A light-emitting diode (LED) refers to a light source (material) that is capable of transforming electrical power into visible light (Hu, Yang & Shin, 2008). A semiconductor, e.g., silicon carbide, emits visible light when electricity is passed through it. LEDs were first discovered in 1962 as ‘electroluminescent’ substances that give out low-intensity red light (Christensen & Graham, 2009).
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Because of this property, LEDs became popular sources of light (illumination) in electronic devices. Unlike their electroluminescent predecessors, modern LEDs can emit light of different colors and wavelengths. They also have wavelengths ranging from infrared to ultraviolet (UV) and high-intensity light, which make them appropriate for various commercial applications.
An LED transmitter is built from a “semiconductor diode (silicon) that has a p-n junction structure” (Christensen & Graham, 2009, p. 364). If an electric current (voltage) is passed through the material, it causes electrons in the diode to interact with the device generating photons, which then produce visible light.
The nature of the semiconductor, i.e., its bandgap determines the color of the light emitted by the diode. LEDs have been applied as sources of light in visible light communication. They are also used to make colored displays in commercial places and airports. Moreover, LEDs have been applied in display screens of devices such as laptops and televisions (TVs). Streetlights and vehicle indicators also use LED-based technology to display light of different colors.
LEDs have several advantages over the other lighting systems. Compared to traditional bulbs, LEDs give out high amounts of visible light, which makes them more efficient. Moreover, LEDs can be made into compact structures with a low surface area (up to 1mm2) and breadth (Ye, Greenfeld & Liang, 2008).
LEDs also consume less electric energy but emit high intensity (bright) light. For instance, research has established that LED TV screens consume 30% less power than CRT-based TV screens (Hu, Yang & Shin, 2008). Moreover, LEDs, if used well, can last for over 30,000 hours or an equivalent of 30 years (Hu, Yang & Shin, 2008). In comparison, incandescent bulbs can only last for up to a maximum period of 3,000 hours.
LEDs have gained application in wireless communication, where they serve as sources of light and transmission media. Visible light technology is a form of wireless communication that uses the light of visible wavelength, that is, 380nm to 780nm (Hu, Yang & Shin, 2008). Its frequency ranges from 382THZ to 790 THz (Komine & Nakagawa, 2004, p. 101). Visible light communication (VLC) relies on a source that emits visible light.
The emitted light is transmitted through the air (medium) into the photodiode or the receiver (Komine & Nakagawa, 2004). To convey information (data) using VLC, the intensity (amplitude) of the light is adjusted depending on the amount of data that is being transmitted. Often, fluctuations in amplitude can pass unnoticed because they are so minute.
By capitalizing on LEDs ability to modulate the intensity of emitted light signals, LED devices to serve as important communication tools. A diode can serve as a communication transmitter and a source of light in VLC technology. The proposed study will examine the applications of LEDs in visible light communication and compare them with those of radio waves and infrared.
There are many sources of light for visible light communication. The proposed study will only focus on the use of LED as a source of light in communication. It will examine the use of LEDs in illumination and data transmission. The main goal of the study is to determine the factors that affect the performance of LED-based communication.
The study will also seek to identify suitable ways in which LEDs can be used in data transmission. To achieve this, the study will evaluate current applications of LED systems in various fields and devices, including automobiles, commercial organizations, and military aircraft. An LED-based communication system (VLC) will be developed to identify the appropriate levels of brightness for LED devices.
The Problems/Issues Addressed
The LED-based communication technologies have been developed as replacements to traditional systems that were based on radio waves. They offer high-speed transmission and data security compared to micro- and radio- waves (Bhalerao, Sonavane & Kumar, 2013). LEDs systems rely on the light of a regulated wavelength that originates from an LED source.
They have found applications in indoor lighting, televisions, and cell phones, among others (Bhalerao, Sonavane & Kumar, 2013). They offer several solutions to many communication problems, including information security, safety, and more bandwidth. These qualities make them useful sources of light in communication. However, the protocols that govern data transmission for visible light transmission are yet to be standardized.
Several factors should be considered when developing the protocols for LED-based visible light communication. One of such factors is the efficiency of the light spectrum used to transmit the data. It refers to the amount of information that can be transmitted over a system within a particular bandwidth (Bhalerao, Sonavane & Kumar, 2013).
