Design And Implementation Of Modulation And Demodulation In Outdoor Remote Visible Light Communication System

Visible light communication(VLC)takes the visible light as the communication carrier and the atmosphere as the transmission medium.Compared with the traditional radio frequency communication,it has significant advantages,such as large communication capacity,anti-electromagnetic interference resistance,no spectrum authentication and strong secrecy.With the progress of LED technology in recent years,features of LED,high electro-optical efficiency,small volume,long service time and short response time for example,also lead visible light communication into a flourishing stage.As one of the main directions of visible light communication,the maximum communication distance of outdoor visible light communication has reached more than 10km,and the maximum communication speed has reached more than 150kbps.However,most of these systems use intensity modulation/direct detection(IM/DD),the most common of which are OOK modulation and PPM.There are few examples of carrier-based modulation methods in actual implementation.In the long-range visible light communication,the channel attenuation is serious and the ambient noise power is large.Thus,the signal at the receiver is weak and the signal to noise ratio(SNR)is very small.Different modulation modes take up different bandwidth,and the SNR required to achieve the same error rate is different.The smaller the transmission power required under the same communication condition,the better the modulation mode is.In this paper,channel attenuation and noise power are analyzed and measured.By comparison,2PSK is chosen,and the modulation and demodulation scheme is designed.Finally,the 2PSK modulation and demodulation function is realized on SCM.The main work of this paper is as follows:1.The channel attenuation is analyzed and calculated from three aspects of atmospheric attenuation,geometric attenuation,and optical attenuation.Atmospheric attenuation is mainly caused by absorption,scattering,and turbulence.The divergence of the beam and the limited area of the receiving lens cause the geometric attenuation.The optical attenuation is related to the optical devices used by the system.By simulation,when the communication distance is 10km,the transmitter half-power angle is 5 degrees,receiving lens of 50mm focal length and 0.95 aperture is adopted,and the atmospheric transmittance is 0.8T,the attenuation of the channel is 106.37dB.2.The noise power spectrum is measured,the communication band is determined and the modulation mode of the minimum transmission power required under the same communication condition is selected.By analyzing the noise power spectrum,the carrier frequency of the communication is set on 250kHz.The power of the ambient light noise and the circuit noise is measured on the photoelectric conversion circuit.Then the signal power of the receiver is estimated according to the channel attenuation,and the SNR at the receiver is obtained.Compared with several modulation methods,2PSK has the minimum transmission power to achieve the target bit error rate(10-5).3.The modulation and demodulation scheme of 2PSK are designed.The communication band is selected by analyzing the power spectrum of ambient light noise.2PSK modulation is realized by direct digital frequency synthesis.A demodulation scheme is designed with reference to the correlation demodulation in the optimum receiver of 2PSK.A new bit synchronization algorithm is proposed and introduced,and a band-pass filter is designed.At last,when the carrier frequency is 250kHz and the communication rate is 5Kbps,the bit error rate simulation is carried out.4.According to the design scheme,the 2PSK modulation and demodulation in the visible light communication system are realized on the circuit.The circuit adopts STM32F446 as the main control and calculation chip.lt controls AD9837 to realize the modulation function and ADS8866 to sample the received signal.Then the sampled data are filtered,carrier recovery,bit synchronization,integral decision and so on,and demodulation is realized.When the bit rate is 5kbps and the two symbols of 2PSK appear alternately,it can be accurately demodulated and output high and low levels alternately.At last,the MAX3232 is controlled,and the serial communication between the circuit and the computer is realized,so is the data receiving and sending.