Adaptive Equalization Circuit Design For Visible Light Communication Systems

Visible light communication（VLC）has advantages such as low power consumption,good security,abundant spectrum resources,and low cost,making it one of the key technologies for future communication research.However,there are two key challenges to achieving high-speed VLC:first,high precision alignment is required between the transmitter and receiver of the high-speed VLC system,and signal attenuation in mobile scenarios may lead to link interruption;and second,the modulation bandwidth of the VLC system is limited.To address the above two problems,this thesis focuses on the adaptive control of the receiving signal attenuation and compensation system frequency response attenuation in the VLC system.Adaptive amplification circuits and analog equalization circuits were designed,and a high-speed real-time VLC system was built on this basis.In addition,the equalization technology based on the convolutional neural network-gated recurrent unit（CNN-GRU）algorithm was studied,which effectively solved the signal distortion problem in the visible light communication system,thereby reducing the bit error rate.The main research content of this thesis shown as follows:1.To address the problem of rapid signal attenuation in mobile scenarios in VLC systems,an adaptive amplification circuit design scheme is proposed.By optimizing the hardware circuit of the VLC system transmitter and receiver modules,including the driver circuit,power supply circuit,transimpedance amplifier,and using feedback control to adjust the receiving signal attenuation,the problem of rapid signal attenuation in mobile scenarios is effectively addressed.Based on the designed circuit,a VLC hardware testing platform targeting mobile scenarios was built,and the experimental results demonstrate that:The receiver was tested at a distance of 1.4 m from the light source,with a horizontal movement range of-0.3 m to 1.1 m,and a vertical movement range of 0.5 cm.The output signal amplitude remained stable throughout the movement ranges.By adding a blue light filter in front of the receiver,the bandwidth of the system was increased from 3 MHz to 4.1 MHz.2.To address the issue of limited high-speed data transmission caused by the low bandwidth of VLC systems,a compensation system that compensates for the high-frequency attenuation of the frequency response was designed.This was achieved by designing an analog pre-equalization and post-equalization circuit.Through experimentation,the performance of the pre-equalization and post-equalization circuits was optimized,resulting in an increase of the VLC system bandwidth from 4.1 MHz to224 MHz.Based on this,a high-speed visible light communication system was constructed.The system was applied to transmit 100 Mbps Ethernet network signals and tested by experimentation,resulting in the following findings:At an indoor transmission distance of 2.5 m,the system can achieve a stable download speed of 95 Mbps.In addition,the system was integrated to realize the application scenario of turning on the lights for internet access,and certified by the Shenzhen Big Data Institute of The Chinese University of Hong Kong（Shenzhen）,it successfully achieved stable transmission of 1Gbps Ethernet signals.3.To address the issue of signal distortion in the hardware system of VLC,a CNN-GRU neural network was employed for equalization restoration of distorted signals in Four-Level Pulse Amplitude Modulation（PAM4）.The network parameters were optimized through backpropagation to minimize the mean square error loss function.Simulation results show that the neural network reduced the system’s bit error rate from10^-2 to 10^4-.Compared with the LMS algorithm,the neural network showed better convergence performance and lower bit error rate,verifying the effectiveness of the neural network in signal equalization.