Describe about the Visible Light Communication?
Visible Light Communication (VLC) falls in the category of the highly developed technology of the optical wireless communication division, in this visible region (0.375um-0.780um) light is utilized as a data transmission medium with enhanced security and higher data rates features in comparison to the predictable technologies similar to Wi -Fi, Wi-max etc., which accomplish the communication by the radio waves. While making use of wireless internet the system bandwidth got bothered at the lower data rate or slow speeds frequently in the case of multiple devices using the same network. To conquer the lack of bandwidth we can make use of light for data transfer and the process is called as “ILLUMINATION DATA ”. The thought at the back is that, infra-red remote is somewhat customized i.e., LED light bulb that bring intensity contrasts which cannot be tracked by normal human perception. It is probable to encode the data in the light by modulating or altering the light to make the LEDs flicker and make it on and off to provide the binary strings of 1s and 0s.At the same time as using combinations of green, blue and red LEDs to change the frequency of light encoding a dissimilar data channel.
It utilizes fast changing light pulses to send out the information wirelessly. The major parts of this system of communication are
Switching the LED in the on and off mode is done to produce binary 1s and 0s strings. Encoding of data can be performed in the light to produce a novel stream of data by changing the LED flickering rate. To be precise, through the process is about modulation of the data signal using the LED light, the illumination of the LED light can be utilized like a source of communication. Due to the fast flickering rate, the output at the LED end comes into view like a constant light to the naked eye. Using suitable techniques for multiplexing it is possible to have data rate more than 100 Mbps.
In case of parallel data communication the VLC data rate can be augmented. In this each LED transmits a dissimilar data stream and combined forms an array.
Fig. 1 demonstrates the standard demonstrator block diagram. The link contains of two DSP boards first one is transmitter (Tx) and second is the receiver (Rx). In exacting, the evaluation board of TMS320C6000 DSP using Texas Instruments C6713 based on floating-point processor that is structured of VLIW very long instruction word architecture is utilized.
Speed of processor is 250MHz. The evaluation boards has 32-bit stereo analogue output and input port with a utmost sampling frequency 96kHz.
Fig.1 Visible light data transmission prototype
For the function of exhibition, data source of digital image is utilized. The produced D/A (digital/analogue) changed modulation signal from the correspondent DSP is feeder to the circuit of optical transmitter which constrains the white LED. At the end of receiver, a circuit containing a photodiode is utilized to change the signal of optical range to the signal of electrical domain. The converted signal is then conceded through an ADC converter trailed by elimination of cyclic prefix and process of demodulation. These processes are headed by a frame synchronization routine
Fig.2 VLC test-bed system using an array of 16 resonated white LEDs.
The VLC transmitter has components like a power supply, an amplifier, a bias-T. The receiver includes a PIN type diode as a photo-detector or APD or Avalanche Photo Diode. LED acting as a source of light from the transmitter side produces visible light radiation and then absorbed by the receiver all the way through free space spread. The signal source and a terminal analyzer component is made up of two-port network analyzer, giving output in form of a small sine wave and calculating the received amplitude too.
The LED modulation bandwidth is limited with the frequency of response. Though, the lifetime of the minority carriers’ in semiconductors have an effect on this reaction frequency.
Consequently, there exists a limit on the LED’s theoretical bandwidth upto 2 GHz. at present; the practical LED bandwidth is much lower than this limit of theoretical value. Therefore, the resulting LED’s lower modulation bandwidth influences its function in the communication with the high-speed bandwidth.
For analysis consider the three different LEDs which are further separated into three categories, which were calculated using same intensity of light, and after that the only variable is the current density. Consequently, the consequences can straightly replicate the association of the 3 dB LED bandwidth and current density. In below Figure 3, the Y coordinates gives amplitude, tells that the proportion of power at output and power at input and it reproduces the LED’s 3 dB bandwidth. Since noise is present in the background the curvature in Figure 3 shows big fluctuations. Although, these curves with non-smooth nature does not moving the in general experimental results trends.
