LED Backhaul Project Engineer Blog

What's Li-Fi (2) Can you communicate with non line of sight?

Last Update: Aug 17th, 2021

---

Introduction: The Biggest Weakness of Li-Fi

One of the problems with Wi-Fi is that it spreads the signal more than desired. This causes two main negatives. The first is that radio waves from unrelated access points (APs) next to each other interfere with each other, slowing down the speed. The other is that the radio waves will be sent to unnecessary places (for example, to neighboring buildings), which is a security problem. On the other hand, Li-Fi, which communicates using light, does not have the two phenomena mentioned above because the range of light radiation can be easily limited. However, Wi-Fi has an advantage that Li-Fi cannot imitate, which outweighs its disadvantages. It is the ability to connect in any installation condition. You can connect in any direction, up, down, left, right, or even when you are in a bag. The same is true for other forms of radio communication, such as cell phones and Bluetooth, so you may think this is obvious, but this is a feature that optical wireless communication cannot imitate. Conversely, the inability to do this is the greatest weakness of optical wireless communication.

LOS and NLOS

This is a term that is often used in communications, but between two points that are in a place where they can see each other directly without any obstructions, or in other words, where there is a line of sight, we call it Line Of Site, or LOS for short. Conversely, a line between two points where there is no line of sight is called a Non-LOS, or NLOS. The reason why Wi-Fi can communicate in NLOS is that radio waves can be reflected, diffracted, or transmitted through walls, etc. This is why Wi-Fi can communicate in the next room or on another floor even if there is an obstacle between the AP and the terminal. This is why communication is possible even if there is an obstacle between the AP and the terminal in the next room or on another floor.

What about light? Light is reflected by walls and ceilings, and you remember doing experiments on light diffraction when you were a child, right? But what about transmission? You can transmit light through a window, but never through a wall, right? Therefore, it is impossible for Li-Fi to communicate in different rooms. Unfortunately, we learned as children that light diffraction does not "bend and go around" enough to be used for communication. So what about reflection? Reflections do happen, don't they? Not only mirrors, but all things reflect light. There is even such a thing as indirect lighting. So, even if there is an obstacle in the way and it is NLOS, it would be possible to communicate with the reflected light hitting the wall or ceiling, wouldn't it?

Ideal scenario

Let's take a look at an ideal scenario, as described in a textbook (although there are only a few textbooks on optical wireless communications).

The figure shows that the light emitted from the transmitter is reflected by the reflecting surface and can be received if it is within the receiving angle. There are three parameters that are key to NLOS communication. One is the angle at which transmission is possible, or the transmission range, also the reception range, and the reflectivity of the wall.

There are two types of LEDs: the chip type, which is mainly attached to the board, and the bullet type, which is probably the one you see most often. For example, the irradiation angle of a bullet-shaped LED ranges from 30 degrees (±15 degrees) to 60 degrees (±30 degrees). The smaller the irradiation angle, the farther the light will reach because the light is focused. On the other hand, if the irradiation range is too wide, the light will be weaker and the communication distance will be shorter. Therefore, it is important to have a good balance. However, the irradiation angle of the LED does not necessarily determine the limit of the Li-Fi irradiation angle. You can use as many LEDs as you like. Therefore, the irradiation angle of the device can be as wide as the number of LEDs (Figure 2). This is the same as LED lighting. Therefore, the transmission range can be made relatively freely.

The reception range is the angle at which a photodiode (PD) can receive light, which is the same as a solar cell, and is based on the fact that a voltage is generated when light shines on a semiconductor, which is the light receiving element. PDs, like solar cells, detect light by using the voltage generated when light shines on the photosensor semiconductor. However, since the area of the light receiving element is several times larger than that of the LED, the directivity is generally looser and the reception range is wider. In order to expand the directionality of reception, it is possible to attach as many PDs as in the case of transmission, but unlike the transmission side, where the same signal is all that is needed, in reception it is not enough to simply synthesize the signals, so care must be taken. In the end, what determines the angle at which transmission and reception are possible is not only the performance of the LED or PD, but also the product size, power consumption, and component cost.

Next, let's talk about reflectance. First of all, there is data on the absorption rate of light by materials. It can be expressed as the ease with which a material is warmed by light. If a material is opaque, the light that hits it is either absorbed or reflected, and the relationship absorptivity + reflectivity = 1 holds. Therefore, if you know the absorption coefficient of an opaque material, you can also know its reflectance. In the near-infrared^*1, for example, silver, which is also used for mirrors, has a reflectance of 99%, while concrete has a reflectance of 35% and carbon only 5%. If the reflectance is 35%, it is not so bad. Even concrete, which seems to have a fairly high reflectivity of 35%, decays to 12% of its original value after two reflections, so the NLOS of Li-Fi is effectively a single reflection environment. And an environment like the one shown in Figure 3 appears to be sufficient for communication, although it is a bit slower than the LOS environment.

