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Experimental works on photocell based visible light communication for audio signal transmission application.

Introduction

Visible Light Communication is the most advanced communication technology using visible light between 400THz and 700THz (http://en.wikipedia.org/wiki/Visible_Light_Communications). This latest technology use light that are visible to human eye to transmit information (http://www.vlcc.net/e/e_about.html). Light is a part of our lives as we are using throughout day and night. For example, there are many devices including lightings in office, home, commercial display, traffic lights, electronic home appliances such as television computers and etc. Recently, all the devices has been fabricated using LEDs (Light Emitting Diodes) because of LEDs have low power consumption and long life span (http://www.marktechopto.com). LED has a special characteristic to light on and off very fast (http://www.vlcc.net/e/e_about.html). With this characteristic, data could be transmitted by lighting LED on and off at ultra speed. This findings will be developed to replace the current communication technology using radio waves.

In the field of communication today, there's no need to explain the useful of wireless communication using radio waves. Although radio waves had been used widely around the world, due to weakness and limitation researchers is exploring the visible lighting as a medium of communication in future. Research is focusing on LEDs that are expected to serve in the next generation lamps. LEDs offers advantageous properties such as brightness, reliability, lower power consumption and long lifetime (Komine, 2004). This devices are not only used for illuminating rooms but also for an optical wireless communication system (Komine, 2004). This system is expected to be the indoor communication of the next generation.

Today, among all wireless communication schemes, transmission using radio waves (RF) or microwaves has dominated. This domination was primarily due to by their availability of high-sensitivity receivers and the ability to provide either broad either broad coverage at low frequencies or line-of-sight propagation at high frequencies (Gareth Parry., 2007). However, RF can support only limited bandwidth because of restricted spectrum availability and interference (Singh, 2002). The Radio Frequency Interference will thus degraded user connectivity in terms of throughput, link quality and range (Far Point Group, 2010). Besides, position detection with cellular telephones and car navigation systems that use radio waves is not very accurate, as there is some difficulty in identifying the exact location of the information terminal (http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920). This drawback becomes more pronounced when equipment is used in underground shopping malls or inside buildings (http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920). As such, visible light in the electromagnetic (EM) wave spectrum was considered to be a potential solution to combat the plight faced by RF (http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920).

The purpose of this proposal is to develop a communication system for next generation using visible light. This system will implement the application of photo cells as a photo detector to absorb the illumination of LED and turn it into electricity. By using solar cells to transmit the signal, the overall system, cost reduction could be reached at optimum level. Moreover the system will use LEDs as source of illuminations and as a medium of indoor communications. This means that the LEDs have duo-function replacing the current lamps for effective illuminations and radio waves as wireless communication medium.

Literature Analysis:

A fundamental analysis done by T. Komine and M. Nakagawa on visible light communication come to conclusion that visible light communication is expected to be the indoor wireless communication of the next generation due to the possibility of transmitting high data rate (Komine, 2004; Tanaka, 2001). In this paper also they discussed about reflection and intersymbol interference. From the analysis, they found that communication performance is degraded severely by intersymbol interference (Tanaka, 2001). Intersymbol interference depended on the data rate and the Field Of Receiver (FOV) as the as the LED lights are distributed within a room and the irradiance of light is wide for function of lighting equipment.

Deqiang, Xizheng and Linpeng had carried out the numerical simulation of optimal lights layout scheme for visible light communication to find the effectiveness of this latest communication system. There are two kinds of communication system being proposed, optical up-link and optical down-link (Deqiang, 2007). Up-link has a small superficial area and narrow angle of irradiance like an electric torch whereby the LED will be lighten up from the bottom to top. Generally the down-link system has a large superficial area and wide angle of irradiance as the light will be located at the ceiling and lighten up from top to bottom. The performance of visible light communication is analyzed using the optimal lights layout scheme in term of the received power and bit error rate(BIT). Base on the numerical simulation results, the received power is very large compared with the infrared wireless communication which will make broadband communication possible (Deqiang, 2007). The effectiveness of the light layout is analyzed based on the received power. As the result from the simulation of optimal lights layout scheme for visible light communication they came out to the conclusion that this technique can be applied to obtain the excellent performance.

A study on challenges and possibilities of visible light communications done by O'Brien, Zeng, Le-Minh, Faulkner and W.Walewski introduced the principles of VLC and outlines some of its major challenges and overview of its application. In this paper there are few problems for implementation that was highlighted. The problems that has been raised up are, the limited bandwidth of LED that will affect the high-speed communication, low bandwidth of the transmitter, the disadvantage of transmitter equalization, providing an uplink to the distributed transmitter structures can be problematic and visible light communication is subject to regulation by non-communication standard (O'Brien, 2007).

