Study On The Channel Model And Optical Receiving End Of LED-based Indoor Visible Light Communication

With the advantages of high brightness, low power consumption and long service life,white LED has gradually replaced the incandescent and fluorescent and would become themain lighting source for next generation. The modulation bandwidth of LED which is up toseveral MHz to several hundreds MHz makes possible visible light communication, andthere is no need for spectrum license. Presently, indoor visible light communication basedon LED has attracted the widespread attention at the home and the abroad researchinstitutions, and research groups and organiations about the visible light communicationhave been establishmented. The main focusing is to improve the communication data rateby modulation mode and channel coding, while having the less research on the receiveroptical end of indoor visible light communication.The receiver optical end is mainly researched in this paper. The receiver optical endshould have the following characteristics: high received power, high signal to noise ratio,wide viewing angle, light and small pattern and low cost.According to the types of white LED, the indoor visible light communication systemcould be divided into two categories: the indoor visible light communication system basedon the white LED that is a blue LED in combination with wide-band phosphor, and theindoor visible light communication system based on RGB LED. In this paper, the channelmodel of indoor visible light communication based on the minimum variance ofillumination has been established, and the design of tilting receiver plane including thenon-line-of-sight link is proposed, and then the design and experiment of opticalreceiving end of reflection volume holographic grating are proposed and experimentallyverified.The main contents are as follows：Firstly, the typical layout of indoor light source has been discussed, and the design oflight source layout based on the minimum variance of illumination including thenon-line-of-sight link has been proposed. The illuminance distribution, received power ofreceiver distribution, and signal to noise rate distribution at the desk including the line-of-sight link and non-line-of-sight are respectively calculated. It can be concluded that theone reflection of walls of non-line-of-sight link could not be ignored, especially the inter-symbol interference caused by wall reflection reduces signal to noise rate of system.Last the mitigation strategy for suppressing the inter-symbol interference has beenproposed.Secondly, the design of tilting receiver plane including non-line-of-sight link has beenproposed based on the layout of only one light or multiple lights. Newton iterated algorithmis employed to calculate and find the optimal tilting angle of receiver at the indoor desk onwhich the non-line-of link is considered, and this method is a fast algorithm that only threeor four searching steps are needed. The received power distribution and signal to noise ratedistribution at the indoor dest are compared and analyzed before and after tilting thereceiver plane. The minimum and average signal to noise rate can respectively increase byabout5dB and2dB. As can be seen from the simulation results, the signal to noise rate hasa significant improvement at the corner of the room, and the signal to noise rate also haslittle improvement.Thirdly, the “curved surface” coupled wave theory for volume holographic gratingwith curved interference fringes has been proposed based on the Kogelnik copuled wavetheory with the both plane waves interference. The formula about the magnitude anddirectional angle of grating vector in arbitrary position of volume holographic grating withcurved grating has been deduced. We found that the wavelength selectivity and angularselectivity may be different in different position of volume holographic curved stripegrating which depend on the angle between the propagation vector of reference beam andsignal beam. The larger the angle, the greater the angle and wavelength selectivity, whereasthe weaker. The diffraction efficiency in the side lobe would be larger than the one in themain lobe in some position for transmission volume holographic curved stripe grating,which will produce serious dispersion during the restruction for white LED, but thedispersion phenomenon would not occur in the reflection volume grating. Therefore, thereflection volume holographic curved fringe grating will be employed to produce theoptical receiving end for the indoor visible light communication based on white LED.Fourthly, the design of two different receiver optical end for off-axis reflection volumeholographic gratings with curved interference fringes is proposed in the indoor visible lightcommunication based on phosphorus fluorescent and RGB white LEDs. For the indoor visible light communication system based on phosphorus fluorescent white LED, thereceiver optical end has been designed with filtering yellow light and focusing bule light,and then the wavelength selectivity and angular selectivity are analyzed. For the indoorvisible light communication system based on RGB white LED, the receiver optical end hasbeen designed with seperating and focusing RGB light in different positions bymulti-wavelength recording volume holographic gratings with curved interference fringes,and the wavelength selectivity and angular selectivity are also analyzed, and then thefunction of seperating and focusing RGB light is experimentally demonstrated. For narrowfield-of view of volume holographic grating, the method of increasing the field of view hasbeen proposed, that is multi-recording of reference beam with the different angles at thedifferent time and multi-recording of reference beam with the different angles at thedifferent positions. The field of view can be effectively entranced by the above methods.