Modeling And Performance Analysis Of Communication System Based On Fluorescent Materials

Increasing communication demands have led researchers to focus on optical wire-less communications(OWC).The indoor visible light communication(VLC)scheme is based on the existing illumination device,light emitting diodes(LEDs)for high-speed communication,and the most common white LED is composed of a blue LED and a yel-low fluorescent substance.In outdoor ultraviolet communication,the high-speed pho-todetector,avalanche photodiode(APD),has poor responsivity in the short-wavelength band.Therefore,it is possible to consider the use of fluorescent substances in the front of APD to convert short-wavelength light to longer,which increases signal responsive power.This dissertation focuses on the role of fluorescent substances in OWC systems,by conducting theoretical,simulation and experimental research.First of all,through the investigation,we understand the specific process of flu-orescence emission at the molecular level,especially the inevitable energy loss in the process of fluorescence emission,and find that the wavelength change of short-wave excitation light in a certain range does not affect the emission spectrum and lifetime of the fluorescent substance.The effects of the thickness of phosphor layer and excitation light power on the excitation light and fluorescence power in the outgoing light were explored,which can help the subsequent modeling of fluorescence effects.Meanwhile,we tested the effect of the fluorescent film on the system bandwidth,and observed that the longer the wavelength of the fluorescent layer,the faster the frequency response curve drops.Then,based on the above results,we established a mathematical model of fluores-cence decay.Each parameter in the model can be determined by the thickness of the phosphor layer,the absorption constant of the phosphor layer,the energy level lifetime of fluorescent substances,and the spectral responsivity of the photodetector.The model can provide reference for the phosphor layer design and detector selection of various types of joint lighting and visible communication systems or fluorescence enhanced receivers.Based on the single exponential model,a more realistic multi-exponential model is proposed.The frequency response curves of various fluorescent bands were tested and fitted with two models.From the aspect of fitness,the double exponential model was slightly better.Furthermore,the effects of fluorescence on on-off key(OOK)modulation OWC system and orthogonal frequency division multiplexing(OFDhM)modulation OWC sys-tem on signal power and bit error rate are analyzed in detail.According to the model,the recovery method of OOK signal with the contribution of the fluorescence is pro-posed,and the bit error rate after signal recovery is simulated.The gain brought by the fluorescence effect is calculated.A blue laser diode based white light VLC platform was built.The signal power gain and bit error rate were tested at different bit rates,and the white light signal was recovered by using single exponential model and double exponential model respectively.The recovery effect of the method is very good,so that the white light OOK signal bit error rate of the 250 Mbps VLC system is reduced from 0.0424 to 1.05 x 10-5.The effects of fluorescence on the noise of the system are briefly analyzed.