Modulated Led:

Audio signals can be carried in radio waves through space and in electrical pulses through metal wires. Other forms of electromagnetic radiation, including visible light can carry audio signals through optical modulation. In modern communications systems such as long haul communication and high-bandwidth communication lines, modulated signals carried on a beam of light is commonly used. The relatively high frequencies of visual light can carry a lot more information than lower frequency radio waves. In optical fiber communication systems often utilize semiconductor light sources such as laser diode (LD) or light emitting diode (LED).

Both have the advantage that the digital modulation at logic circuit voltages can be directly applied to the operating bias current of the diode, which in turn varies the output power

Intensity Modulation:

In optical communications, intensity modulation is a form of modulation in which the optical power output of a source is varied in accordance with some characteristic of the modulating signal. In intensity modulation, there are no discrete upper and lower sidebands in the usually understood sense of these terms, because present optical sources lack sufficient coherence to produce them. The envelope of the modulated optical signal is an analog of the modulating signal in the sense that the instantaneous power of the envelope is an analog of the characteristic of interest in the modulating signal. Recovery of the modulating signal is by direct detection, not heterodyning.

[FIGURE 1 OMITTED]

An important conclusion that can be drawn from the data shown in Figure 1 is that the maximum bandwidth of direct modulation links is roughly 20 Ghz (Cox, 1997).

Photodetectors:

Photodetectors in use today are based on the p-i-n structure; consequently the emphasis will be on this type of detector in the discussion below. Long wavelength-i.e., 1.3and 1.55 nm-avalanche photodiodes have been developed, but they have approximately an order of magnitude lower avalanche gain at the same excess noise factor than silicon avalanche photodiodes (Cox, 1997). As is evident from the summary of reported results shown in Figure 2 (Shun Liu, 2007) photodetectors are available with 3-dB bandwidths which are more than sufficient for most link applications. However, the highest bandwidths come at a penalty to the detector responsivity. As will be seen below, the square of this parameter is a term in the equation for link gain. At any optical wavelength, the ideal responsivity of any photodetector is simply the electron charge divided by the photon energy (where is Planck's constant and is the speed of light in a vacuum).

[FIGURE 2 OMITTED]

At 1.3 nm this works out to about 1.05 A/W. Therefore, at low frequencies, where some commercially available 1.3- nm photodetectors have fiber-coupled responsivities of 0.9 A/W, this factor contributes only about 1.3 dB to the link RF-to-RF loss (as shown on the right-hand axis of Figure 2), whereas at higher frequencies a typical photodetector responsivity of 0.1 A/W would contribute 20.4 dB to the link loss. Another photodetector tradeoff is between linearity and optical power. All IMDD links have a reason to operate at high optical power. For direct modulation, the maximum modulation frequency of diode lasers is proportional to the square root of optical power (Way., 1987).

For external modulation, the link gain is proportional to the square of optical power, as will be discussed further on. Unfortunately, photo detector nonlinearities begin to dominate over modulation-device nonlinearities at high optical power densities. Recent results suggest that photodiode nonlinearities can be reduced by increasing the bias, since at high detector currents the series resistance and/or space-charge effects reduce the fraction of the bias voltage which actually appears as a field across the junction at higher optical powers.

In this paper we focused on the experimental work of visible light communications using the Photocells. From the experiment done, we found that photocell work as the photo detector that can change the light illuminations into electricity and finally convert it into signal.

Methodology: Transmitter And Receiver Circuit:

Basically a communication system could be explained through the Figure 3. As drawn by the this system could be divided into two parts, one is the transmitter and the second is the receiver. Focusing on the transmitter circuit, an input signal will be injected in a driver circuit and will be change to a light signal using the light source (LED). Then the signal will be transmit through the fiber optic medium to the receiver. The receiver basically consist of the photo detector, signal amplifier and signal restorer. The photo detector senses the luminescent power falling upon it and converts the variation of this optical power into a correspondingly varying electric current (Keiser, 2000). The optical signal is generally weakened and distorted when it emerges from the end of the fiber, an amplification of the optical signal and signal restoration will done to get the output signal. This is a basic diagram of the communication system using the fiber optic as a medium.

[FIGURE 3 OMITTED]

In this paper, some modification of the communication has been made through simple experimental works. The circuit is simple and similar to what has been reported in (http://www.exploratorium.edu/square_wheels/modulated_led,). The experiment was carried out using air as the medium of transmission and the photocell as the photodetector. Back to the basic circuit, this system has two parts, that is transmitter and receiver. Refer to the circuit Figure 4.

