| With the continuous evolution of wireless communication technology,the increasing demand of users for high-speed and large capacity multimedia wireless data communication services leads to the increasing shortage of radio spectrum resources,and as the main technology of wireless communication,radio frequency(RF)communication also faces serious electromagnetic interference,signal instability,narrow bandwidth,high energy consumption and other inherent defects.Visible light communication(VLC)technology just in time to meet the trend of technology evolution,using the wavelength range of 380-780 nm visible light band as a data transmission carrier to transmit signals in the medium.Compared with RF,VLC spectrum resources are almost infinite,with the advantages of strong anti electromagnetic interference ability,green security,low cost and high confidentiality.However,there are still many challenges for VLC technology from laboratory to practical,such as the lack of recognized channel model,LED nonlinearity,multipath interference and background light interference.Among them,the signal distortion caused by LED nonlinearity is one of the main bottlenecks restricting the development of VLC technology,and the suppression technology of LED nonlinear distortion is the research hotspot nowadays.This paper aims to analyze and suppress the influence of LED nonlinearity on system performance.The specific contents are as follows:1.The error vector magnitude(EVM)and bit error rate(BER)performance of visible light communication system(VLC)will be deteriorated by the nonlinear characteristics of LED,so the influence of nonlinear distortion must be analyzed and studied.The principle of optical orthogonal optical frequency division multiplexing(O-OFDM)visible light communication system based on nonlinear LED is introduced,and the error rate expression of the system is derived.It is proposed to divide the LED nonlinear distortion into nonlinear distortion in the working area and clipping distortion outside the working area.For the clipping distortion,the subcarrier attenuation factor of load information is calculated according to Bussgang theory,and then the clipping noise variance superimposed on the subcarrier is obtained.For the nonlinear distortion in the working area,the equivalent is the driving signal plus the nonlinear distortion noise.The total average power of the nonlinear LED output deviates from the ideal linear signal minus the DC power as the variance of the noise.Monte Carlo simulations analyzed the effects of nonlinear distortion on constellation,EVM,and BER performance.The results show that: compared with the ideal case,the two kinds of LED nonlinear distortion have caused different degrees of degradation to the system performance.Under the same signal-tonoise ratio and DC bias,it can be found that two kinds of distortions lead to significant performance degradation.2.Aiming at the non-linear distortion in the working area of LED,a method of restraining the non-linear distortion of LED by equalizing the slope of LED linearization function in the receiving end of O-OFDM system is proposed.By introducing the slope equalization module into the performance analysis of the receiver of O-OFDM system,the bit to noise ratio and bit error rate of the O-OFDM system are obtained.The relationship between the total power and the slope of the nonlinear distortion noise and the functional relationship between the DC power and the slope are derived.By subtracting DC power from total noise power,the relationship between noise variance and slope on subcarrier is obtained,and the optimal slope value is derived to minimize noise variance.The results show that the optimal slope can minimize the variance of nonlinear distortion noise in the range of slope.When the SNR is more than 15 dB,the BER performance under the optimal slope is improved obviously. |
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