LED Backhaul Project Engineer Blog
Answers to FAQ (Part 1)
Last Update: November 9th, 2021
Introduction
Our company manufactures and sells optical wireless communication devices called LED backhaul, and is also an importer of Li-Fi devices from OLEDCOMM of France. We exhibit these products at various exhibitions and talk to various customers, but most of them have the impression that "I didn't know there was such a thing," which reminds me that optical wireless communication is still not well known at all. I have already explained about optical wireless communication in general on our company website, but this time I would like to introduce some of the frequently asked questions from new customers and their answers. The first one is about our product, LED backhaul, and you can find the basic explanation about LED backhaul in here.
FAQ and Answer about LED backhaul
Q. Why are there two lenses?
A. It is a transmitting lens and a receiving lens.
The white one on the right is the transmitting lens, and the black one on the left is the receiving lens. The lenses are exactly the same, but the receiving lens has a filter to prevent sunlight from affecting it, so it looks black. Incidentally, the lens used has a short focal length despite its large diameter, so if it were made with an ordinary convex lens, it would be thick even with high-grade glass for lenses, but by using a fresnel lens, we were able to make it thinner even though it is made of inexpensive acrylic.
Fig. 1 Transmitting lens and receiving lens
Q. What color of light does the LED backhaul use to communicate?
A. They use near-infrared light, which is invisible to the eye.
However, because the central wavelength is 850nm, which is quite close to visible light, some of the light emitted by the LED is also visible light, so it appears slightly red when it enters the front of the optical axis in a situation where the surroundings are dark. In terms of performance (in the air), visible light such as red or blue light is almost the same, but we use invisible infrared light to avoid glare.
Q. How is the weather affected?
A. Rain doesn't have much of an effect, snow has a different effect than snow itself. Fog has the greatest impact.
The LED backhaul uses a 10cm diameter lens, so the communication channel can be thought of as a 10cm diameter cylinder. The amount of light attenuation due to rain or snow is proportional to how much rain or snow occupies that cylinder as an area when viewed from the side. Therefore, the more rainfall or snowfall, the greater the attenuation, and the longer the distance for the same amount of rainfall, the greater the attenuation, and for the same amount of water, the attenuation is greater for snow than for rain (due to the larger size of the grains). In fact, this concept is not only used for optical wireless communication. How much light attenuates? is exactly the same as "How far ahead can you see with the naked eye? is exactly the same as "how far ahead can you see with the naked eye? How far ahead can you see? In meteorological terms, this is called visibility. The definition of "visibility" is "visibility = 1000m" when you can see 1000m ahead, but the actual visibility is measured by a special measuring device (visibility meter) based on the attenuation of specific wavelengths. This range of vision is most commonly used at airports. Since it is one of the criteria for determining whether or not to take off and land, every airfield has a rangefinder. This is a bit of a digression, but the communication speed and range of the LED backhaul is basically proportional to this visibility. The effect of rain, snow, and fog on communication is basically determined by how the visibility is affected by these factors.
The relationship between rainfall and attenuation in LED backhaul is shown in the figure below. The effect of rain is not as large as you might think. In fact, even if it is raining very hard, it is unlikely that you will not be able to see 200 meters ahead. Even during typhoon No. 19 in 2019 (the typhoon that flooded the Futakotamagawa River and other areas), we did not receive any calls from customers saying that they were cut off. (By the way, we are still collecting data on rainfall and attenuation, and we plan to publish more detailed data in a future blog.)
Fig. 2 Relationship between precipitation and attenuation.
On the other hand, when considering the effects of snow, it is not as simple as rain. If it simply snows, the light attenuation will be proportional to the amount of snowfall, but snow has another major problem: it will deposit on the lens. the LED backhaul itself has a communication module inside that generates heat, which melts the snow on the lens to some extent. The LED backhaul itself generates heat from the communication module inside, which melts the snow on the lens to some extent, but not enough to melt the snow in cooler temperatures, and if there is wind, the snow will land on the lens. Since the snow will directly block the lens, the attenuation will be very large regardless of the viewing angle. Another snow problem is the "ground snowstorm". In a blizzard, not only the falling snow, but also the snow flown by the wind can cause considerable loss of visibility. If a "whiteout" occurs, where the visibility is almost zero, the LED backhaul will be completely blocked and communication will be impossible. In this way, snowfall alone can make communication possible, but snowfall and blizzards can make communication impossible.