In this experiment, the transmitter part is built using 4 components. The first component, which is super bright LED, is used as the light audio transmission modulation. LED work to modulate the amplitude of the laser based on the amplitude of the audio signal that we are trying to transmit. Standard 3.5mm audio jack, the audio signal, which is fed to the circuit, is sent to the 470-ohm resistor and 9-volt battery. The LED is provided by a steady DC current by the battery and under the effect of the battery alone, the LED glows with a fixed brightness. The resistor function is to limit the current so the LED does not burn out. The Figure 5 showed the experimental works.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

For the second part of this system, the receiver, it is built using solar cell and speaker connected. The solar cell is connected to the radio in order to get the output signal. The experimental set up is shown in Figure 6.

[FIGURE 6 OMITTED]

Weak fluctuating radio signal is added to the constant signal from the battery after the audio jack is connected to audio source. The LED still glows, but now it blinking in synchronizes with the audio signal, as varies amount of current passing through it. Electrical signals is generated when the blinking light hitting the solar cell. This signal varies in synchronization with the original audio signal. Then, the signals are fed to the speaker, recreating the original sounds from the audio source. This means that the audio signal is transmitted from the LED to the photocell through air medium.

Results and discussion

The constructed first circuit was tested in the laboratory to analyze some characteristics designed circuit. We were tested the circuit by varying 3 parameters to get the output to easily analyze the characteristics of the circuit. Those three parameters are frequency, distance and resistance.

The obtained results are as shown below for three characteristics that we were carried out the tests. First, the power received for variable frequencies were analysed. Figure 7 shows the relationship of frequency of transmission signal and power launched by the LED. From the result and ploted graph we can conclude that the power is increase if the frequency increase. The voltage is constant for this experiment which is 2.5 Volts. The total power received are directly proportional to the input frequency. However the as we increase the input frequency, the power received is began to be linear.

Besides that, voltage characteristics was analysed by varying the distance as shown in Figure 8. The output voltage is decreased when the distance is increased. This shows that the voltage is inversely proportional to the distance. The frequency was constant at 1kHz throughout the experiment to avoid any other effects on the results. Even though the expected results were obtained but the received voltage level is in millivoltage which is considered very low. The lower voltage due to the some unexpected noise which occured during the experiment.

Finally, current versus resistance characteristics was tested by varying the resistance value. The resistance value was controlled by using potentiometer which connected to the circuit as shown in the circuit description part. As expected, the current value was reduced when the resistance value increased as shown in Figure 9. Figure 10 shows the some captured signal during the experimental work by using oscilloscope.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Conclusion:

The wireless communication based on photovoltaic receiver has been demonstrated successfully. The result shows the system works excellently for the low data rate transmission such as audio based application. The application is widely used in area of advertisement, entertainment, announcement and communication. Although there are many large bandwidth broadband application proposed for home and other small world application such as Videoconferencing (110 kbps), E-Learning (110 kbps), Telemedicine (110 kbps), Video Telephony (70 kbps), Near Video on Demand (1 Mbps), Movies on Demand (1 Mbps), Audio on Demand (110 kbps), Telegaming (40 kbps), Home Shopping (40 kbps) and other (SRC., 2006) but the short range communication which less energy consumption and with high bandwidth offering is still required to increase the system efficiency as well as for green awareness (Douseki, 2004).

References

Cox, C. III and R. Helkey, 1997. Techniques and Performance of Intensity-Modulation Direct-Detection Analog Optical Links. IEEE Transactions on Microwave theory and techniques 45(8).

Deqiang, D. K. Xizheng, X. Linpeng, 2007. "An Optical Lights layout Scheme for Visible-Light Communication ",Electronic Measurement and instruments, 2007. ICEMI '07.8th International Conference pp: 2-189(2): 194.

Douseki, T., 2004. "A batteryless optical-wireless system with white-LED illumination, " Proc. Of the 15th IEEE International Symposium on Personal, Indoor and Mobile radio Communicatios, 4: 2529-2533.

Far Point Group 2010. "The Invisible Threat: Interference and Wireless LANs".

Gareth Parry and Paul Stavrinou, 2007. "Nature Photonics-Dominic O'Brien".

http://en.wikipedia.org/wiki/Visible_Light_Communications

http://www.vlcc.net/e/e_about.html

http://www.marktechopto.com/products/Application-Of-LEDs/consumer- electronics-and-general-indication-ofleds.cfm

http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920

http://www.exploratorium.edu/square_wheels/modulated_led,

Komine, T., M. Nakagawa, 2004. "Fundamental analysis for visible light communication system using LED Lights".Consumer Electronics,IEEE, 50(1): 100-107.

