early days of LED lighting, some trichromatic LEDs were used due to curiosity (and many had the ability to change the color of the light). However, due to their low color rendering, they are no longer used as LED lighting (*1).

What is used in LED lighting? In LED lighting, blue LEDs are combined with phosphors that glow yellow when exposed to blue light. In LED lighting, blue LEDs are combined with phosphors that emit yellow light when illuminated by blue LEDs, resulting in the blending of the complementary colors of blue and yellow. Compared to LEDs, phosphors can produce light with a wider range of frequencies, and in some cases, multiple phosphors can be combined to produce a light with a higher color rendering quality than trichromatic LEDs, which use LEDs alone to produce white. (*2) Incidentally, fluorescent lamps, which were the mainstay of lighting before LEDs, also produce white light by applying ultraviolet rays to phosphors. Therefore, it is no exaggeration to say that modern lighting is realized not by "light emitters" that shine directly like LEDs, but by "phosphors" that receive light and emit a different kind of light.

fig.3 Spectrum of blue LED and yellow phosphor (example)

White light and communication

Phosphor is used for this lighting, but it is a big problem in optical wireless communication. The phosphors are slow** (take a long time). Some LED lights even take a few seconds after being turned off for the yellow light to gradually fade away. As for phosphors, the higher the power (which is also true for LEDs), the slower they tend to be. There are some high-speed phosphors, but they are used for detecting weak light, and there is no such thing as a phosphor that is high enough power to be used for lighting, yet close to the speed of an LED. At best, there is one that is one-tenth the speed of an LED (i.e., it takes 10 times longer). Unfortunately, it is difficult to use LEDs for high-speed communication if the phosphor makes them even slower (and the two types of light are shifted). If we had no choice but to abandon color rendering and use red-green-blue trichromatic LEDs, we would have another problem. This is because of the obvious reason that the output fluctuates when it is communicating. It doesn't matter which of the three colors is used for communication, but in order to get a stable white, each color needs to be stable and well adjusted. If the strength fluctuates as it communicates, it will be difficult to coordinate with the other colors. If the output of one color is unstable, the problem of "not even looking white" will occur before color rendering. In fact, it seems that it is difficult to achieve both communication and white color, and I once saw an experimental device at the Fraunhofer HHI Laboratory that communicated with trichromatic LEDs, but at the time I had the impression that it was not white, but rather bluish.

Phosphors are slow, and the color rendering properties of trichromatic LEDs are extremely low, so I think you can see that it is very difficult to communicate with white illumination using LEDs. As mentioned above, Li-Fi was originally "communication by illumination light," but nowadays, there are also types of Li-Fi that are "attached to the ceiling but are not illumination (they do not glow)" or "illumination light and communication light are completely different (illumination light does not communicate, but actually communicates by invisible infrared rays, etc.). Therefore, it is no longer possible to say that "Li-Fi = lighting. The aforementioned Li-Fi MAX from OLEDCOMM is one such Li-Fi that is not lighting.

Summary

LEDs for lighting need to be white light, but not only white light, they need to contain not only the three primary colors of red, green, and blue, but also various other colors like the rainbow in order to improve the "color rendering property".
In order to mix various colors, phosphors are essential, but phosphors have a reaction speed that is only about one tenth that of LEDs, making them very difficult to use for high-speed communications.
When trying to communicate with tri-color LEDs at the expense of color rendering, if the output power is not adjusted extremely well, colors that are "not even white" will be produced!
Since it is difficult to communicate with white LEDs, more and more Li-Fi systems are using invisible colors such as infrared rays to communicate.

(*1) The red-green-blue trichromatic LED is very good for display applications where "white is all that is needed" and is used in color destination indicators, LED aurora vision, and micro LED displays that are expected to become popular in the future.

(*2) In order to improve color rendering, various types of LED lighting are now available, such as those using violet LEDs with even shorter wavelengths than blue and multiple phosphors, and those combining multiple colors of LEDs and phosphors.