25-07-2012, 03:50 PM
Visible light communication
[attachment=28940]
INTRODUCTION
Visible light
Visible light is the form in which electromagnetic radiation with wave lengths in a particular range is interpreted by the human brain. Visible light is thus, by definition, comprised of visually perceivable electromagnetic waves. The visible spectrum covers wave lengths from 380 nm to 750 nm.
At the lower end of the spectrum there are violet-bluish tones and light at the other end of the spectrum is interpreted to be distinctly red. Note that some animals exist whose vision merges into the ultraviolet (< 380 nm) or the infrared (> 750 nm).
Motivation
Using visible light for data transmission entails many advantages and eliminates most drawbacks of transmission via electromagnetic waves outside the visible spectrum. For instance, few known visible light-induced health problems exist today, exposure within moderation is assumed to be safe on the human body. Moreover, since no interference with electromagnetic radiation occurs, visible light can be used in hospitals and other institutions without hesitation.
Furthermore, visible light is free. No company owns property rights for visible light and thus no royalty fees have to be paid nor does expensive patent-license have to be purchased in order to use visible light for communication purposes. Visible light can serve as an entirely free infrastructure to base a complex communication network on.
VLC is mostly used indoors and transmitted light consequently does not leave the room when the doors are closed and the curtains drawn, because light cannot penetrate solid objects such as walls or furniture. Therefore, it is hard to eavesdrop on a visible light based conversation, which makes VLC a safe technology if the sender intends to transmit confidential data.
The most important requirement that a light source has to meet in order to serve communication purposes is the ability to be switched on and off again in very short intervals, because this is how data is later modulated.These rules out many conventional light sources, such as incandescent lamps.
Over the course of the last years, usage of LEDs1 has risen sharply. LEDs are often built into traffic and braking lights, but they also push conventional illumination methods (such as incandescent lamps) aside generally (LEDs are applied in more and more flashlights, headlights, status displays etc.) and might replace these other light sources entirely in the near future. LEDs fulfill the above requirement in that they can be switched on and o_ quickly. Thus they are well suited to modulate data into visible light. In order to receive data sent out in this way, photodiode receivers or CCD/CMOS sensors can be used which are typically built into digital cameras.
Figure 2 shows a general overview of the process of sending and receiving data described below.
Visible Light Communications Consortium
The Visible Light Communications Consortium (VLCC) which is mainly comprised of Japanese technology companies was founded in November 2003. It promotes usage of visible light for data transmission through public relations and tries to establish consistent standards. A list of member companies can be found in the appendix. The work done by the VLCC is split up among 4 different committees:
(1) Research Advancement and Planning Committee
(2) Technical Committee
(3) Standardization Committee
(4) Popularization Committee
TECHNOLOGY
Transmitters
Every kind of light source can theoretically be used as transmitting device for VLC. However, some are better suited than others. For instance, incandescent lights quickly break down when switched on and off frequently. These are thus not recommended as VLC transmitters. More promising alternatives are fluorescent lights and LEDs. VLC transmitters are usually also used for providing illumination of the rooms in which they are used. This makes fluorescent lights a particularly popular choice, because they can flicker quickly enough to transmit a meaningful amount of data and are already widely used for illumination purposes.
However, with an ever-rising market share of LEDs and further technological improvements such as higher brightness and spectral clarity,
LEDs are expected to replace fluorescent lights as illumination sources and VLC transmitters.
The simplest form of LEDs are those which consist of a bluish to ultraviolet LED surrounded by phosphorus which is then stimulated by the actual LED and emits white light. This leads to data rates up to 40 Mbit/s.
RGB LEDs do not rely on phosphorus any more to generate white light. They come with three distinct LEDs (a red, a blue and a green one) which, when lighting up at the same time, emit light that humans perceive as white. Because there is no delay by stimulating phosphorus first, Data rates of up to 100 MBit/s can be achieved using RGB LEDs ([Won et al. 2008]).
In recent years the development of resonant cavity LEDs (RCLEDs) has advanced considerably. These are similar to RGB LEDs in that they are comprised of three distinct LEDs, but in addition they are fitted with Bragg mirrors which enhance the spectral clarity to such a degree that emitted light can be modulated at very high frequencies. In early 2010, Siemens has shown that data transmission at a rate of 500MBit/s is possible with this approach.
It should be noted that VLC will probably not be used for massive data transmission. High data rates as the ones referred to above, were reached under meticulous setups which cannot be expected to be reproduced in real-life scenarios. One can expect to see data rates of about 5 kbit/s in average applications, such as estimation [Haruyama et al. 2008]. The distance in which VLC can be expected to be reasonably used ranges up to about 6 meters [Won et al. 2008].
Receivers
The most common choice of receivers is photodiodes which turn light into electrical pulses. The signal retrieved in this way can then be demodulated into actual data. In more complex VLC-based scenarios, such as Image Sensor Communication [Iizuka and Wang 2008], even CMOS or CCD sensors are used (which are usually built into digital cameras).
MODULATION
In order to actually send out data via LEDs, such as pictures or audio _les, it
Is necessary to modulate these into a carrier signal. In the context of visible light communication, this carrier signal consists of light pulses sent out in short intervals.
How these are exactly interpreted depends on the chosen modulation scheme, two of which will be presented in this section. At first, a scheme called sub carrier pulse-position modulation is presented which is already established as VLC-standard by the VLCC. The second modulation scheme to be addressed is called frequency shift keying, commonly referred to as FSK.
