For numerous centuries techniques
based on visible light – hence eyesight – have been used to deliver messages –
information over various distances. From beacons to ship – to – ship
communications and from heliograph to photophone [1] by Graham Bell, even
todays traffic lights have been using visible light in order to establish
communication links. Today optical
communications is one of the emerging technologies but mostly when referring to
this, we imply the usage of guided, mostly near infrared light through optical
fibers or even integrated circuits. Visible light communications (VLC) is a
free space optics (FSO) technique which instead of using infrared or near
infrared guided light uses visible light which propagates in free space.
Visible light covers the radio spectrum from 380 – 740nm. VLC is in fact a part
of a broader developing technology called FSO or OWC (optical wireless
communications). What actually makes VLC different is that visible light is
used.
Up to date there are various
standards – techniques used in order to establish wireless links, for
information transmission and reception. WiFi, WiMax, Bluetooth, Zigbee, GSM,
3G, 4G, NFC, DVB are only some of the most popular technologies that are
incorporated in any electronic device (mobile phones, laptops, tablets,
satellite TV receivers etc.) needs connectivity with other devices. These techniques
are quite mature; they achieve high data rates and they have already penetrated
into the market remarkably successfully and rapidly. Then why have new technologies
been developed, especially if in practice they differ quite a lot, such as VLC?
Recent analysis have shown that we are running out of spectrum due to the
massive usage of our data – hungry electronic devices while at the same time
the power consumption of these devices is worryingly high. In order to get a
small flavor of the former argument, a glance at the spectrum allocation reveals
how densely sliced it is and moving a step forward the prices for buying a
small slice confirm how valuable that is.
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| Figure 1 |
In figure 1 the most commonly used
commercial standards are placed on a graph, depicting their data rates and
achievable distances. Most of them are located in the RF area of the spectrum
and some others in the IR. The green area shows the current capabilities of
OWC. It is very provoking and tempting to say that all the overlapping and currently
used standards can be immediately replaced by this new technology as they
fulfill the requirements for both range and rates and they additionally show
great potential but this is not the actual goal (at least for now) of VLC or
OWC. The current target of VLC is to become a complement of RF standards,
increase network capacity and be used where the conventional techniques are not
applicable or advisable while at the same time face other important issues like power consumption. Maybe in the future, even the not so far future, the increasing capabilities of this technology will allow the gradual replacement of other RF technologies.
References
[1] A. G. Bell, "The Photophone", J. Frank. Inst. 4,
1880, 237-248
