There has been a lot of media coverage about Visible Light Communications in the past month or so. This has led to some interesting discussions and comments being left. Some are enthusiastic and excited, some are curious, some are intrigued and others are just stupid or based on ignorance. I thought it would be helpful therefore to provide some of my answers to the frequently asked questions and maybe dispel a few myths.
1. What is the difference between Visible Light Communications (VLC) and Optical Wireless Communications (OWC)?
VLC is really a subset of OWC. Optical wavelengths include infrared, visible and ultraviolet light, whereas VLC is relevant to just the visible part of the spectrum. The reason that VLC has become a technology in itself is due to the fact that is can be seen by the human eye and so VLC provides illumination as well as communication. Traditionally OWC has been concerned just with communications.
2. What happens when I turn the light off?
The simple answer is that if you turn the light off (completely) then you will have no illumination and no communications. However, if you do not need the illumination due to there being sufficient ambient light the lights can be automatically dimmed to a low level sufficient for communications and which will not actually brighten the area more than necessary. The ambient light is constant whereas the artificial light is modulated so the two types of light are easily separated at the receiver. So the less simple answer is, yes it will still work with the lights “off”.
Definitely, yes it will! If the lights are on anyway there is virtually no additional energy required to transmit data. The receiver will consume some energy but this will be less than for radio systems. When illumination is not required it may not seem efficient to have them on dimmed when they could be off. However, if you turned off the lights completely you would need to use radio (e.g. WiFi) to communicate. Only 2% of the WiFi power is actual transmitted by the radio so these are inefficient communications devices. A light bulb can provide communications only where it is required and so overall less power is required for VLC with the lights “off” compared to the lights being fully off and the WiFi turned on.
4. If I get in the way of the light will communications stop
In a practical scenario this should not be a problem, light travels in straight lines but is also reflected easily. The light that can be detected at a receiver may take many different paths from the transmitter (i.e. the light bulb) but if you can receive illumination you will have communications. For example, if you point your digital camera at an artificially lit room the camera’s optical sensor can see the reflected light, it does not need to be pointed directly towards the light source. Obviously if you completely cover the detector (like covering the camera lens) then the communications will be interrupted.
5. Can I get two-way communications with VLC?
There are several ways to achieve two-way communication using VLC. The methods depend very much on the application, but in general both a receiver and transmitter are placed at either end of the communications link. We call this combined unit a transceiver. With a transceiver we need to ensure that the data transmitted is received by the other end of the link, rather than being received by the same end of the link. There are a number of ways the two paths of data can be isolated from each other.
Optical isolation: The optics of the transceiver can be arranged such that there is no optical path between the emitter and the detector. With simple optics and component positioning this is relatively easy to achieve.
Wavelenth division multiplexing (WDM): As the name implies a different wavelength is used to isolate the two paths. The receiver end has an optical filter so that only the correct wavelength (or colour) of light is received. As an example the downlink from a light fixture would be white visible light, the uplink back to the light fixture could use infrared.
Time Division Duplex (TDD): The uplink and downlink are isolated by time. i.e. only one transmitter-receiver pair is active at a time and the direction of information is governed by a clock signal. The uplink and downlink capacity can be asymmetric. Typically there can be considerably more downlink traffic compared with uplink.
Radio: Radio can be used for the uplink where only a small capacity is needed. With finite radio spectrum and large demands for downlink capacity, both technologies can be combined and capacity bottle necks avoided.
Broadcast: For some types of service there might be no need for two-way communications, i.e. these are broadcast services. The distribution of media – video, radio etc. often takes this form. Other services such as indoor positioning do not need an uplink either.
6. How does the data get to the light fixture?
In any communications technology there is the issue of the backhaul. The data must get to and from the transceiver devices. There are a number of ways to do this. The simplest is just to run a cable to the fixture or the driver for a number of fixtures. For lighting circuits it is normally simple to run Ethernet cables in the ceiling void. Cables in walls and floors are much more difficult and expensive. The second practical solution is to use Power Line Communications (PLC). PLC enables the existing mains power lighting cables to be used to carry the data to the fixture.
7. Can we be sure that VLC is safe?
Visible light itself is necessary for life on earth and so tends to be inherently safe for humans. Of course VLC modulates the light (a fast flickering that cannot be detected by the eye) but we are used to fast flickering lights – such as from TVs, computer screens and fluorescent lamps. Modern screens have been designed to refresh at frequencies above those that the human eye can detect and VLC modulation frequencies are above these still.
There have been studies that show positive and negative psychological effects of different colours of light. These studies are now looking at how light can be used to improve moods, productivity and help us to relax. They have also identified colours that should be avoided. VLC itself does not change the colour of the light so there ought to be no unpleasant effects introduced.
8. Will it be faster than wireless data using radio?
I believe that early VLC products will provide data rates that match or exceed WiFi. Reports of the early LVX System deployments are that they do not yet match WiFi data rates, although I cannot confirm this myself and I have not yet received a reply from an email to LVX.
Practical systems that are currently working in labs, in the UK at the University of Edinburgh and in Germany at the Heinrich Hertz Institute, are currently achieving 100Mbit/s and deploying this in practical products would provide a clear advantage over current WiFi.
Due to the large bandwidths freely available and improvements in the modulation methods and optical components, VLC ought to leave radio based wireless data systems behind in the speed stakes quite quickly.
Radio spectrum is full. Demand for wireless data continues to grow at 108% per year. Spectral efficiencies of radio systems are growing at only 12% per year. Radio systems require more system components (RF circuits, antennae, etc) than VLC systems making them more costly. Radio interference is difficult to constrain but visible light is not. There are security and health issues with radio communications that are not issues for light communications, etc, etc. If we rely solely on radio we are heading for a lot of pain.
10. When will VLC be ready commercially?
VLC products are close to market. The first mover, LVX System, already appears to have a pilot deployment in St Cloud, Minnesota. The University of Edinburgh spin-out, VLC Ltd, plans to offer products during 2012. The Boston University spin-out ByteLight has been incoporated. I believe that 2012 will mark the beginning of the VLC industry.