Inhibit The Study Of Nonlinear Distortion. Ofdm System

Mobile communication has experienced the first generation (1G) using analog technology of the voice system and the second generation (2G) adopting digital system. Now we have entered the third generation (3G) mobile communication which can also support the voice and data services. The development and evolution issues of the next generation mobile system become the focus of the current research. The fourth generation mobile system (4G) will have the features of higher speed and more capacity.The Orthogonal Frequency Division Multiplexing (OFDM) is an attractive technique for multi-carrier parallel transmission scheme due to its high bandwidth efficiency and good capability to resist multi-path interference, so it has been widely used in several areas such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting and Wireless Networks (WLAN), etc. However, there exists a major weakness in the OFDM technology, that is, there exists higher Peak-to-Average Power Ratio (PAPR) when the signal is transmitted. Although the probability of peak power is inferior, in order to transfer the OFDM signal with no distortion, it makes a high request for the linearity of high-power amplifier (HPA) in transmitter. In addition, the nonlinear characteristics of the amplifier will also make the nonlinear distortions, serious signal spectrum and signal distortion within the system; therefore, it is necessary to compensate the nonlinearities introduced by the amplifier.Therefore, this thesis focuses on how to effectively reduce the OFDM signal PAPR value and improve the power amplifier nonlinear distortions. This article details the basic principle and key techniques, analyzes the cause of high PAPR of the OFDM and the amplifier model. On the basis of theoretic analysis, two different PAPR reduction structures are proposed, which are digital baseband signal processing including clipping technique, coding technique,scrambling technique and the nonlinearity distortions mitigating including back-off, feed-forward, feedback and pre-distortion. Research shows that clipping technique algorithm aims directly at the PAPR reduction of OFDM signal, while Amplitude and Phase Predistortion algorithm puts its emphasis on the power amplifier linearization. These two kinds of technology are complementary, so the combination of these two techniques is also studied and some useful conclusions are achieved through the analysis and simulation.