One only needs to read some of the daft comments posted at the foot of online visible light comms articles to understand that there are a large number of misconceptions out there. Here is my Li-Fi myth buster top 10.
1. The lights flicker.
We subtly modulate the current supply to the LED devices at relatively high speeds. We are not harshly switching the LEDs on and off, and we are not modulating at speeds anywhere near those perceptible to the human eye. Your TV and computer displays do flicker at just higher than perceptible rates, the same is true of some LED dimming technologies. VLC does not flicker the lights like this, it will not give you a headache!
2. You cannot dim the lights.
There are VLC patents pending on methods to dim the LED while maintaining high data rates until the current is dimmed to about 50%. After that the data rates will begin to diminish in a very graceful manner. So yes, you can dim the lights and maintain communications reliably.
3. VLC is uni-directional (downlink or broadcast only).
VLC can be used for transmission in either direction. The uplink and downlink can be isolated in a number of ways – wavelength, time, code and also by spatial or optical isolation. For practical and cost reasons VLC might be implemented for downlink only since this is where bottlenecks exist with existing technologies, e.g. Wi-Fi may already provide a reliable uplink where congestion is less likely and Li-Fi provides a high capacity uncongested downlink.
4. There will be interference from sunlight.
It is relatively simple to eliminate the vast majority of interference from natural and artificial sources using optical filters (which avoids receiver saturation). After the photo-detector further analogue and digital filtering ensure remaining interference is negligible.
5. Lights need to be on so this is inefficient.
To use VLC the lights do need to be on. However in the vast majority of industrial, commercial and retail environments the lights are on when the area is occupied. Given that the lights are usually on, VLC transmission power comes free as it is already used for illumination so this is highly efficient.
In domestic environments we do tend to switch off lights during daylight. Where the lights would have been off the power required for VLC is not free but the lights only need to be dimmed up to transmit data. The illumination need not be above ambient levels so will not be noticed. The power consumed is comparable with the watts/bit for radio transmission and so on aggregate even in domestic environments there is a significant net saving in power.
6. You must have line-of-sight.
Line of sight is a definite advantage because the signal will be stronger. However, if you look under the table you can still see despite there being no line of sight from the sun or from artificial sources. If a VLC receiver can collect photons, it can receive data, albeit at a lower data rate if light levels are low. Radio technology is similar in that indirect signals have a lower power and hence the data rate reduces. Visible light can be reflected but generally does not penetrate materials which can be a security advantage and perhaps a coverage disadvantage. Radio can suffer multipath interference from non-line of sight reflected signal cancelling each other by being in anti-phase – leads to signal fading. VLC signal always add and cannot cancel each other and cause fading which is a significant advantage.
7. This is a disruptive technology.
VLC is often regarded as a disruptive technology relative to radio technology. I do not believe this should be considered to be the case. I believe VLC is totally complementary to radio. In the same way as Wi-Fi is seen as complimentary to cellular data, VLC or Li-Fi is complementary to Wi-Fi. Cellular data is automatically off-loaded to Wi-Fi when in-doors, in the office or home. Cellular operators insist that smart phones used on their networks are Wi-Fi enabled for this reason, and for quality of service reasons we tend to turn this feature on automatically to the relief of the cellular operators. Unfortunately and consequently Wi-Fi is now becoming heavily congested. Li-Fi can provide a high speed, high density bearer onto which the congested Wi-Fi downlink traffic can be off-loaded. VLC is radio’s friend we should not be considered enemies.
8. You need special LEDs
Specialist LEDs with ideal characteristics for VLC would be great. However, solid state LED lighting is currently being sold based on its performance for illumination purposes (colour temperature, efficacy, CRI, lifetime, etc). Communications performance is not even a secondary consideration, so it is wholly unrealistic to expect the lighting industry to factor this into designs at this stage.
In a practical sense we can achieve excellent results with COTS LED devices, if better devices are available great, but to implement VLC we can use existing LED devices. When VLC becomes a significant part of the LED industry then we can start to influence the specification of these devices.
9. VLC is a complex technology
VLC is a very simple technology since it uses direct modulation and direct demodulation . Infra-red remote controls are very low-cost for exactly the same reason. On the other hand radio technology is complex since it requires radio frequency circuits to modulate the data onto the radio bearer and then it requires an antenna system to transmit the signal. The radio receiver is often even more complex requiring an antenna system, radio receiver and carrier synchronisation circuits. Therefore VLC is much simpler than the equivalent radio system.
10. It will never work!
VLC technology has been proven to work by a number of companies and research establishments. The reliability of lighting systems has rarely been questioned but the reliability of wireless communications is increasingly in question. To my mind the question we need to be asking is; “Could we ever make wireless communications as reliable as lighting technology?”
Surely the answer to the above question is – “If we use lighting technology to deliver wireless communications then it will become as reliable as the lighting itself”. At least that is my dream!