Research And Implementation Of Transceiver Technology In Visible Light Communication System

With the development of wireless communication technology and people's pursuit of green life,indoor visible light communication becomes one of the research hotspots of wireless communication.Indoor visible light communication has the advantages of no electromagnetic interference,being harmless to human body,flexible construction and low cost.The system light source is white light LED for general lighting,and the wide use of LED light has laid a solid foundation for the application of indoor visible light communication,but there are still many key technical problems to be solved urgently.Among these problems,there are two most critical questions: one is that indoor light signal transmission is affected by the multipath effect,and the bit errors are easy to occur.The other is that the general lighting LED bandwidth is too narrow to meet the demand for high-speed communication.In order to solve these two problems,the OFDM technology in wireless communication is introduced into the indoor visible light communication system to improve the system performance.To Implement the design of indoor VLC system,the main research contents of this paper include the following points:Firstly,the principle and development status of indoor visible light communication system are briefly introduced,and the basic structure of the system is proposed.Secondly,the basic characteristics of white LED are studied,including the current-voltage characteristic,the illuminance characteristic and the bandwidth characteristic.Then the selection of LED is discussed.According to the current-voltage characteristic,the threshold voltage of LED is 2.5V,and LED is required to work in the linear region in order to ensure that LED can transmit light signal effectively.According to the illuminance characteristic,the light intensity which is proportional to the current attenuates with distance in exponential form.Therefore,LED fits to be driven by constant current.According to the bandwidth characteristic,it is known that the bandwidth of LED is 2MHz,and digital modulation technology is needed to improve the transmission rate.Thirdly,introduce common digital modulations,and use OFDM in indoor light communication system.MATLAB simulation result shows that OFDM can effectively reduce error rate of the system and the influence of the multipath effect.Fourthly,according to the characteristic parameters of LED,the hardware circuit of the transmitter and the receiver is designed.The transmitter includes voltage-current conversion circuit,constant current drive circuit and Bias Tee circuit.The receiver includes preamplifier circuit,high-pass filter circuit,main amplifier circuit and low-pass filter circuit.The circuit meets the design requirements through the drawing and testing of PCB.Fifthly,use FPGA to implement OFDM modulation and demodulation.OFDM modulation includes scrambling,interweaving,16 QAM mapping,serial-to-parallel conversion,fast Fourier inversion,parallel-to-serial conversion,adding cyclic prefix and adding window function.The submodules of OFDM demodulation module correspond with the inverse process of modulation module.The digital circuit part of FPGA is connected with the hardware transceiver circuit of LED,and the transmission rate of the system is tested.Test result shows that the system can transmit data at the rate of 8Mbps compared with 2Mbps transmission rate of the single-carrier,and the system effectively improves the transmission speed.