VLC – A Brief Technology Description

Visible Light Communications are meant to be used by and between all kinds of terminals. From fixed terminals like TVs, PCs, printers etc. to mobile terminal like mobile phones, tablets, laptops and so on and of course the infrastructure. Figure 1 depicts this regime where each device can send and receive data from another of from the infrastructure.

Figure 1

Information is ultimately sent and received in the form of 1s and 0s as in any other case of communication. The archetype, the simplest form possible, VLC use the existence or absence of light to denote the information sent. The existence of light manifests the bit one for example while the absence of light states 0. These are the ON and OFF states and this modulation is called ON – OFF Keying (OOK). Practically the light source is driven by a drive circuit which enables the electric signals to be translated into light intensity changes. The alternation between states is realized so fast that human eye cannot perceive the flickering light. In figure 2 the states of light are shown schematically. At the receiver the transmitted 0s and 1s are detected by the photodetector which in turn translates them back into electrical signals (changes in electric current). The electrical signals are then amplified and forwarded to the rest of the system. In figure 3 this simple model is shown.

Figure 2

Figure 3

  

Like the conventional RF data transmission, OOK or amplitude modulations cannot reach very transmission rates due to inherent limitations. In more complex systems advanced modulation schemes like FDM (frequency division multiplexing), DMT (discrete multi-tone technique), WDM (wavelength division multiplexing) are used in order to reach higher data rates and combat other limitations and issues like range, noise, interference etc. A VLC link implemented by means of WDM is presented in figure 4. Some information and the implications in the modulation schemes are presented below.

Figure 4

The progress of LEDs led to an evolution in VCL field by enabling the transition from slow response incandescent bulbs and fluorescent lambs to faster response white light sources. Two are the main types of LEDs used for transmitting wireless data and they are depicted in figure 5. Blue LED and phosphor has two distinct emitting bands. In practice the blue response is used for data transmission due to its better response (offers higher bandwidth) while the rest is used for illumination only (offers lower bandwidth). The phosphor however limits the bandwidth thus it cannot be used for very demanding applications. For this family of LEDs OOK is used and a blue filter is necessary at the receiver to reject the phosphorescent components. On the other side, RGB (red green blue) triplet LEDs present three discrete bands, each corresponding to one of the colors thus making WDM or DMT potential candidates for their modulation. The more complex modulation schemes also yield higher data rates but the downside is that these LEDs are more expensive.

Figure 5

Moving forward to even more complex configurations MIMO (multiple input multiple output) techniques can also be implemented. In the next figure an example is given. MIMO implementation can combat the strong need for alignment or lower the SNR threshold but these techniques are quite sophisticated and demand strong signal processing.

Figure 6

Sources

[1] G. Cossu, A. M. Khalid, P. Choudhury, R. Corsini, E. Ciaramella, "3.4Gbit/s visible optical wireless transmission based  in RGB LED", Optics Express, Vol.20, No.26, 2012

[2] http://www.renesas.com/edge_ol/features/10/index.jsp

[3] http://bemri.org/visible-light-communication.html

[4] http://www.slideshare.net/hossamzein/visible-light-communication

[5] http://soe.northumbria.ac.uk/ocr/

[6] http://bemri.org/component/docman/doc_download/422-visible-light-communications-achieving-high-data-rates.html?Itemid=23