Why VLC?

"The time is right for VLC, I strongly believe that"

 Harald Haas, University of Edinburgh, UK

Well, ok… It has been stressed that VLC can be deployed for numerous applications in various places, but is this enough for it to grow up and penetrate the market similarly to RF based communications? What are its critically strong points? Where does it outmatch its opponents, if for example WiFi, Bluetooth, UWB etc. can be considered as such? In order to make such an assessment, the weak – problematical points and bottlenecks of the current dominant standards should be considered.

Capacity

Perhaps the most critical feature of a telecommunication link (of course it depends strongly on the application) is the achieved data rates or in other words the capacity of the channel. The WiFi standard 802.11n offers the impressive 600Mbps, UWB 802.15.3a the quite high 110Mbps, Bluetooth v.3.0 + HS the decent 24Mbps, ZigBee standard 802.15.4 the very low 250Kbps. But they all have one inherent limitation, very limited bandwidth because Bluetooth, WiFi and ZigBee are mainly operating at 2.4GHz (WiFi also operates at 5GHz band and ZigBee at the 900MHz band) and UWB from 3.1GHz – 10.6GHz thus a very narrow part of the RF spectrum which is already extremely utilized and crowded. Even though most of these rates are currently adequate and cover our needs this is not a panacea. Our needs in terms of throughput and bandwidth are rapidly increasing (more that 100% increase every year), RF spectrum is not wide enough to accommodate them and congestion is an unavoidable consequence as there is a saturation pattern in data rate increase. On the other hand VLC covers a linewidth from 380 – 740nm which corresponds to more than 300THz. Even if the whole spectrum from 0 to 100GHz was available and we had the means to fully exploit it, VLC available spectrum is more than 3000 times larger. The larger bandwidth in turn yields potentially higher data rates. Another important factor is that visible spectrum is an unlicensed band like the 2.4GHz band for WiFi. But that is a very narrow slice of the RF part of the spectrum. The rest of it is almost completely reserved for other applications and purposes like satellite communications, military applications, TV and radio broadcasting, mobile communications, aeronautical radio navigation and so on. An unlicensed band also means no extra costs for licenses and no bureaucratic obstacles for its usage.

 

Efficiency

Present RF technologies are characterized by very high inefficiency. Typically the percentage of power that is utilized is below 10% and the rest 90% is wasted. It is very enlightening to mention that in many cases more power is consumed in order to cool down devices like modems and routers than the actual transmitting power. On the other side VLC comprises two functionalities, namely illumination and wireless data transfer. In practice the data transfer comes as a byproduct of illumination – the visible light piggybacks the data transmission. The net result is that negligible power is consumed for data transmission purposes therefore giving a good alternative to replace inefficient and power – hungry RF based technologies. Even in the presence of light during the daytime when normally no lights would be on, replacing the traditional WiFi with illumination would be much more energy efficient. The VLC high efficiency originates from the evolution of lighting solutions. LED technology offers illumination with much higher efficiencies than the old fluorescent lamps and even more from the incandescent bulbs while at the same time enable high switching rates which yield the capability of high speed modulation. Today’s LEDs reach efficiency of more than 50%, more than 10 times the efficiency of incandescent bulbs, luminous efficacy up to 200, quadruple of fluorescent lambs, and their theoretical limits are even higher. Moreover LEDs have lifetime more than 50000 hours and are much cheaper.

  

Availability

In a previous post regarding the potential applications of VLC, it is emphasized that it is applicable in many occasions. The most significant part though is that in many of these occasions RF based communications are not allowed or applicable thus giving a direct alternative. Airplane cabins, hospitals, power plants with explosive and flammable fuels and so on, do not permit the use of conventional technologies due to interferences with other communication systems and radars, health hazards etc. The imposed barrier can be overcome by using VLC. Visible light is not a subject to electromagnetic interference and also does not cause itself interference because its spectrum is located very far from the spectrum used by traditional technologies. Moreover visible light is not prominently affected by other sources placed at the same room – place and not at all by sources located in other rooms or when an obstacle is place between them. Visible light is also perfectly safe while this is not the case for RF. Apart from these applications availability can be expanded in other cases as well, like in public places by setting hotspots thus offering increased connectivity.

Security 

Many times a desired property of WiFi for example is long range and it is usually achieved. 802.11 standards offer ranges of tens of meters depending of course on the propagation environment. RF waves have inherent properties like diffraction, reflection, propagation through objects like walls that allow such applications to cover long ranges despite large attenuation. The downside of long range is that an outsider can quite easily intercept thus rendering the network or communication link not secure. On the other hand visible light does not penetrate walls and is very directive compared to RF waves. By just constraining - focusing the light beam or just interpose an obstacle between the light source and a potential eavesdropper one can make sure that the communication link is safe.

Infrastructure

While VLC may seem kind of exotic or complicated to some people the fact is that it is a very simple, straightforward technology and most important the entire infrastructure exists and is already in place. Tens of billions light bulbs are used daily and are ready to be replaced by more efficient and cheaper LEDs accompanied by a simple microchip that will perform the data modulation and the transmitters are ready to go. 

Sources 

[1] F. Deicke, W. J. Fisher, M. Faulwasser, "Optical Wireless Communication to Eco - System", Future Network and Mobile Summit Conference Proceedings

[2] M. V. Bhalerao, S. S. Sonavane, V. Kumar, "A Survey of Wireless Communication Using Visible Light", International Journal of Advances in Engineering and Technology

[3] http://www.ted.com/talks/harald_haas_wireless_data_from_every_light_bulb

[4] http://www.hhi.fraunhofer.de/fields-of-competence/photonic-networks-and-systems/research-topics/optical-indoor-networks/optical-wireless-communication.html