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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?

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  • $\begingroup$ Marked John Rennie's answer as the solution because of its clear explanation, although I do not want to leave CuriousOne's answer unmentioned as it adds extra information which improved understanding. Due to my lack of rep. points: +1 for both :). Thanks! $\endgroup$ – Lennart Sep 10 '14 at 9:39
  • $\begingroup$ Your question seems to be missing basic information. Are you doing digital or analog communication? Are you using a multiplexing scheme? $\endgroup$ – akrasia Sep 10 '14 at 9:41
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    $\begingroup$ It is a more fundamental question, not aimed at digital or analog communication specifically. I wanted to know how FM is done in VLC. $\endgroup$ – Lennart Sep 10 '14 at 9:44
  • $\begingroup$ Just for background (I know this is not what you are asking): there are optical communication systems which do indeed phase modulate the light carrier itself - these are so called coherent optical communications systems - the word "coherent" here meaning that it is the phase of the light that encodes information - and they use light-analogues of all kinds of FM/PM subsystems, such as optical phase locked loops, where a laser under closed loop control is brought into coherence with another optical carrier. $\endgroup$ – Selene Routley Sep 10 '14 at 12:10
  • $\begingroup$ @WetSavannaAnimalakaRodVance Thanks for the added information! The term 'coherent optical communication' opened a new door for me in the world of scientific papers. $\endgroup$ – Lennart Sep 11 '14 at 9:07
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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.

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  • $\begingroup$ Would this also be possible with RF? $\endgroup$ – Lennart Sep 10 '14 at 9:07
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    $\begingroup$ @Lennart: yes, but with a limited data transmission rate. If the frequency of your radiation (light or RF) is $\nu_r$, then the frequency of the carrier wave $\nu_c$ can't be higher than $\nu_c \le \nu_r/2$, and for practical reasons it will be substantially lower. The data rate $\nu_d$ in turn can't be higher than $\nu_d \le \nu_c/2$ and again will normally be considerably lower. So the data rate ends up being a lot lower than the original radiation frequency. The frequency of light is so high this doesn't matter, but for RF it will limit the data rate. $\endgroup$ – John Rennie Sep 10 '14 at 9:16
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Physics first: light and radio waves are the same thing, just at vastly different frequencies. Radio works between $1\text{ Hz}$ and approx. $(10^{11}-10^{12})\text{ Hz}$ (at the moment). The frequency of visible light is around $10^{15}\text{ Hz}$. The range between these frequencies is usually called infrared radiation and is of little interest for communication.

Communication with visible light does usually NOT modulate the carrier wave at $10^{15}\text{ Hz}$ directly. Instead we modulate an RF signal onto that light (as an amplitude modulated signal) and then we modulate the phase or frequency of that RF signal. The reason for that is because we don't have easy ways to mix light (it can be done, but requires complicated optical setups). This leaves you with pretty much the same choices of modulation technique as with any other IF scheme. For data communication, of course, one can modulate the codes directly, because the semiconductor lasers are fast enough for that (LEDs are not).

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  • $\begingroup$ So it is basicly a form of Frequency Modulation through Amplitude Modulation? $\endgroup$ – Lennart Sep 10 '14 at 9:05
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    $\begingroup$ Yes, in most technical implementations that's exactly what it is. Having said that, one can modulate frequency, phase and polarization of light directly, and it's being done for experimental purposes all the time, but mostly in interferometer applications, where the original light source (i.e. the local oscillator signal in terms of radio) is also available. The main problem in communications applications with direct modulation would be the need to phase lock a local oscillator (laser) to the modulated light wave, which is hard and expensive. $\endgroup$ – CuriousOne Sep 10 '14 at 9:12
  • $\begingroup$ Thanks, CuriousOne! One question, what do you mean with your last remark about data communication? $\endgroup$ – Lennart Sep 10 '14 at 9:24
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    $\begingroup$ In data communication over glass fiber there is usually no need for a carrier modulation because the channel gain is almost constant and the codes that are being used are already DC-free (or almost DC-free) for baseline recovery. Dispersion of the signal due to rise and fall times and optical dispersion in the fiber is also tolerable, which is not the case for optical free space communication, which is usually done with LEDs, rather than lasers and which suffers from much larger dynamic range issues, limited bandwidth and stray light which can shift the baseline. $\endgroup$ – CuriousOne Sep 10 '14 at 9:35

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