Visible Light Communication
Visible light communication (VLC) is a wireless method that enables high-speed transmission of data with visible light. This data is transmitted by modulating the intensity of light given off by a light source. The signal is received by a photodiode device that transforms the data into forms that are readable and readily-consumed by end users.
Using Visible Light in Data Transmission
In the discussion of light, this usually means the entire electromagnetic spectrum, which comprises everything from gamma rays to radio waves. Only a small portion of the entire electromagnetic spectrum can be seen by the human eye, which is aptly called visible light.
The use of visible light in the transmission of data holds many different key advantages over technologies that make use of radio frequency. Its biggest advantage is the size of the entire visible light spectrum, which is 10,000 times larger than the entire radio spectrum, which is also too congested due to its overuse. With mobile traffic expected to increase sevenfold by the year 2021, the vast size of the visible light spectrum, which carries 300 THz of license-free bandwidth carried on visible wavelengths, certainly makes VLC a viable option.
Aside from the size of the visible light spectrum, light travels 186,000 miles per second, which is way faster than the 344 meters per second traveled by radio waves in air. This means that communication using light is virtually instantaneous, which also makes VLC the fastest means of communication among those commercially available in the market.
Data is transmitted in VLC systems by modulating light. At slow speeds, this will be seen as a constant flickering of light, which breaks down data into a system of ones and zeroes that will be converted into consumable data through a transceiver. However, the speed of data transmission is highly dependent on the speed of the flickering. For this reason, light emitting diodes (LED) are used as the primary light source in VLC systems. LED bulbs are semiconductors, giving them the ability to handle ultra-fast modulation of light occurring at speeds undetectable by the human eye.
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Characteristics of VLC
There are some characteristics that are unique to VLC. These characteristics include:
The nature of light is that it is unable to pass through opaque walls. This makes it easy to confine signals to within a single room, which increases the level of security of the network.
Many believe that because VLC systems use light, any blockage can severely hinder its ability to transmit data. That is definitely not the case as it is not dependent on line of sight. In fact, studies have shown that they can still perform in rooms that are severely obstructed.
Safe in hazardous environments
VLC can be used as a practical alternative for areas where RF signals are perceived as a hazard. Aside from using non-RF technology to deliver data, the light source used in these systems emit low energies, ensuring their safe use. These “hazardous” environments include hospitals, airplanes, or mines.
The architecture of VLC Systems
There two integral parts to VLC systems: the transmitter and the receiver. These parts consist of three common layers: the physical layer, the MAC layer, and the application layer. For the purposes of this discussion, only two layers will be discussed for the sake of simplicity.
VLC transmitters basically mean the source of the light. The evolution of LED lighting makes this technology possible and has made solid-state lighting – lighting that does not use electric filaments, plasma, or gas. That is because LEDs far surpass incandescent and fluorescent light sources in terms of reliability, power requirement, and luminous efficiency. The efficiency of LEDs, as well as the white light they emit and wavelength converters, make LEDs the best choice for a VLC light source.
There are many different spectra in which white light is produced by LED light. The most commonly used method for producing white light is trichromatic (red, green, and blue), more commonly known as RGB. They are advantageous in that they are able to produce high bandwidths, allowing higher data rates. They are, however, very complex and difficult to modulate. Other methods for white light generation are dichromatic (blue and yellow) and tetra-chromatic (blue, cyan, green, and red).
Receivers for VLC systems generally consist of an optical filter, optical concentrators, and an amplification circuit. Light is emitted from the VCL transmitter in order to give transmit data. However, this light is generally weak due to beam divergence because LEDs generally illuminate large spaces. This weaker signal is picked up by the optical concentrator and amplifies the signal. The signal is then detected and picked up by a photodiode, which is converted into a photocurrent. Silicon photodiodes, PIN diodes, and avalanche photodiodes are used for VLC systems.
VLC systems are vulnerable to interferences such as sunlight and other forms of illumination. For this reason, optical filters are added in order to eliminate noise from the received signal. And in the case of stationary receivers, photodiodes are employed. Imaging sensors are used for cases where mobility is required (e.g. VLC systems in vehicles) because of the larger field of view. These are, however, slow and energy-intensive. That is why it is necessary to have a trade-off between cost, speed, and complexity when considering the use of either a photodiode or an imaging sensor.
The physical layer provides the physical specifications of the VLC device as well as the relationship between the device and the medium used for data transfer. A summary of the interactions that occur within the physical layer goes like this: input bits of the data stream passes through a series of processes before it reaches the light source. This light source then emits photons through an optical channel through light signals. These signals are received by the photodiode device, which is demodulated and transcribed into output data.
The Media Access Control (MAC) Layer is responsible for the transmission of the packets of data received to and from the network. Its basic function is to provide a way for each node within a network to communicate with other available nodes. In other words, the MAC Layer points the packets of data towards the directions in which they need to go. In VLC systems, the MAC Layer is put in charge of the following tasks:
- Mobility support
- Dimming support
- Security support
- Visibility support
- Schemes for mitigation of flickering
- Color function support
- Network beacons generation if the device is a coordinator
- VPAN disassociation and association support
- Providing a reliable link between peer MAC entities
Terms Associated with VLC
VLC is a broad term that can encompass all form of transmission of information using a visible light signal. But there are also other similar terms that have come to be associated with VLC but have different functions or meanings. These terms include:
LiFi is a term used to describe high-speed networks that make use of visible light to transmit data. LiFi, which means light fidelity, makes use of LEDs to emit light signals to transmit data. These signals are then received by a photodiode device attached to the device to provide access to the data, which can be images, videos, documents, or the internet. LiFi is considered a complementary technology to WiFi, which can be used to provide relief to the already congested radio spectrum in providing internet access to the general public.
Free Space Optical (FSO) Communication
Similar to VLC, free space optical communication also involves the use of light in the transmission of data. However, the light used in FSO is not constrained to just visible light. The use of ultraviolet (UV) and infrared (IR) in communication also falls within this category. Unlike VLC, however, illumination is not required. This means that instead of LED lights, narrow beams of focused light such as laser diodes are used for transmission.
Optical Wireless Communication (OWC)
OWC is a general term that is used to refer to all types of optical communication without the use of cables (for instance, fiber optic cables). This means that visible light communication, LiFi, free space optical communication, and infrared remote control all fall under OWC.
Visible light communication is a revolutionary piece of communication technology that takes advantage of the surge in popularity of solid state lighting to open up numerous additional channels in which to transmit data. A VLC system is composed of two parts: the transmitter and the receiver. In VLC, light emitted from a LED light (transmitter) through rapid light modulation is received by a receiving device, which is then translated into usable data. This can then be separated into three layers: the physical layer, which basically dictates the relationship between the device and the medium, the MAC layer, which points the data received and process to the direction in which they need to go, and the application layer.
Together with LiFi and infrared remote controls, VLC falls under the category of Optical Wireless communication – a term used to describe all forms of light communication that do not employ the use of cables and other physical media.