Study Of Spectral Selective Detector For Visible Light Communication

Light emitting diodes(LEDs)have been used widely in visible light communica-tions(VLC).LEDs also have the ability to response to light and can be utilized as the receiver in VLC.However the properties of LEDs as the receiver that different from Si photodiodes have not been deeply explored.In this paper both the electro-optical and photoelectronic responsibilities of LEDs are investigated.The relations between the central wavelength and range of the response spectrum and the emission spectrum of LEDs are analyzed.Besides,the noise model of the LED receivers is clarified.The noise obeys sub-Gaussian distribution and belongs to the low-pass color noise.The shot noise does exist,but its variance is much smaller than the variance of other signal-independent noises.Next we study the photoelectronic responsibility of LEDs when they are lighting with the forward voltage.The results show that LEDs can response to optical signals even when lighting,but the response intensity exponential decays.The LED-LED visi-ble light communication performance is bad when the signal to noise ratio(SNR)is low.The communication performance becomes the better with the increase of the SNR,and eventually can exceed the one of the time division multiplexing.Reducing the propor-tion of high level of the signal can improve the performance.Furthermore the influence of the forward bias current on the electro-optical re-sponse spectrum of LEDs is experimentally investigated.The bandwidth of LEDs in-creases with the forward bias current.The logarithm of 3dB bandwidth versus the lumi-nous efficacy is approximately distributed over a negative slope line.Besides,the bias voltage can affect the signal peak to peak voltage(Vpp)that actually added on LEDs.It is indicated that the actual Vpp on LEDs can be seen as the divided voltage which is dependent of the ratio between the LED dynamic impedance and the bias tee output impedance.On the other hand,the more optical power received by the LED receiver,the lower bandwidth of the LED-LED VLC link is.With the LED equivalent circuit and the carrier transport model,we theoretically analyze the optical power dependent AC impedance of the LED receiver and its output current signal.Then we simulate the influence of these nonlinear characteristics of LEDs on LED-LED visible light commu-nications.For designing the multi input multi output(MIMO)LED-LED VLC system,we propose the Vpp constrains over the singular value diversity(SVD).To make the volt-age on the LED transmitter after SVD processing within voltage constrain,there would be SNR loss.When interference is small this loss might exceed the SVD gain.How-ever,for large interference the SVD method still work.The voltage allocation for LED transmitters can improve the SVD gain.At last but not the least,an asymmetric multi color LED-LED MIMO half-duplex VLC system with four pairs of LEDs is designed.Based on the orange red and red LED links,the offline test is investigated experimentally and the voltage allocation strategy is verified.Without equalization and channel coding,the communication symbol rate can reach 12.5MHz for 4PAM modulation with 15cm Tx-Rx distance,and 25MHz for OOK modulation with 20cm Tx-Rx distance.