As given in figure 3 that the LED A’s 3 dB bandwidth was 10.5MHz if the biasing current is 20 mA and the significance quickly increased to value of 44MHz for the biasing current of 100mA. In adding, the other LED’s B and C also established this regulation and the consequences were revealed in Figures 3(b) and (c), correspondingly. Consequently, the calculated 3dB bandwidth of three dissimilar sizes LEDs were all enhanced considerably with the amplified current density. This occurrence can be established with the probability of bimolecular recombination, that was relative to the carrier density of injected into the dynamic volume.
Fig. 3 The normalized frequency response of (a) LED A, (b) LED B and (c) LED C measured under different bias currents
Summary and Conclusions
In this report, a dimension association of modulation individuality for VLC systems is explained and the dissimilar-sized blue LEDs with different bandwidths have been accounted. The results obviously discloses alike linear association between 3dB bandwidth and its LED current density. This occurrence can be credited to the bimolecular recombination likelihood that is relative to the density of injected carrier into the dynamic volume. Consequently, growing the LED current density is a possible method in VLC systems to improve the rate of data transmission. Furthermore, this experiment demonstrates that series resistance of high value is one main concern that puts restriction on modulation speed of LED. Therefore, additional study will spotlight on optimizing layout of device in addition to dropping material bulk resistance to decrease the resistance.
References
1. H.L. Minh, D. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, Y. J. Oh and E. T. Won, “100-Mb/s NRZ Visible Light Communications Using a Postequalized White LED” inIEEE Photonics Technology Letters, Vol. 21, No.15, August 2009.
2. F. L. Jenq, T. J. Liu and F.Y. Leu, “An AC LED Smart Lighting System with Visible Light Time-Division Multiplexing Free Space Optical Communication,” 2011 Fifth International Conference on Innovative Mobile and Internet Services in Ubiquitous Computing.
3. Z. W, J. Chau and T. Little, “Modeling and Designing of a New Indoor Free Space Visible Light Communication System,” Networks and Optical Communications (NOC) in16th European Conference,July 2011, pp. 72-75.
4. J. Cao, Z. Liang and Z.Ma, “White LED Modulation Bandwidth and Modulation Characteristics of the Study” Hans Journal of Wireless Communications, 2012.
5. J. VuÄÂić, C. Kottke, S. Nerreter, K.-D. Langer and W. Waleski, “513 Mbit/s Visible Light Communication Link Based on DMT-Modulation of a White LED,” Journal of Light Technology, Vol. 28, No. 24, 2010.
6. J. VuÄÂić, C. Kottke, K. Habel and K.-D. Langer, “803 Mbit/s Visible Light WDM Link based on DMT Modulation of a single RGB LED Luminary,” Optical Fiber Communication Conference and Exposition (OFC/NFOEC), 2011 and the National Fiber Optic Engineers Conference, Conference Publications, March 2011, pp.1-3.
7. Z. Xu, H. Liang, Z. Zhong, L. He and X. Gu, “Effect of High-Power LED Spatial Light Intensity on Amplitude-Frequency Characteristics of VLC,” Optical Communication Technology, Vol. 35, 2011.
8. R. D. Koudelka and J. M. Woodall, “Light Emitting Devices with Increased Modulation Bandwidth,” Yale University, 2011.
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10. K. Ikeda, S. Horiuchi, T. Tanaka and W. Susaki, “Design Parameters of Frequency Response of GaAs-(Ga,Al) As double Heterostructure LED’sfor optical communications,” IEEE Tansactions on Electron Devices, Vol. ED-24, No. 7, 1977, pp. 1001-1005.
11. J. Grubor, S. C. J. Lee, K.-D. Langer, T. Koonen, and J. W. Walewski, “Wireless high-speed data transmission with phosphorescent whitelight LEDs,” inProc. Post Deadline Session Eur. Conf. Opt. Commun. (ECOC 2007), Berlin, Germany, 2007, pp. 1–2, [06.11].
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