Currently I am doing research into visible light communication from an Embedded Systems background (MSc), but I am struggling to relate the concepts of modulation of radio to visible light. I already apply modulation to a working VLC system, but the underlying system still puzzles me.
My goal of this post is to create an understanding of the underlying fundamental concepts, so if there are flaws in my assumptions below please point me to it.
Radio
In RF communication, two modulation techniques are used: amplitude modulation and frequency modulation. All other modulation techniques are variations on these types.
These modulation techniques change the electromagnetic field in terms of coherent photons with a specific frequency and amplitude (source).
Visible light communication
In VLC, amplitude modulation is the most common technique (changing the intensity), but frequency modulation is also used. As frequency relates to color, some research uses RGB LEDs to do this in combination with color filtered photodiodes.
(1) However, and this is where the problem is, some research applies frequency and phase modulation using a single white LED. They use a photodiode as a receiver which is sensitive to intensity but not to color variations, so how is frequency modulation then possible?
(2) It seems that VLC has a third kind of main modulation technique in comparison with RF, is this correct?
(3) If so, what makes light so different from RF then, as it also is electromagnetic radiation? And can this third type be applied to RF?
Best Answer
Frequency modulation in VLC is not done by changing the frequency of the light itself. The light intensity is pulsed at a high frequency to create a carrier wave. This carrier wave is then frequency modulated to transmit the information, typically using frequency shift keying.
That's why it works with white light, where the frequency of the light isn't well defined.