01-03-2013, 04:54 PM
Seminar Report On VISIBLE LIGHT COMMUNICATION
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ABSTRACT
Nowadays, a lot of researchers are working on the development of light-emitting diode (LED) lighting system. The LED lighting system can achieve lower power consumption and has a longer life-time compared to the fluorescent lamp system.
In this project, the characteristic of short transient time in turning the light on/off processes was further investigated. A high-speed wireless communication system, which is embedded in our LED lighting system, was built. The duplex communication system consists of both downlink and uplink media through different frequencies of lights.
Several experiments were conducted in the visible light communication system. In this communication system, off-the-self components were taken part in building the driver circuit and the performance of the system was evaluated, such as, data transmission rate, data transmission distance and the field of view of the transmitter.
INTRODUCTION
VISIBLE LIGHT
Visible light is the form in which electromagnetic radiation with wave lenghts 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.
HISTORY
The idea of using visible light for data transmission is not entirely new. Using smoke signals to transfer messages goes back several thousand years and was used by many di_erent cultures, e.g. Native Americans and Romans ([?; ?, donald-son1988signalling] [Sterling 2007] and [Crown 1974]). Lighthouses are employed to help ships navigate through dangerous coastal areas by sending out visible beams of light in periodical intervals. The “Pharos of Alexandria" was arguably the first tower which served as lighthouse and was one of the Seven Wonders of the World. Its construction dates back to 300 BC [Rawlins 2008].
The _rst sophisticated attempt to harness visible light for transmitting data was carried out by the Scottish scientist and inventor Alexander Graham Bell who is credited with inventing - among other devices - the photophone ([Gilleo ] and [Eitane et al. 2005]). The photophone was a device that allowed to transmit data on rays of sunlight and was completed in February 1880 by Bell and his assistant Charles Tainter. Figure 3 shows a schematic representation of the photophone which will help to describe its operation. Incoming sunlight is focused through a lense a on a very thin mirror b which is made to vibrate by a person's voice channeled through a mouth piece. Light beams that are thus reected from the vibrating mirror contain the modulated speech signal. This light then travels through lense c to a parabolic mirror e in whose focal point a selenium cell d is positioned. Selenium has the convenient property that its conductivity is inversely proportional to the amount of light falling on it. An attached telephone g can therefore demodulate the signal back into audible waves.
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 fluourescent lights and LEDs. VLC transmitters are usually also used for providing illumination of the rooms in which they are used. This makes uorescent lights a particularly popular choice, because they can icker 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 [Won et al. 2008], LEDs are expected to replace uorescent lights as illumination sources and VLC transmitters.
MODULATION
In order to actually send out data via LEDs, such as pictures or audio files, 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 subcarrier 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. A detailed account on modulation can be found in Sugiyama et al. [2007]. They also explore how to combine pulse-position modulation with illumination control.
PULSE-POSITION MODULATION
To successfully carry out subcarrier pulse-position modulation (SC-PPM) a time window T is chosen in which exactly one pulse of length T/k is expected. Thus, subcarrier pulse-position modulation can also be described as parameterized form, i.e. SC - kPPM. k has to be a power of two, i.e. k = 2l for some l. Then there are k = 2l different points of time for the pulse to occur. Suppose a pulse is registered at some point k’ ≤ k. The data represented by this pulse is then simply the number k’ written as k-digit binary number
CONCLUSIONS AND OUTLOOK
It has been shown that even though most existing e_orts are still in a very early stage, VLC is a promising technology with a wide _eld of prospective applications. An ever-growing interest in VLC throughout the world can be expected to lead to real-world applications in the future. In some _elds of application it poses a favorable alternative to conventional solutions (infrared, WLAN etc.). The main goals for the future are increasing the transmission rate and improving standardization.
[attachment=52353]
ABSTRACT
Nowadays, a lot of researchers are working on the development of light-emitting diode (LED) lighting system. The LED lighting system can achieve lower power consumption and has a longer life-time compared to the fluorescent lamp system.
In this project, the characteristic of short transient time in turning the light on/off processes was further investigated. A high-speed wireless communication system, which is embedded in our LED lighting system, was built. The duplex communication system consists of both downlink and uplink media through different frequencies of lights.
Several experiments were conducted in the visible light communication system. In this communication system, off-the-self components were taken part in building the driver circuit and the performance of the system was evaluated, such as, data transmission rate, data transmission distance and the field of view of the transmitter.
INTRODUCTION
VISIBLE LIGHT
Visible light is the form in which electromagnetic radiation with wave lenghts 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.
HISTORY
The idea of using visible light for data transmission is not entirely new. Using smoke signals to transfer messages goes back several thousand years and was used by many di_erent cultures, e.g. Native Americans and Romans ([?; ?, donald-son1988signalling] [Sterling 2007] and [Crown 1974]). Lighthouses are employed to help ships navigate through dangerous coastal areas by sending out visible beams of light in periodical intervals. The “Pharos of Alexandria" was arguably the first tower which served as lighthouse and was one of the Seven Wonders of the World. Its construction dates back to 300 BC [Rawlins 2008].
The _rst sophisticated attempt to harness visible light for transmitting data was carried out by the Scottish scientist and inventor Alexander Graham Bell who is credited with inventing - among other devices - the photophone ([Gilleo ] and [Eitane et al. 2005]). The photophone was a device that allowed to transmit data on rays of sunlight and was completed in February 1880 by Bell and his assistant Charles Tainter. Figure 3 shows a schematic representation of the photophone which will help to describe its operation. Incoming sunlight is focused through a lense a on a very thin mirror b which is made to vibrate by a person's voice channeled through a mouth piece. Light beams that are thus reected from the vibrating mirror contain the modulated speech signal. This light then travels through lense c to a parabolic mirror e in whose focal point a selenium cell d is positioned. Selenium has the convenient property that its conductivity is inversely proportional to the amount of light falling on it. An attached telephone g can therefore demodulate the signal back into audible waves.
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 fluourescent lights and LEDs. VLC transmitters are usually also used for providing illumination of the rooms in which they are used. This makes uorescent lights a particularly popular choice, because they can icker 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 [Won et al. 2008], LEDs are expected to replace uorescent lights as illumination sources and VLC transmitters.
MODULATION
In order to actually send out data via LEDs, such as pictures or audio files, 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 subcarrier 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. A detailed account on modulation can be found in Sugiyama et al. [2007]. They also explore how to combine pulse-position modulation with illumination control.
PULSE-POSITION MODULATION
To successfully carry out subcarrier pulse-position modulation (SC-PPM) a time window T is chosen in which exactly one pulse of length T/k is expected. Thus, subcarrier pulse-position modulation can also be described as parameterized form, i.e. SC - kPPM. k has to be a power of two, i.e. k = 2l for some l. Then there are k = 2l different points of time for the pulse to occur. Suppose a pulse is registered at some point k’ ≤ k. The data represented by this pulse is then simply the number k’ written as k-digit binary number
CONCLUSIONS AND OUTLOOK
It has been shown that even though most existing e_orts are still in a very early stage, VLC is a promising technology with a wide _eld of prospective applications. An ever-growing interest in VLC throughout the world can be expected to lead to real-world applications in the future. In some _elds of application it poses a favorable alternative to conventional solutions (infrared, WLAN etc.). The main goals for the future are increasing the transmission rate and improving standardization.