Fog has a greater effect on visibility than rain or snow. For example, in the center of Tokyo, it is unlikely that rain or snow will cause visibility to drop below 1,000 meters, even taking into account the guerrilla rains of recent years. Therefore, it can be said that fog has the greatest impact on the communication of LED backhaul in a normal installation environment. As a rough guide to the effect of fog on communication, please consider that communication will become impossible when the visibility drops below the communication distance due to fog. According to past data from the Japan Meteorological Agency, visibility in urban areas such as Tokyo and Osaka rarely drops below 200 meters, so if the communication distance (including margins) is 150 meters or less, there is no need to consider disconnection due to fog.
Q. What are the effects of sunlight?
A. There is almost no effect under normal use.
The LED BH is equipped with a filter to block sunlight. The LED BH is equipped with a filter to block sunlight, thereby minimizing the effect of sunlight. For more information, please see here.
The LED BH is a lens-based light collection system. Under strong sunlight, if the LED BH is pointed straight at the sun, the receiver element (photodiode) inside may be burned out. This is the same phenomenon that occurs when a magnifying glass is used to focus light and burn paper. (Incidentally, the same thing happens with cameras, so please be careful.)
Figure 3: Photodiode burned by sunlight.
Q. Can a laser pointer jam communications?
A. Theoretically, it can be jammed, but practically it is difficult and pointless.
In general, laser light is very strong, so if the laser light hits the light receiving part (photodiode) of the LED backhaul, the communication speed will decrease, communication will be cut off, or in the worst case, the equipment may break down. However, this is subject to the condition that the laser is irradiated from a close distance and straight to the communication direction (optical axis). As mentioned in the answer to the question "Effects of sunlight", there is a filter installed on the receiver side to prevent sunlight from hitting the receiver, which blocks light other than in the front direction, and also prevents light from hitting the receiver unless it is within the receiving range of the receiving lens. The LED backhaul uses infrared light, but the receiver is equipped with a "filter" to cut off unwanted light, thus reducing the influence of visible light lasers.
Furthermore, the sale of high-power laser pointers is prohibited in Japan by the Consumer Product Safety Act, so using a laser with an output below the law to affect LED backhaul requires illumination from a very short distance.
As mentioned above, if you are trying to interfere with LED backhaul communications with a laser pointer, you will have to meet a variety of conditions and it will be quite difficult to interfere. It is much easier to directly block the light from the communication than to use a laser to jam it.
Q. Is it possible to communicate underwater?
A. No, it is not.
We are often asked this question, probably because underwater communication using optical wireless communication has been attracting a lot of attention in recent years. However, the LED backhaul uses near-infrared light, which is absorbed by water, so it is not suitable for underwater communication. The LED backhaul housing is equivalent to IP67, so it will work for a short time under water, but no communication will be possible. Short wavelengths of green to blue light are suitable for underwater communication.
Q. Does it affect the operation of the infrared remote control?
A. No, it does not.
First of all, the LED backhaul itself transmits the light very narrowly in the direction of the same LED backhaul device with which it is communicating. Therefore, it is not very likely that the light will hit the infrared remote control receiver. The wavelength of the light is 850nm for LED backhaul and 940nm for infrared remote control, which is the same infrared light but slightly different. Also, the infrared remote control is pulse modulated, which makes it very noise-resistant. From the point of view of the infrared remote control, the modulation of the LED backhaul is too fast and is just a DC component of light, i.e., a part of the ambient light, and the output of the LED backhaul itself is weak compared to the ambient light such as sunlight or lighting, so if the infrared remote control is very strong against ambient light noise, it will have little effect. The output of the LED backhaul itself is weak compared to ambient light such as sunlight or lighting, so an infrared remote control, which is very resistant to ambient light noise, will have little effect.
Finally.
We hope to continue answering questions like this in the future. If you have any questions about optical wireless communication, please post them in the CONTACT section and we will answer them (as much as possible).
