VLC stands for Visible Light Communications. Basically it allows to use light in the visible spectrum to carry digital signal. Intuitively it is like switching on and off the light and associating to each on the value 1 and to each off the value 0 (or the reverse...). If you do this switching fast enough our eyes won't be able that there has been a "switch off" at all, due to the latency of light on the retina (this is what happens when we watch television: we perceive a fluid video whilst, in effect, there is a sequence of images being displayed, 25 or more depending on the standard used).
To make this feasible you need a light source that can be modulated (switched on and off) like LED. An incandescent bulb would not work since there is a thermal latency when you switch the light off and the incandescent filament remains such for a little while. You won't be able to modulate any signal at a reasonable frequency (People in the middle age modulated light signals with hand held mirror but the communication speed was very low).
VLC has attracted the interest of several parties, including Municipalities. With the incentive to substitute street illumination based on incandescent bulbs (or gas bulbs) with much more energy wise LED it becomes possible to use those same light poles as beacons to broadcast data. We are not there, yet, but technology is progressing.
The problem with the modulation of data onto a visible light signal used for illumination is that the modulation should not affect the quality of the perceived illumination (by our eyes). This result in the need to keep the light "on" for a sufficient time to trick our eyes into seeing a continuous illumination (not a flickering one). To do this, and transmit quite a bit of data, you need to increase the modulation frequency but present LEDs are not good for high frequency modulation (the ones we have in our televisions, cell phones and computer screens need only support a 50-100Hz on off cycles....).
Here is where the results obtained at King Abdullah University of Science and Technology (KAUST) comes in. Researches at KAUST have managed to create perovskite nano-crystals associated to a normal red phosphor. They can produce white light at a color temperature of 3236 K, which is in the normal range for illumination. The nano crystal support a very high modulation frequency, 40 times higher than normal phosphor, making it possible to transmit data at 2Gbps. It is a pretty good bandwidth if you imagine creating one 2Gbps access point from every light bulb around a city. Notice that this is supporting only downstream data communications but it can be good enough for many services where the illumination acts as a data beacon.
Furthermore, in a 5G architecture one can have the downstream data arriving from the city illumination at the spot where you are whilst the upstream data can go through a radio network access point. This will be the beauty of 5G: managing into a single communications sessions several communications media and channels.
One more reasons for Municipalities to become players in the 5G arena.