The Visible Light Communication System: Making the most out of our traditional lighting system

The first time I heard about the visible light communication system, the first technology I thought about was Laser but exposure to laser might not align well with our health in the long run. As an experienced network engineer, I've worked with many radio communication equipment that make use of electromagnet waves and of concern are; their ability to serve for a reasonable amount of time and also their operating speeds.

^{EM waves within the visible range. Image source: Wikimedia. Creative Commons Attribution-Share Alike 3.0 Unported license.}

On the average, signal generators available in radios fail to last for a very long time and for this reason, many antennas come with hundreds of other tiny antennas, extending the general lifetime of the radio. The serving lifetime of sectorial antennas used in telecommunications are determined by the lifetime of half of the tiny signal generators in the radio. This is very sub optimal and economically demanding as transmission and distribution radios are always very expensive and requires replacement usually within a year and half in environments where high availability is of concern.

Well, say hello to visible light communication system (VLC). The visible light just as the name imply are those part of the electromagnetic waves that are visible to human eyes. Electromagnetic spectrum is display of the complete set of electromagnetic radiations with respect to either their wavelength or their speeds. In accordance with their wavelength distributions, electromagnetic waves withing the range of approximately, 390 to 700 nm are visible to the human eye and are hence referred to as visible lights.

A very popular lighting system used for the VLC is the light emitting diode. The LED has gained lots of popularities in recent years and has found applications in many imaging equipment like the printer, medical imaging equipment, and even in advanced storage systems like the latest 5D data storage system. Properties of LED that contributed to its popularity are power efficiency, very scalability and most especially, long life span. These properties of LED made it an excellent component in the visible light communication system.

Most modern network systems adopt Cisco's three layer approach to network design. Here, we have the core layer, the distribution layer and the access layer. The core layer just as the name imply is the center of the whole network system and handles tremendous amount of traffic and due to the fact they handle most of the network switching, high availability solutions like setting up redundant links are implemented at the core layer while the distribution layer takes care of network security. Of importance is the medium of connection between these layers. Most connections between core and distribution layers are mostly fiber optics since fiber technology is the fastest so far.

Yes, fiber optics solved connectivity issues at the internet core or backbone but can we say the same for network last-mile or the network end users? No matter the speed of cable connections, everyone would happily opt for slower, comfortable and easy to use wireless connectivity over the fast but cumbersome wired connections. But the fact still remains that wireless area networks are always bottlenecks in a network. The radios in our mobile devices are designed to make use of frequencies between 2.5 GHz and 5 GHz but its obvious we're at the edge of their performance due to their limited bandwidth which is still being shared among many network technologies and the hunger for faster internet surfing speeds is still on the rise.

One might ask, what about frequencies above 5 GHz? Heard about Wireless gigabit abbreviated WiGig? It is a wireless access technology within the access layer (wireless within end-users) which operates at gigabit speeds.By using frequencies above 10 GHz, it was possible to extend wireless access speeds up to 7 Gigabit per second but why is this WiGig technology available everywhere? Remember the inverse relationship between frequency and wavelength?

Velocity (v) = Frequency (f) * wavelength(λ)
this implies that f = v/λ

The result of increased frequency and reduced wavelength is short operating distance due to increased signal error rate.

The generally, visible light communication system are made up of three parts which are the transmitter, the medium or channel and the receiver, for better understanding, I will discuss each in detail.

Optical Data Transmitter

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The objectives of visible light communication system is to provide illumination and also leverage on such system to also provide faster data communication when compared to technologies like wireless fidelity and the likes. Hence, the transmitter in this case is the light source. Two major light sources are used for VLC and they're light emitting diodes (LEDs) and laser diodes though the later is rarely used due to cost possible health risk(s).

By satisfactorily answering the following questions, the concept of LED as data transmitting device would be clear. These questions are:

• Why use LEDs instead of other domestic light sources like incandescent bulbs?
• Where's the lights coming from?
• Can LED radiating pattern support data transmission?

One of the technology that widely employed LED "services" is the solar and battery inverter systems for the major reason of power efficiency. LEDs not only give off brilliant light but does this efficiently. More so, no one would like to adopt a network system that requires frequent changing of part(s), in fact, VLC was designed to reduce management overhead. Take for example, the Luxeon light emitting diode which can serve for over 2, 500 days which is approximately 7 years, is majorly used in implementing VLC systems.

LED is a diode, which implies that it is a creature of semiconductor, a P-N junction device to be precise. This means combining a positively charged semiconductor with a negatively charged semiconductor in a process known as doping.

Without a potential difference at the terminals of the diode, there exists a space between the two different charge carriers (Holes being the majority carrier for the positively charged region and electrons being the minority carrier for the negatively charged region) and this space is known as the depletion region.

When a positive potential is applied to the positive end of the P-N junction and a negative potential applied at the negative charged end of the P-N junction, this is known as a forward bias. When the P-N junction is forward biased, both the minority and the majority carriers tends to rush to the depletion region and this would cause recombination of both the electron and proton. Whereas the electrons are in a higher energy level, the protons have lower energy level and this recombination process causes a tremendous release of energy in the form of photon (light).

The P-N junction of LEDs are usually enclosed in hemispherical glasses. These glasses have varying refractive indices. This implies that light leaving the LED will do so differently from light entering the LED. Due to this radiating pattern of LEDs, of importance to us is the wavelength of the emitted light. We can determine the wavelength of the emitted light using energy equation as follows:

λ = h*c/e
where h is known as plank's constant
c is the speed of light and
e = energy at the depletion region or the band gap energy.

