Application Of Digital Predistortion Technology In WCDMA Repeater

WCDMA is one of the 3G standards, for which a large-scale commercial market in the world has been formed. However, it has multi-level signals which have high peak-to-average power ratio, thus the linearity techniques for power amplifier (PA) in both base station and repeater of this system are still attracted great attention. Currently, many methods of linearity have been proposed both in industry and academia, such as feedforward, negative feedback, and predistortion. Among these methods, adaptive predistortion, especially the digital baseband predistortion (DPD), embraces favorable bandwidth feature and efficiency. The reason lies in the fact that DPD avoids the complicated radio frequency signal processing, while only deals with the baseband signals at low frequency to facilitate the hardware implementation. Thus, this thesis will focus on the digital baseband predistortion technique.First, this thesis describes the nonlinear feature of PA, builds models of PA, and analyzes the effects of the nonlinearity upon the signal transmission, which justify the research necessity of DPD. Predistortion technique includes analogy predistortion and digital predistortion, where the latter has a better performance and portability.Second, we discuss the theory of digital predistortion, and present two DPD algorithms, i.e., Hammerstein DPD and polynomial DPD with memory. The latter is investigated in detail, which achieves better balance between complexity and performance.Third, the simulations of the polynomial DPD with memory in WCDMA system are presented, including both the float-point and fixed-point simulations. Therein, the fixed-point simulations include the results with various quantization bits. It is shown that DPD algorithms enjoy high efficiency and strong adaptability, which improve the Power Spectral Density (PSD) metric for 10dB, and reduces the system Error Vector Magnitude (EVM) efficiently.Fourth, this thesis introduces the digital front end (DFE) of soft radio. With the application of DPD technique in DFE, a universal DPD design framework is derived, which is very practical and easy for promotion. Furthermore, a hardware system with the Xilinx FPGA is designed for the implementation and test of DPD. It is discovered from the test results that DPD technique improves PSD metric for 10dB, which means an excellent linearity performance and can satisfy the linearity requirement of multi-carrier WCDMA system.