LED Visible Light Communication System Design Based On OOK Modulation

Light Emitting Diode(LED)has become a leader in the lighting industry due to its low power consumption,low cost,and long life.Therefore,scientific researchers are envisioning the introduction of LEDs into the field of visible light communication to have dual functions of lighting and communication.In addition,because of the existence of electromagnetic radiation,radio frequency communication is not allowed in application fields such as hospitals and airports.Therefore,LED visible light communication,which without electromagnetic radiation,will become a good substitute for radio frequency communication.However,due to the low inherent bandwidth of LED and the low TIA bandwidth of the receiver in visible light communication.the speed limitation of visible light communication still needs to be solved.Therefore,it is particularly important to broaden the LED-3db bandwidth for the development of visible light communication.Due to the effect of equivalent junction resistance and barrier capacitance of PN junction,the output of LED presents low-pass characteristics.The equalizer used in this paper is a second-order active high pass filter circuit.The sufficient gain of medium frequency and high frequency will be used to increase the output response of LED.so as to expand the LED bandwidth to match the transmission rate required by the system.At the receiving end,the bandwidth of TIA is limited to 3-4MHz due to the requirements of system stability(self-oscillation caused by deep negative feedback system and open loop gain of operational amplifier)and high feedback gain of TIA(feedback resistance is large enough),So this article also involves the post-equalization circuit of the Trans-impedance Amplifier(TIA)at the receiving end.By adopting the designed equalization circuit,the transmitting and receiving bandwidth of the visible light communication system has been significantly improved.,the bandwidth of the visible light communication system is significantly improved.In the design of this article,firstly,a pre-equalization circuit of the LED is designed at the transmitting end,and the equivalent junction resistance,turn-on voltage and equivalent junction capacitance measurement methods of the LED are also given.Secondly,a set of LED communication system is designed,including LED transmitting system,channel transmission model and TIA receiving system.The LED emission system uses white light LEDs,the expanded-3dB bandwidth is 114MHz,and the T-type bias(BiasTee)is used to couple the modulated AC signal and DC bias to the LED,and the LED will emit according to the model of Lamb radiation.The white light energy radiates to the surrounding 180-degree hemisphere range.In the propagation path,the relationship between the optical power and the propagation distance is the inverse square of the distance.When the light propagates to the receiving end,the photodiode will convert the received optical power into the form of photocurrent,and finally the photocurrent signal is converted into voltage by the TIA circuit and output.Finally,combined with the noise model of the receiving end,the SNR and BER of the receiving end are estimated,and the system SNR and BER under different LED transmit powers are summarized for readers' reference.The experimental results show that the 100Mbps LED visible light communication system is feasible.