Scattering

In reality, however, this is not the case. This is because the reflectance does not take into account the scattering of light that comes from the shape of the material. Unabsorbed light hitting a wall will be reflected, but if the surface of the material is uneven, the light will be scattered and diffused during the reflection process. If the surface of the material is uneven, the light will be scattered during the reflection process, making it impossible to use much of the reflected light that could have been used by Li-Fi. Of course, some light can be newly usable due to scattering, but in total, the amount of light that flies in directions unrelated to communication becomes overwhelmingly large, and communication is greatly degraded.

For example, if the same metal has an extremely flat surface like a mirror, it will scatter very little, but if it has a rough surface finish like a hairline finish, it will scatter more easily. There is a good way to describe whether a material surface scatters light or not. It's called shiny. A floor that has been polished to a shine is very shiny. However, an unwaxed, scratchy floor is not shiny.

Now, imagine the living room of your house. Do you see anything shiny? The only things that are shiny are glass objects and the TV, right? If the walls of your house are shiny, you are living in a very unusual house. The wallpaper in your house, your desk, your office furniture, etc., are all "not shiny on purpose. If the wallpaper and desks are smooth and shiny, the sun and lights will glare into your eyes, making it difficult to live. Think of the wallpaper as being deliberately uneven and treated with what LCDs call "non-glare" (by the way, LCD non-glare reduces reflectivity by a factor of 10). And if it scatters, it will attenuate the light coming into the receiver. That's why the rooms where normal people live are surrounded by things that don't shine = scatter easily, which actually makes NLOS communication a very difficult place for Li-Fi. Now, some of you may ask, "Doesn't Wi-Fi scatter? Some of you may be wondering, "Doesn't Wi-Fi scatter? Radio waves also scatter, but not as much as light. The reason for this is the difference in wavelength: the wavelength of Wi-Fi 2.4GHz is about 12cm, while the wavelength of light is 900nm for infrared rays. Unlike light, which is scattered even by the slightest bump in the wallpaper, light with a wavelength of 12cm is not scattered to the extent that it looks rough.

More communication distance issues

Another reason that makes Li-Fi NLOS communication difficult is the communication range. the AP side can use any amount of power it wants, it can emit light with the output power of an LED bulb, but the child side cannot. The AP can use any amount of power it wants, and it can emit light with the output power of an LED bulb, but the child unit does not, and it must be connected to a PC via USB and run on the power of the USB. In such an environment, using 5W or 10W just to light up the LED would not be feasible for a communication device. Moreover, it would be difficult to communicate without expanding the transmission range to some extent, even at the expense of communication distance, by reducing directivity. Because of this, the current Li-Fi can only communicate at a distance of about 2.5 meters, even at LOS. Even at that distance, if you are communicating in a LOS environment from ceiling to desk, there is no problem. However, if you take into account the reflection, scattering, and attenuation that occurs when trying to communicate at NLOS, you will unfortunately only be able to communicate at very close distances. And NLOS at such a close distance would be almost useless. In the future, if we can develop a system that can change the direction of the transmitted light and track it, the communication distance will increase due to the ability to narrow the light, and perhaps NLOS will become more meaningful. (This sounds like beamforming for 5GNR millimeter wave...)

There is also the problem of ISI^*2 due to reflected waves even with light, but with the current Li-Fi, the communication distance at NLOS is so short that it hardly needs to be considered. (All Li-Fi nowadays are OFDM, so it is easy to prevent ISI.)

Summary

We have seen whether NLOS communication in Li-Fi, especially communication by reflected waves, is possible. And the conclusion is that it is theoretically possible, but currently not realistic. The reasons for this are as follows.

• Indoors, most non-metallic materials have low reflectance, and if they reflect more than once, they will be attenuated significantly.
• Ordinary rooms and offices are made of materials that are easily diffused to avoid glare, and the attenuation there is also large.
• In particular, the short communication distance of a child device, combined with the above two points, makes it difficult to use reflected waves.

Therefore, Li-Fi currently in circulation is almost entirely based on the premise of LOS communication. However, even if Li-Fi is only capable of LOS communication, it has various advantages. In the next article, we will take a look at some of these advantages.

---