Modeling And Experimental Study Of Visible Light Communication System

With the rapid development of multimedia technology and the popularity ofintelligent network technology, people can be in the wireless network coverage area ofany place access to global information network, high-speed access to information.Although, it has greatly enriched people’s lives, but people are increasingly demandingin wireless transmission rates gets much faster and the quality gets much higher. Inmicrowave communications, the limited spectrum resources become very scarce due tothe suddenly increase of the requirement of information transmission. In thisbackground, visible light communication (VLC) technology as the traditional radiocommunications technology complements caused widespread concern and studied byglobal research. Visible light communication technology is the use of light emittingdiode (LED) used in actual lighting scintillation signal light and shade change fast, thehuman eye can not see, to transmit information. Compared with the power of radiofrequency communication, visible light communication system of green environmentalprotection, harmless to the human body, without the need of bandwidth resources, isalso very high security; therefore, visible light communication technology is generallyconsidered as having good development prospects.The visible light communication system model is researched in this paper, Thevisible light communication system based on OOK (On-Off Keying) and based onOFDM (Orthogonal Frequency Division Multiplexing) is established by the theoreticalbasis of system model, the performance of the communication system designed isexperimented respectively and the linear distortion based on phosphor LEDcommunication system is were measured and compensated. The full text includes the following research as:1. Visible channel and visible light communication system are modele. First ofall, the characteristic of LED frequency response is discussed; the frequencyresponse curve is experimentaled. The light intensity distribution that is pointto point by a single LED is modeled by Lambert distribution, and the visiblelight detector is modeled,the signal noise ratio is deduced, the frequencydomain response of the channel is studied, the nonlinear distortion of thevisible channel is modeled, the visible channel model is established and theproblem of the optimized layout is discussed based on the above analysis，avisible light communication system is modeled, a general expression ofvoltage signal which is outputted is deduced. The OOK visible lightcommunication based on the binary amplitude shift keying and imitatesinfrared encoding technique is simulated; the results verify the accuracy of themodel.2. The OOK visible light communication system is designed and integrated, asingle LED light-emitting model is established, the system structure andprinciple are described. The VLC sending and receiving devices are designedto achieve a point to point data transmission by the use of a single white LEDlight and visible light detector with OOK modulation. The program is writtenand the relevant agreements are formulated to achieve a system of softwareand hardware integration.The communication error rate which influenced bycommunication distance, SNR, light intensity and other parameters is studied.The results show that, under the selected hardware and related parameters, thebest illumination line is about200lx, the bit error rate is around10-4when thecommunication distance varies from0to200cm as the illumination line is200lx.3. An experimental system based on fluorescent LED is established, a network analyzer is then used to analyzed amplitude frequency response curve of thesystem, the linear distortion performance is analyzed. Based on theattenuation characteristics of the amplitude frequency response, the first, thesecond and third order high pass flters are designed as linear distortioncompensation circuits. Experimental results indicate that by using the thirdorder post equalization circuit, the Linear distortion of the system is obviouslyimproved, the decrease of the fain is improved form0.3-45MHz with3rdorder compensation. To ensure relatively flat gain characteristics of thecommunication system and obtain a small gain variation of less than10dB,the bandwidth is enlarged from1.8MHz before compensation to41MHz (therange is3~44MHz) with3rd order compensation indicating an increase factorof2, within this frequency range a large gain of-30dB is also achieved.Therefore, by compensating the linear distortion of the communication system,its bandwidth can be effectively enlarged. The proposed experimental andanalytical results in this paper canprovide data basis for system parameteroptimization and further improvement in communication performance.4. The OFDM communication system is modeled, the model diagram of theelectrical sending and receiving ends based on SIMULINK of OFDM systemare given, and the main parameters of the simulation model are determined.The channel estimation techniques, digital modulation techniques, RS(Reed-Solomon Code) error correction encoding technique are used in OFDMsystem modeling, the visible communication system based on OFDM is errorrate which constructed at a given source and detector parameters, thecommunication error rate which influenced by the modulation, form a pilot, apilot impact than, channel coding, the communication distance and otherfactors is studied. Experimental results show that a significant increase inerror rate with the increase in the communication distance of the communication system. Under the same conditions, the characteristics in theQPSK (Quadrature Phase Shift Keyin) modulation system are superior8QAM(Quadrature Amplitude Modulation) modulation scheme, but it also should benoted that, the communication rate of this system is faster under8QAMmodulation. The bit error rate of the system is significantly reduced with RScoding which also plays a very positive impact to the performance of acommunications system. The bit error rate of the communication system canbe reduced to10-5or less, and the communication distance of up to0.9mwhen the QPSK modulation, the introduction of RS channel coding, a pilot inthe form of a pilot block, and the pilot ratio is set to1/3are utilized.