Keiser, G., 2000. "Optical Fiber Communication," Mc Graw-Hill Education.

O'Brien, D.C., L. Zeng, H. Le-Minh, G. Faulkner, J.W. Walewski, S. Randel, 2007." Visible Light Communications: challenges and possibilities", Electronic Measurement and instruments. ICEMI '07.8th International Conference pp: 2-189(2): 194.

Singh, C., J. John, Y.N. Singh, K.K. Tripathi, 2002. "A Review on Indoor Optical Wireless Systems", -IETE Technical Review.

Shun, Liu, AtsuhiMinato., 2007. A New Lighting for Communication System for audio signal for white LED. J. Light and Vis. Env., 31(2): 65-69.

S.R.C., 2006. Broadband Applications. http://www.connectingcommunities.info/article.cfm?id=211

Tanaka, Y., T. Komine, S. Haruyama and M. Nakagawa, 2001. "Indoor visible communication utilizing plural white LEDs as lighting ," Proc. Of the 12th IEEE International Symposium on Personal, Indoor and Mobile radio Communicatios, 2: 81-85.

Way., W., 1987. Large signal nonlinear distortion prediction for a singlemode laser diode under microwave intensity modulation. J. Lightwave Technology LT-5: 305-315.

Corresponding Author: Mohammad Syuhaimi Ab-Rahman, Institute of Space Science (ANGKASA) Universiti Kebangsaan Malaysia 43600 UKM Bangi, Selangor, Malaysia.

(2) Mohammad Syuhaimi Ab-Rahman, Luqman (1) Al-Hakim Azizan, (1) Hadiguna and (1) Mohammad Hazwan Harun

(1) Spectrum Technology Research Group (Spectech) Depart. Of Electrical, Electronics and System Engineering, Faculty of Engineering and Built Environmental Malaysia.

(2) Space Science Institute (ANGKASA) Universiti Kebangsaan Malaysia.
Table 3.1: Technical data for LEDs.

                          [I.sub.F]   [V.sub.F]   [V.sub.F]   [V.sub.F]
Type             Colour     max.         typ.        max.        max.

Standard         Red        30mA        1.7V        2.1V          5V
Standard         Bright     30mA        2.0V        2.5V          5V
                 red
Standard         Yellow     30mA        2.1V        2.5V          5V
Standard         Green      25mA        2.2V        2.5V          5V
High intensity   Blue       30mA        4.5V        5.5V          5V
Super bright     Red        30mA        1.85V       2.5V          5V
Low current      Red        30mA        1.7V        2.0V          5V

Type             Colour     Luminousintensity   Viewingangle

Standard         Red        5mcd @ 10mA         60[degrees]
Standard         Bright     80mcd @ 10mA        60[degrees]
                 red
Standard         Yellow     32mcd @ 10mA        60[degrees]
Standard         Green      32mcd @ 10mA        60[degrees]
High intensity   Blue       60mcd @ 20mA        50[degrees]
Super bright     Red        500mcd @ 20mA       60[degrees]
Low current      Red        5mcd @ 2mA          60[degrees]

Type             Colour     Wavelength

Standard         Red          660nm
Standard         Bright       625nm
                 red
Standard         Yellow       590nm
Standard         Green        565nm
High intensity   Blue         430nm
Super bright     Red          660nm
Low current      Red          625nm

[I.sub.F] max: Maximum forward current, forward just means with the LED
connected correctly.

[V.sub.F] typ.: Typical forward voltage, VL in the LED resistor
calculation. This is about 2V, except for blue and white LEDs for
which it is about 4V.

[V.sub.F] max.: Maximum forward voltage.

[V.sub.R] max.: Maximum reverse voltage. You can ignore this for LEDs
connected the correct way round.

Luminous intensity: Brightness of the LED at the given current measured
in millicandela (mcd)

Viewing angle: Standard LEDs have a viewing angle of 60[degrees],
others emit a narrower beam of about 30[degrees].

Wavelength: The peak wavelength of the light emitted, this determines
the colour of the LED measured in nanometer (nm).
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Title Annotation:Original Article
Author:Ab-Rahman, Mohammad Syuhaimi; Azizan, Luqman Al-Hakim; Hadiguna; Harun, Mohammad Hazwan
Publication:Advances in Natural and Applied Sciences
Article Type:Technical report
Geographic Code:9MALA
Date:Apr 1, 2011
Words:3261
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