Study On Key Techniques Of Visible Light Communications

With the continuous innovation of smart mobile devices and the increasing popularity of short video and live streaming services,the traditional radio frequency communication technology faces a shortage of spectrum and expensive licensing due to the increasing demand for mobile data traffic.Light-emitting diodes(LED)-based Visible light communication(VLC)technology has become a new wireless optical communication technology(OWC),which provides the advantages of unlicensed spectrum,high transmission data rate,no conventional electromagnetic radiation,energy saving and environmental protection.The applications of LEDs including illumination,data communication and positioning,and therefore the research of the VLC technology is critical.At present,a lot of problems need to be solved during the development of VLC technology.The application and development of VLC technology are hindered by high frequency attenuation,propagation loss and complex outdoor environment of VLC channel.It has always been the focus of VLC technology research to deeply study VLC channel characteristics and improve system performance,which is of great significance to the future application of VLC technology.The dissertation first studies the channel characteristics of VLC,and some schemes are proposed to solve the existing problems.The VLC systems were designed and built to verify the performance of the VLC system.The main innovations are summarized as follows:(1)Combining theory with experiment,the VLC channel characteristics of indoor and outdoor environment are modeled and analyzed.The effects of high frequency attenuation,propagation loss and outdoor sunlight background noise characteristics on the VLC system performance are given.The accuracy of the model is verified by experiment.(2)A digital circuit-based feedforward equalization scheme for high speed real-time VLC system.A digital pre-distortion waveform shaping based feedforward equalization(DPDWS-FFE)scheme is proposed to solve the high frequency attenuation problem of indoor high-speed VLC system,which has the characteristics of low power consumption and low cost.The experimental results show that the 3-dB bandwidth of VLC system is increased from 13 MHz to 106 MHz,and the available transmission data rate is 262 Mbps over a transmission distance of 5 m with a bit error rate(BER)of less than 1.0×10-6,which uses a white phosphorous LED and the non-return to zero on-off keying(NRZ-OOK)modulation.(3)An analog circuit-based feedforward equalization scheme for high speed real-time VLC system.An added circuit based feedforward equalization(AC-FFE)scheme is proposed to solve the high frequency attenuation problem of indoor high-speed VLC system.AC-FFE has lower cost and less power consumption compared with DPDWS-FFE.The experimental results show that the 3-dB bandwidth of VLC system is increased from 16 MHz to 91 MHz,and the available transmission data rate is 180 Mbps over a transmission distance of 2.3 m with a BER of less than 1.0 × 10-6,which uses a white phosphorous LED and the NRZ-OOK modulation.(4)The characteristics of solar background noise in outdoor environment are measured and analyzed,and a joint scrambler and filter(JSF)scheme is proposed to address this problem.The experimental results show that the proposed JSF shceme can effectively suppress the solar background noise,and the available transmission data rate is 8 Mbps over a transmission distance of 70 m with a BER of 3.2 ×10-6,which uses a white phosphorous LED and the NRZ-OOK modulation.(5)An adaptive PD array receiving scheme is proposed for flexible VLC systems.The experimental results show that the field of view of the receiver is increased to 70°,the VLC system can move within a certain range,and the transceiver does not need to be strictly aligned,and the available transmission data rate is 180 Mbps over a transmission distance of 2.3 m with a BER of 2.87×10-6,which uses a white phosphorous LED and the NRZ-OOK modulation.On the basis of the above work,this dissertation designs and implements a high-speed indoor VLC system,a medium-distance outdoor VLC system,and an adaptive receiving indoor VLC system.A LED driver,receiver and amplifier,clock data recovery,equalization filter and other system modules are designed for the real-time VLC system.