[attachment=28940]
INTRODUCTION
Visible light
Visible light is the form in which electromagnetic radiation with wave lengths in a particular range is interpreted by the human brain. Visible light is thus, by definition, comprised of visually perceivable electromagnetic waves. The visible spectrum covers wave lengths from 380 nm to 750 nm.
At the lower end of the spectrum there are violet-bluish tones and light at the other end of the spectrum is interpreted to be distinctly red. Note that some animals exist whose vision merges into the ultraviolet (< 380 nm) or the infrared (> 750 nm).
Motivation
Using visible light for data transmission entails many advantages and eliminates most drawbacks of transmission via electromagnetic waves outside the visible spectrum. For instance, few known visible light-induced health problems exist today, exposure within moderation is assumed to be safe on the human body. Moreover, since no interference with electromagnetic radiation occurs, visible light can be used in hospitals and other institutions without hesitation.
Furthermore, visible light is free. No company owns property rights for visible light and thus no royalty fees have to be paid nor does expensive patent-license have to be purchased in order to use visible light for communication purposes. Visible light can serve as an entirely free infrastructure to base a complex communication network on.
VLC is mostly used indoors and transmitted light consequently does not leave the room when the doors are closed and the curtains drawn, because light cannot penetrate solid objects such as walls or furniture. Therefore, it is hard to eavesdrop on a visible light based conversation, which makes VLC a safe technology if the sender intends to transmit confidential data.
The most important requirement that a light source has to meet in order to serve communication purposes is the ability to be switched on and off again in very short intervals, because this is how data is later modulated.These rules out many conventional light sources, such as incandescent lamps.
Over the course of the last years, usage of LEDs1 has risen sharply. LEDs are often built into traffic and braking lights, but they also push conventional illumination methods (such as incandescent lamps) aside generally (LEDs are applied in more and more flashlights, headlights, status displays etc.) and might replace these other light sources entirely in the near future. LEDs fulfill the above requirement in that they can be switched on and o_ quickly. Thus they are well suited to modulate data into visible light. In order to receive data sent out in this way, photodiode receivers or CCD/CMOS sensors can be used which are typically built into digital cameras.
Figure 2 shows a general overview of the process of sending and receiving data described below.
Visible Light Communications Consortium
The Visible Light Communications Consortium (VLCC) which is mainly comprised of Japanese technology companies was founded in November 2003. It promotes usage of visible light for data transmission through public relations and tries to establish consistent standards. A list of member companies can be found in the appendix. The work done by the VLCC is split up among 4 different committees:
(1) Research Advancement and Planning Committee
(2) Technical Committee
(3) Standardization Committee
(4) Popularization Committee
TECHNOLOGY
Transmitters
Every kind of light source can theoretically be used as transmitting device for VLC. However, some are better suited than others. For instance, incandescent lights quickly break down when switched on and off frequently. These are thus not recommended as VLC transmitters. More promising alternatives are fluorescent lights and LEDs. VLC transmitters are usually also used for providing illumination of the rooms in which they are used. This makes fluorescent lights a particularly popular choice, because they can flicker quickly enough to transmit a meaningful amount of data and are already widely used for illumination purposes.
However, with an ever-rising market share of LEDs and further technological improvements such as higher brightness and spectral clarity,
LEDs are expected to replace fluorescent lights as illumination sources and VLC transmitters.
The simplest form of LEDs are those which consist of a bluish to ultraviolet LED surrounded by phosphorus which is then stimulated by the actual LED and emits white light. This leads to data rates up to 40 Mbit/s.
RGB LEDs do not rely on phosphorus any more to generate white light. They come with three distinct LEDs (a red, a blue and a green one) which, when lighting up at the same time, emit light that humans perceive as white. Because there is no delay by stimulating phosphorus first, Data rates of up to 100 MBit/s can be achieved using RGB LEDs ([Won et al. 2008]).
In recent years the development of resonant cavity LEDs (RCLEDs) has advanced considerably. These are similar to RGB LEDs in that they are comprised of three distinct LEDs, but in addition they are fitted with Bragg mirrors which enhance the spectral clarity to such a degree that emitted light can be modulated at very high frequencies. In early 2010, Siemens has shown that data transmission at a rate of 500MBit/s is possible with this approach.
It should be noted that VLC will probably not be used for massive data transmission. High data rates as the ones referred to above, were reached under meticulous setups which cannot be expected to be reproduced in real-life scenarios. One can expect to see data rates of about 5 kbit/s in average applications, such as estimation [Haruyama et al. 2008]. The distance in which VLC can be expected to be reasonably used ranges up to about 6 meters [Won et al. 2008].
Receivers
The most common choice of receivers is photodiodes which turn light into electrical pulses. The signal retrieved in this way can then be demodulated into actual data. In more complex VLC-based scenarios, such as Image Sensor Communication [Iizuka and Wang 2008], even CMOS or CCD sensors are used (which are usually built into digital cameras).
MODULATION
In order to actually send out data via LEDs, such as pictures or audio _les, it
Is necessary to modulate these into a carrier signal. In the context of visible light communication, this carrier signal consists of light pulses sent out in short intervals.
How these are exactly interpreted depends on the chosen modulation scheme, two of which will be presented in this section. At first, a scheme called sub carrier pulse-position modulation is presented which is already established as VLC-standard by the VLCC. The second modulation scheme to be addressed is called frequency shift keying, commonly referred to as FSK.