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It is important to determine the bandwidth rate of LEDs to measure the transmission efficiency of the communication system. Another important factor is system endurance. LEDs, just like incandescent bulbs, overheat when used for a long time. Their temperature can rise to over 45°C when in use (Bhalerao, Sonavane & Kumar, 2013). The heat affects the performance and durability of LEDs. Thus, research on how to control the increase in temperature can provide new ideas, which can be incorporated into the design of LEDs.
The main purpose of the proposed project will be to investigate the factors that affect the performance of LEDs. The findings will be incorporated into the design of visible light communication devices that rely on LED systems.
To achieve this purpose, the proposed study will evaluate the performance of various LED devices, assess the communication protocols for VLC, and identify the LED heat measurement approaches. It is anticipated that this study will generate sufficient empirical data that will inform designers and researchers on LED-based communication.
One of the issues that the proposed study will look into is the heat measurement approaches of LED systems. Research shows that overheating affects the performance, durability, and reliability of LED devices (Komine & Nakagawa, 2004). The study will also focus on the specifications and protocols that govern visible light communication. In this regard, the main research question that will be addressed by this research is: what factors affect the performance of LED devices that are used in visible light communication?
The research will be an exploratory study, which will aim to generate in-depth knowledge regarding the factors that affect the performance of LED-based communication devices. It will explore the efficient transmission protocols and specifications for VLC and temperature requirements for LEDs. The study will also examine the benefits of LEDs over other technologies, including radio waves and microwaves.
In this study, multiple devices that rely on LED technology will be examined, including televisions, cell phones, and laptops (3 research units). The three units will help determine the durability and performance of LEDs in various settings. They will also reveal the protocols that define visible light communication and the specifications required to enhance bandwidth efficiency. To this end, the study will rely on both primary and secondary data.
The proposed research will employ two methods to collect relevant data: (1) interviews and (2) secondary data sources. Semi-structured questions will be used to seek information from users, researchers, and developers regarding the performance of LEDs and the VLC protocols and specifications.
Information about the effect of temperature on the performance of LEDs will also be sought from secondary sources (peer-reviewed articles and books). The researcher will use the information collected to describe the efficient bandwidth for data transmission and the specifications for LED-based visual light communication.
This section will review previous studies that are related to the use of LEDs in communication. It will first compare the benefits of LED-based visual light communication and other technologies used to transmit data.
The benefits of LED-based VLC
According to Ye, Greenfeld, and Liang (2008), VLC differs from other technologies (radio waves) in five main ways: capacity, efficiency, data security, cost, and safety. Radio waves have a small spectrum, which limits their capacity to transmit wireless data (Ye, Greenfeld & Liang, 2008). Thus, they cannot support media applications that transmit large datasets.
In contrast, the LEDs provide the infrastructure that allows VLC to transmit more data, which makes them applicable to many media devices (Saadi et al., 2012). In terms of efficiency, LCD transmitters consume less power but transmit more data per unit voltage compared to radio waves. Moreover, LEDs provide data transmission and lighting simultaneously, which makes them energy efficient.
VLCs also do not require costly RF units to transmit data (Saadi et al., 2012). They rely on LEDs for illumination and data transfer. Furthermore, radio waves are known to cause electromagnetic interference, which may affect the performance of the hospital and aeronautic equipment (Saadi et al., 2012). Thus, radio waves pose safety and health challenges to users. In contrast, VLCs rely on LEDs for illumination. LEDs emit light that is not hazardous to human health or operations.
LEDs are also safe about data transmission. While radio waves can be intercepted, the data transmitted via LED cannot be lost (Ye, Greenfeld & Liang, 2008). Moreover, compared to infrared, the data transmission rate for VLC is higher. VLC can transmit data at a speed of 100Mb/s while infrared transmission can only reach a maximum speed of 20Mb/s (Vitta, Pobedinskas & Zukauskas, 2007).
Additionally, infrared can only transmit data up to a distance of 4m while LED-based VLC can transmit data for longer distances provided there is light. Infrared and VLC technologies also differ in noise sources. While for infrared technology, the light wavelength affects its quality, VLC is affected by sunlight. Another difference between VLC and infrared technologies is that while infrared facilitates transmission only, LEDs not only transmit data but also serve as sources of illumination.
