Research On Theory And Technology Of Wide Spectrum Optical Communication Using Multi-color LEDs

With the rapid growth of data transmission requirement and limited radio resources,visible light communication(VLC)has attracted significant interests.By modulating the information to380nm-780 nm visible spectrum,VLC can not only meet the lighting requirement,but also provide high-speed transmission service.Compared with radio frequency(RF)communication,VLC offers a lot of advantages such as high speed,no interference with sensitive electronic equipment,secure indoor transmission and low cost.VLC generally uses the light-emitting diode(LED)as the transmit antenna.There are generally two kinds of LED: one is phosphor converted LED(PC-LED),which excites phosphor to form yellow light and then mixes to get white light;the other is multi-color LED(MC-LED),which forms white light by mixing several monochromatic lights.Compared with PC-LED,MC-LED can not only adjust the color temperature of illumination but also greatly increase the system capacity.Multi-color VLC(MC-VLC)faces the following problems.1)The overlaps of multi-color spectra are inevitable,resulting in cross-color interference.How to design the bandwidth(BW)and center wavelength(CWL)of optical filters is a key problem in MC-VLC.2)Thin-film optical filters are usually used in MC-VLC.However,the passband of a thin-film optical filter shifts towards a shorter wavelength as the angle of incidence(AOI)increases,leading to a degraded performance.How to achieve a stable performance in the scenario with random AOIs is a challenge for MC-VLC.3)MC-VLC using orthogonal frequency division multiplexing(OFDM)is an important scheme for high-speed transmission,but the improvement of data rate is limited by the non-ideal characteristics of LED(low-pass and nonlinear characteristics)and the lighting conditions(brightness level and color temperature).How to improve the transmission rate under these constraints is also an important problem.This dissertation focuses on the above issues,and proposes effective solutions.Firstly,this dissertation studies the optimization of BW and CWL of optical filters in MCVLC.In specific,three criteria are considered: maximum signal-to-interference-plus-noise ratio(SINR),minimum total mean square error(MSE),and maximum mutual information(MI).Under these three criteria,the optimization problems of BW and CWL of optical filters are established,and effective algorithms are proposed.Simulation results demonstrate that the optimized optical filters achieve better performance than the existing empirical optical filters.In addition,this dissertation also compares the optical filters that are designed under different criteria.Secondly,to address the passband shift issue with the varying AOIs,robust optical filter design is considered.This dissertation considers two common concepts of robustness: statistical robustness and worst-case robustness.The goal of statistical robustness is to optimize the average performance over all AOIs,and the worst-case robustness is to optimize the performance at the worst-case AOI.Based on these two concepts of robustness,the optical filter optimization problems are established,and effective algorithms are proposed.Simulation results demonstrate that the proposed robust optical filter can improve the performance at large AOIs,and thus support the mobility of receiver well.Thirdly,in order to further improve the performance under random AOIs,this dissertation considers the transceiver design for MC-VLC.The transceiver design are from two aspects.On one hand,we use the optical filters with the number larger than that of the LEDs,and a novel structure of optical filter bank is proposed.On the other hand,we consider the design of transmit precoder and receive equalizer,and an algorithm with high performance and low complexity is proposed.Simulation results show that the structure of optical filter bank in along with the transmit precoder and receive equalizer design algorithm can make MC-VLC system achieve good performance at all AOIs.Finally,this dissertation analyzes and optimizes the MC-VLC systems using direct-currentbiased optical orthogonal frequency division multiplexing(DCO-OFDM).Considering the lowpass and nonlinear characteristics of LED,the achievable rates of pre-equalization and postequalization of DCO-OFDM are compared and analyzed.Then,simulation and experimental tests verify the correctness of the theoretical results.Then,under the constraints of illumination brightness level and color temperature,the direct-current(DC)bias and alternating-current(AC)power of MC-LED are optimized to maximize the sum rate,and an effective algorithm is proposed.Simulation results show that the optimized DC bias and AC power can not only meet the required lighting conditions but also improve the sum rate of MC-VLC system.