This implies that we can control the wavelength of the emitted light by controlling the energy of the band gap. The band gap energy is dependent on the type of semiconductor material used.

The Photodetectors

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The LEDs provides the clocking and hence, serves as the transmitter while also illuminating the surrounding. At the receiver end are photodetectors. The idea at the receiving end of the system is to reproduce the electrical pattern at the terminal of the transmitters (LEDs). The idea of photoelectric emission is applied in reversing the trend here at the receiving end.

Since the electrical signal at the P-N junction diode is given off in the form of light, we might ask, do all the generated photons leave the depletion region? For an ideal P-N junction diode, all the photons at the depletion region are assumed to escape the P-N junction but this is not the case in real LEDs because of some physical factors which can absorb some of the generated photons. This boils down to how efficiency of the LED and this is known as extraction efficiency. This makes the electrical power input to the LED to be unequal to the optical energy at the surface of the LED.

There are several light sensitive materials that can transform incident light at them to electrical equivalent using the process of photoelectric effect. Photoelectric effect is the ejection of electrons from the surface a material as a result of incident light ray on such material.

The widely used receiver or photoelectric is the photodiode, thanks to their high power efficiency, their physical sizes and the ability to quickly respond to light stimulations.

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Intensity Modulation and the Optical Channel

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Many radio systems make use of different modulation techniques and these techniques enables each medium to effectively deliver the message signal they’re conveying. The modulation technique for the optical wireless communication such as visible light communication system is the intensity modulation. This form of modulation is chosen due to the fact that light from LEDs are not coherent. Now before I explain what intensity modulation is let’s get to understand what coherent light source is.

As stated earlier, the recombination reaction between electrons which are at higher energy level with protons which are at lower energy level at the depletion region produces photons. These photons which are emitted have different phases and hence, LEDs are known as non-coherent light source. Conversely, lights from more precise source like the laser are coherent. This is because lasers do not instantaneously produce photons, rather, a photon stimulates the emission of another photon and the photon which is emitted by this stimulation is in phase with the ‘parent’ photon. This new photon goes ahead to further stimulate the release of another photon and the sequence is continuous.

In intensity modulation, a waveform is generated by changing the intensity of light source (LED light) since the emitted light does not have definite phase. The signal (network traffic) that is being modulated is discrete in nature but the conveying signal (light) is analog in nature. Once a signal has been modulated and conveyed, a demodulation technique is required and in this case, a technique called direct detection is used to demodulate the signal encoded in the intensity of emitted light.

In direct detection demodulation technique, a photoelectric emission is carried out by photodetectors and the generated current due to incident light is directly proportional the intensity of the incident light. The photodetectors are designed in such a way that their surface area are large enough to accommodate the wavelength of any modulated incident light.

Issues Associated with VLC (Pros and Cons)

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At this point, I know I must have succeeded in raising many questions about this technology, but hey, it has been implemented and has a commercial name like the Light Fidelity, Li-Fi. Questions like, Can it be used outdoor? What about reflections and interference? Is it truly mobile? Is it scalable? Is it available in the market?

Well, sorry to be the bearer of the bad news, it cannot be used outdoors but I assure you, with its speed of operation, you wouldn’t mind getting it for your office or your home moreover, more researches are on-going in this field to extend its functionality.

Also, the designers considers this as a “security advantage” since your neighbor can’t get close to the light coming out from your room to have a share of your supersonic LAN, lol. It can only be used “per enclosure”, this means that in the situation where by you want to deploy it through your home or office, each compartment will need to have its own VLC access point.

On the issue of reflection and interference, I’ll say it’s both good and bad for us. It is good for us in the sense that VLC system suffers little to no signal interference when deployed appropriately but it does suffer from reflection issues known as >multipath distortion. This is due to the availability of reflective surface in its operating area which is definitely unavoidable since the wall of a room is a reflective surface for light beams. This distortion is generally viewed as noise and some amplification and filtering techniques are used to reduce the effect though it is still available in negligible magnitude.

Within its enclosure, VLC systems allows the user to be mobile to a great extent and can be viewed also as a scalable system to a great extent. This is because, within an enclosure, there’s a minimum amount of light (transmitters) needed for its operation. The amount of transmitters (LEDs) can be increased to meet the size of any enclosure, say, large halls.

Summary

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The need to create a fast communication system with speeds comparable to that of optical fiber system led to the invention of visible light communication system. Wireless technologies such as Wi-Fi, telephone cellular networks and even our Bluetooth system share the same channel and is difficult creating a system with higher bandwidth without compromising its operating distance.

The VLC system is here to mitigate the bottleneck introduced in our fast internet backbone system and to enable internet nodes experience the same or almost the same speed as the backbone.

I would say that this technology is a must have for any data center. Please, feel free to ask any further question(s) using the comment section.

REFERENCES

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References presented below was done without considering any order with the rest of the references presented in line with the text. All images presented above are creative commons license images and their distributions clearly stated.

1. Visible light communication system ~Wikipedia
2. Working principles of LEDs ~toppr
3. Intensity modulation ~wikipedia
4. Photodiode ~wikipedia
5. Li-Fi ~wikipedia
6. Visible light communication technology ~visiblelightcomm

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