The LEDs that are used in visible light communication can be categorized into two groups: white LEDs and colored LEDs (green, red, and blue light). Colored LEDs emit light with a spectrum range of 480nm to about 750nm (Vitta, Pobedinskas & Zukauskas, 2007). On the other hand, white LEDs emit a mixture of colored light, which makes them more preferable for illumination purposes. They can be made by combining three colored diodes (red, green, and blue), which emit light of different colors at the same time giving rise to white light.
Yellow light can also be produced when the blue diode is coated with phosphor. In this case, an electric current makes the diode to emit blue light, which is then converted to yellow light.
The yellow light combines with the blue light to generate white light. According to Vitta, Pobedinskas, and Zukauskas (2007), the white light generated this way is less expensive but has a low transmission speed because the phosphor takes a considerable amount of time to convert blue light into yellow light. VLC can utilize either white or colored LEDs. However, white LEDs made from phosphor may not be suitable for communication because a considerable amount of time is spent during the conversion of blue light to yellow light.
To transmit information effectively through the VLC systems, data modulation is important. Several methods can be used to modulate data in VLC systems. One such method is on-off keying (OOK). In this method, modulation is achieved by “turning the LED on and off” (Burmen, Pernus & Likar, 2008, p. 124). It uses binary numbers ‘1′ to indicate ‘lights on’ and ‘0′ to indicate ‘lights ‘off.’ One advantage of the OOK method is that its coding is simple.
However, it is not possible to achieve high-speed transmission using this method (Burmen, Pernus & Likar, 2008). The second method is known as pulse width modulation (PWM). In the PWM method, data is transmitted in coded pulses (Burmen, Pernus & Likar, 2008). Unlike OOK, PWM allows simultaneous transmission of many data bits through pulses. It is also easy to encode data using this method.
However, the method is slower, which makes it less efficient compared to OOK. The third data modulation method is pulse position modulation (PPM). In this case, the “data is encoded based on the position of the pulse within a frame” (Burmen, Pernus & Likar, 2008, p. 124). This approach allows the transmission of more data, but it takes a long time to transmit information compared to the other methods. This property makes it less efficient for use in VLC systems.
Another method that can be used to modulate data in VLC systems is the variable pulse position modulation (PPM). The method transmits data in pulses but provides for the control of the data transmitted in each pulse (Burmen, Pernus & Likar, 2008). Pulse amplitude modulation (PAM) is another data modulation method. In PAM, data is transmitted via pulse amplitudes.
The method allows for the transmission of more data bits in a single pulse (Burmen, Pernus & Likar, 2008). Thus, PAM is more efficient in VLC data transmission than OOK. However, it is complex to code data using PAM. Color shifting keying (CSK) is another method that can be used to modulate data transmission in VLC systems. It is particularly used in red, blue, and green LEDs. In this method, data is transmitted via a particular light. However, CSK requires complicated transmission and reception systems.
VLC data can also be transmitted using orthogonal frequency division multiplex schemes (OFDM). This method is often used in optical communications and ‘Wi-Fi’ and digital TV transmission (Burmen, Pernus & Likar, 2008). It relies on several carriers to transmit data at different frequencies, which makes the method efficient in the transmission of a broad spectrum of data. However, OFDM is complex and costly to use in VLC systems. Data can also be transmitted through the spatial modulation (SM) method. This method involves the use of optical signals. It uses multiple light sources (LEDs) to transmit several data bits simultaneously.
Challenges of LEDs in VLC Technology
LEDs have many potential applications in visible light communication. However, several technical problems limit their usage in VLC technology. One of the challenges is that LED-based communication follows a definite line of sight (LOS). Although this enhances the strength of the signal, the fact that LED lights cannot penetrate most objects reduces the transmission coverage. Visible light emitted by LEDs cannot pass through walls as radio waves do (Saadi, Wattisuttikulkij, Zhao & Sangwongngam, 2012).
This prevents data from being transmitted to devices placed in different rooms. According to Saadi et al. (2012), data transmission outside the LOS affects the rate of communication, as energy is lost when the receiver reflects light.
