Research On Predistorter In Wideband Wireless Communication System

Orthogonal frequency division multiplexing (OFDM) with both high spectrum efficiency and robustness to multi-path fading is widely used in wideband wireless communication systems. However, its high Peak-to-Average Power Ratio (PAPR) will result in out-of-band spectral regrowth and in-band distortion when OFDM signal passes through a power amplifier with nonlinear range. Thus, it is significant to further conduct research on the nonlinear character of amplifiers and implement the linearization technology in more ways. In addition, the technology of digital predistorter is very promising for its superiority in accuracy, complexity and adaptability. This dissertation focuses on the digital predistorter in OFDM systems. The main contains are as follows:1. Introduce the main HPA models and the AM-AM and AM-PM characteristics of them.2. Introduce the structure of conventional polynomial predistorter, including direct learning structure and indirect learning structure. Simulation shows the advantage and disadvantage of each structure. The basic principles of LS algorithm ,LMS algorithm ,NLMS algorithm and GNGD algorithm are described.Simulation shows the result of comparision of each algorithm in complexity, convergence rate and convergence value. GNGD algorithm is improved for the purpose of noise control. Simulation shows that comparing with the traditional GNGD algorithm , convergence rate of the new algorithm increase one-third times, and convergence value decrease 1dB.3. For the surge of the condition number as the coefficient matrix dimension increases, orthogonal polynomial predistorter model is researched, which decrease the condition number greatly.4. For the requirement of increasing sampling rate because of the increasing of band wide,the method is researched on the base of subsampling in feedback loop. Improving the present zero-padding,simulation shows that improved method represent better than zero-padding.