Research On The Reduction Of PAPR Of OFDM In VLC System

In order to achieve high speed of data transmission, the visible light communication system usually use OFDM modulation mode. Inevitably, OFDM modulation brings high Peak-to-Average Power Ratio problems. Because of the nonlinearity of LED, the PAPR of the OFDM signal makes the performance of VLC system worse. This article discussed reducing the PAPR of OFDM signal and the ways to improve the nonlinearity of LED characteristics from two sides of the OFDM signal itself and the LED's nonlinearity. The main work of this paper is as follows:Firstly, this paper focus on the characteristics that the optical system uses intensity modulation/demodulation, discusses two ways to generate the VLC-OFDM signal:ACO-OFDM and DCO-OFDM. Then introduces the distribution characteristics of OFDM signal based on the traditional distribution of PAPR of OFDM signal. And modeling of the key components of LED. Then this paper models the nonlinear characteristics of LED. The simulation of the DCO-OFDM system shows that the DCO-OFDM signal with high PAPR will deteriorate the performance of system.Secondly, with the consideration of the DCO-OFDM signal, the paper applied direct clipping method to the DCO-OFDM VLC system. The direct clipping method is simple and easy to implement, and the effect of reducing the PAPR of DCO-OFDM signal is Significant. But the theoretical analysis and simulation shows that while this method reduce the PAPR effect is obvious, but it poses a serious clipping distortion, which can deteriorate the performance of the system. Then this paper introduce a simple method to mitigate the distortion of the clipped signal to improve the performance of the system.Thirdly, the method of companding and expansion is applied to the VLC communication system. The companding and expansion method has more application in the field of PAPR reduction of traditional OFDM signal. The paper improve the traditional exponential companding based on the distribution characteristics of DCO-OFDM signal The simulation shows that this methods can reduce the PAPR of DCO-OFDM signal, and has a very good performance.Finally, based on nonlinear characteristics of the LED nonlinearity, digital predistortion method is applied to linearize the characteristics of the LED. Mainly introduces the adaptive digital predistortion method, combining the lookup table. Then put forward the digital predistortion method based on look-up table. Simulation shows that predistortion can improve the BER performance of system effectively. Because of the fact that the transmitter should not be too complicated in practical application, then we can use the alternative way to simplify the lookup table digital predistortion.