This means that the LEDs (light sources) must have enough power to emit strong signals to support communication. In contrast, weak light signals reduce the rate at which the receivers collect photons, which slows down communication. Indirect signals have limited energy, which reduces the data transmission rate (Saadi et al., 2012).
Another challenge in LED communication is the lack of duplex transmission. Duplex systems allow communication between two receivers. VLC transmission cannot allow duplex communication, as data from the LEDs is broadcast out to many receivers simultaneously. Thus, developing a communication channel for VLC is a problem.
In recent years, methods such as flash LED and IR have been developed to facilitate upstream data transmission for VLC (Saadi et al., 2012). Research has also focused on the use of radiofrequency (RF) as a VLC transmission channel (Saadi et al., 2012). Until now, no definite solution has been identified for upstream VLC transmission.
Transmitter sources also pose a big challenge to VLC transmission. The current LEDs lack specific qualities for optimal VLC transmission. LEDs only provide illumination in VLC devices, as their communication performance is low (Saadi et al., 2012). The designs of modern VLC devices consider both lighting and communication performance of the LEDs. Dimming control is another problem associated with visible light communication.
It means that if LED lights are ‘off’ or dimmed, no communication occurs, as VLC transmission relies on illumination. Thus, communication can only occur when the lights are on. However, when the LED lights are dimmed (daytime), the communication link is interrupted, and thus, no data transmission can occur. One way dimming can be achieved without interrupting transmission is by lowering the brightness of LED lights (Saadi et al., 2012). However, more research is required to develop techniques for facilitating transmission under dimmed lights.
Another challenge facing VLC technologies is sunlight interference. The transmission beams are affected by sunlight, which results in poor quality communication. However, according to Ye, Greenfeld, and Liang (2008), optical filters have been developed that prevent interference from artificial and natural light sources. The filters ensure that the interference does not affect the quality of the LED lights. Equalization of the channel also presents a big challenge to VLC transmission.
The channel can be equalized at two different points: (1) the receiver and (2) the transmitter (Vitta, Pobedinskas & Zukauskas, 2007). It is not clear which equalization technique gives the highest data transmission rate. An optimal equalization technique would increase the data transmission between the transmitter (LED source) and the VLC receiver.
As aforementioned, visible light technologies face challenges related to modulation. High bandwidth modulation systems require SNR channels (Saadi et al., 2012). Discrete multi-tone modulation (DMT) schemes that transmit data at 100Mb/s have been developed (Saadi et al., 2012). Simple modulation allows for expanded channel bandwidth, while complex modulation results in less data transmission channels. VLC technologies are also faced with regulatory challenges.
As a requirement, the LEDs used in communication have to meet certain safety standards. They are subject to regulation by various regulators, including environmental, health, and automotive agencies. Thus, VLC standards must incorporate different standards developed by the various regulatory agencies. Developing common regulatory standards is a challenge because the regulatory frameworks are many and diverse. Moreover, linking the regulatory techniques used in different countries is a challenge to the visible light communication.
Duplex Transmission between the Receiver and the Transmitter
Visible light technology is a broadcast communication. However, in some instances, it may be important to send data back and forth within the transmission medium. Park, Kim, Lee, and Park (2006) describe three approaches that can be used to circumvent this problem. The first method involves placing LEDs (photodiodes) in both the receiver and transmitter. Moreover, a VLC receiver is placed in the transmitter device to detect the response signal. However, the approach is costly because a high amount of energy is required by such a scheme.
The second approach involves the use of a ‘retro re-ector,’ which reflects the LED light to the transmitter without any data loss (Park et al., 2006). In this method, the light is modulated before being re-transmitted back to the LED source. This approach is less costly compared to the first method discussed before.
However, Park et al. (2006) state that retro re-ectors have low data transmission rates compared to LED transmitters. The third approach involves the use of either radio or infrared transmitters in place of LED sources. Although this method allows for data transmission at a high rate, it is prone to electromagnetic interference, which may affect the quality of the data transmitted (Park et al., 2006).
