Research On Peak-To-Average Power Ratio Reduction In VLC-OFDM System

In the recent years, visible light communication (VLC) transmitting data by light emitting diodes (LEDs) has attracted considerable attention due to its beneficial attributes such as the convergenceof illumination and communication, harmlessness to human bodies, high communication security and high data rates. Orthogonal frequency division multiplexing (OFDM) is one of the key technologies in broadband wireless communication. OFDM allows equalization to be performed with single-tap equalizers in the frequency domain, which reduces design complexity and equalization cost. Besides, it can achieve high spectral Efficiency and be efficiently implemented by using fast Fourier transform (FFT). These advantages results in the application of OFDM to VLC systems.Despite many merits of OFDM, it inevitably brings the drawback of high peak-to-average power ratio (PAPR), leading to nonlinear distortion caused by electronic devices. Hence it is urgent to reduce PAPR in OFDM systems. Since the VLC signal is real and positive, conventional PAPR reduction methods are not well suited for VLC systems and should be modified accordingly to support transmission in VLC channels. The aim of this dissertation is to develop efficient PAPR reduction methods for VLC-OFDM systems based on the statistical properties of PAPR and its effects on system performance. Specific work of this dissertation is as follows.Firstly, this thesis derives the closed-form expression for the complementary cumulative distribution function (CCDF) of PAPR of the Nyquist sampling signals and continuous signals in the two basic optical OFDM systems:DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM). The comparison showed that the PAPR problem is more serious in ACO-OFDM system than that in DCO-OFDM system. Besides, this thesis establish the nonlinear model of the LED based on the measured data, which reflect the necessity of PAPR reduction.Secondly, this thesis studies a time-domain clipping technique for PAPR reduction of OFDM signals. Combined with the characteristics of ACO-OFDM system, a simple and easy to realized clipping method is applied to the ACO-OFDM system and its impact on the system performance is analyzed. In addition, in order to effectively eliminate the clipping noise, the DAR method is also extended to the ACO-OFDM system. Through the combination of two ways can effectively reduce the PAPR and improve the BER performance of the system under the influence of LED.Thirdly, the dissertation studies a time-domain companding technique for PAPR reduction of OFDM signals. On the analysis and comparison of the existing companding transforms and their problems, a recoverable multistage linear companding(RMLC) transform novel companding technique is proposed, which can significantly reduce the PAPR of the ACO-OFDM signal. The proposed method makes full use of the specific structure of ACO-OFDM signals in the time domain, where half of the positions are forced to zeros within an OFDM symbol. The zero-valued positions are utilized to carry the companding factor, which makes it possible to recover the transmit signal by using MAP detection. Simulation results show that the proposed RMLC method achieves a significant PAPR reduction, while maintaining a competitive bit-error rate performance compared with the conventional schemes.Finally, two of the probability-based PAPR reduced method are researched. They are SLM method and PTS method. The probability-based methods usually need high computation complexity. Considering the Hermitian symmetry of the ACO-OFDM signal in the frequency domain, a novel implemented structure is designed. The proposed structure can greatly reduced the computaional complexity of IFFT in SLM and PTS by selecting part of the effective frequency signal first. Simulation results show that the improved structure not only can reduce computational complexity, but also maintain the PAPR reduction performance.