Potential Applications of LEDs in Communication
LEDs have many advantages over the radio- and micro-waves. Thus, they have found many applications in various fields. In particular, LED-based systems are applied in the healthcare sector, the automobile industry, air transport, and military communication. In aviation, passengers are discouraged from using communication devices that emit radio waves, as this may affect the safety of air travel (Manninen, Hovila, Karha & Ikonen, 2007).
Therefore, LED-based lights are used to illuminate airplanes, as these are safer than radio waves. Moreover, research is underway to develop VLC transmitters that can be used to transmit data and illuminate the aircraft. This has the potential of enhancing the safety of air travel (Manninen et al., 2007).
Another potential application of VLC systems is aesthetic lighting. Commercial buildings and infrastructure require energy-efficient, aesthetic lighting. LED-based visible lights can provide aesthetic lighting within a room or building (Manninen et al., 2007). Radiofrequency (RFs) and microwaves cannot be used in environments such as mines or nuclear stations because of safety concerns (Lindemann & Maass, 2009).
Thus, communication in these environments is a problem. In this regard, LED-based VLC offers safe communication and lighting technology in these environments. Visible light communication involves a controlled beam of light emitted by LED sources. Thus, fast and secure data connectivity between devices can be achieved using VLC technology.
LEDs have also found use in traffic lights. Moreover, car indicators, which enhance road safety, also use LED-based communication systems. Military vehicles and unmanned aircraft communicate via VLC systems that transmit a large amount of data in a fast and secure manner (Ye, Greenfeld & Liang, 2008).
In hospitals, communication that uses radio waves has been replaced with LED-based VLC, which does not interfere with medical equipment. VLC has also found application in communication under the sea. Radiofrequency (RF) and microwaves cannot transmit data underwater, and thus, LED-based VLCs are increasingly being used for communication among divers.
In this literature review, a survey of the benefits, challenges, and applications of LED-based VLC has been provided. LEDs have many potential applications in several fields. However, several challenges limit the use of this technology. The proposed study will examine the specifications and protocols that govern LED-based communication. It will also investigate the effect of heat on the performance, durability, and reliability of LED-based devices.
Conclusion and Recommendation
Presently, RFs and microwaves dominate communication systems. However, in recent years, research has focused on alternative ways of illumination and communication. LEDs are fast replacing RFs because they are energy-efficient sources that support communication and illumination.
The literature review has shown that LEDs have many advantages over radio waves and microwaves. They transmit more data per unit time compared to the other technologies. They are also energy-efficient, cheap, safe, and secure technologies for transmitting data from an LED source to a receiver.
However, technical challenges have prevented this technology from being fully exploited in visible light communication. LEDs cannot pass through solid objects and thus, can only be used for communication that occurs within one room or building. LED-based transmission involves broadcast technology. This means that the receiver cannot send back data to the transmitter. LED-based VLC also have challenges related to modulation and dimming control. LED beams are also prone to interference from the sunrays.
Current research in this area seeks to circumvent these challenges to optimize the spectral transmission of VLC systems. Another area that has attracted interest is the standardization of LED-based communications. Currently, various regulatory bodies have different standards for LED devices, depending on the nature of the field. Thus, the proposed study will investigate the universal standards that enhance the efficiency of LED-based communication.
The literature review shows that information transmission via LEDs must involve visible light. It means that, for communication to occur, the LED source must remain ‘switched on’ even during daytime. However, the light emitted by the LEDs is often too bright, and thus, many people may not find it comfortable to work in a room illuminated by LED. In this regard, this study will investigate the ways of reducing the brightness of the emitted light during the daytime to allow people to work.
To achieve this, a communication system will be designed that regulates the intensity of emitted light without affecting the amount of data that is transmitted from the source to the receiver. Appropriate levels of brightness should allow people to work comfortably and facilitate communication during the daytime.
LED-based visible light communication has a potential of expanding the bandwidth, providing secure and fast data transmission channels, transmitting data in a safe and environmentally friendly way, and facilitating broadcast communication between different devices. In this regard, it is recommended that developers of VLC technologies explore how the LEDs can be incorporated into current devices to develop hybrid communication systems.
The hybrid systems can help overcome the problems associated with LEDs and allow communication devices to function in multiple environments. For instance, RFs can be used to transmit data during daytime when LED lights are considered too bright while LEDs can be used at night to transmit data and illuminate the room.
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