Research Of Wideband Adaptive Predistortion Technology For Power Amplifiers

The power amplifier (PA) is an essential element in modern wireless communication systems. However, it is inherently a nonlinear component. The nonlinearity induces in-band and out-of-band error distortions that cause adjacent channel interference and degradation of bit-error-rate performance. Moreover, modern communication systems, such as wideband code-division multiple access (WCDMA) and world interoperability for microwave access (WiMAX), usually use spectrally efficient modulation schemes to transmit large quantities of information within a limited band. As a result, the signal envelop of the systems vary rapidly and have a large PAPR, accordingly aggravate the impact of strong nonlinear memory effect in transmitter. Therefore, PA linearization is often the necessary methods to reduce the bit error rate, suppress spectral regrowth and adjacent channel interference, and enhance the PA efficiency, also speed up the communication system design and verification process, which have the important theory value and broad application prospect.Among all linearization techniques, digital predistortion (DPD) is the most promising technique because of its high accuracy, energy-efficient operation, and flexibility. Therefore, it is widely employed in modern communication system. In this dissertation, the technology of behavioral modeling and adaptive digital predistortion of PA with nonlinear memory is studied in broadband communication systems. The main contents are as follows:1. The basic theory of nonlinear characteristics of power amplifier are researched, the major technical indexes evaluating nonlinear characteristics of the power amplifier are described in detail, and the familiar memoryless behavioral modeling and behavioral modeling with memory of PA are summarized. These lay an important theoretical basis for the adaptive digital predistortion technology research.2. The measurement and simulation setup used for model extraction and validation is established. The measurement results of nonlinear characteristics of PAs directly determine the precision of the extracted behavioral model. The measurement setup used for behavioral modeling extraction and verification under the broadband communication signal excitation is constructed from two aspects. One is the real-time test setting based on Agilent measure instrument, the other is the system simulation setting based in ADS2006. And the measure and simulation results of major characteristics of real PA are presented.3. This thesis presents the behavioral modeling of broadband PA for predistortion application using enhanced nonlinear auto-regressive moving average (NARMA) model. The parameter vector of the proposed model is updated using the common least squares (LS) algorithm and recursive least squares (RLS) algorithm respectively. The model has been validated using ADS2006 simulated and real measured data. The modeling performance is assessed in terms of in-band and out-of band errors, such as normalized mean square error and adjacent channel error power ratio, and it is compared with a conventional NARMA, a memory polynomial with respect to accuracy and computation complexities. The results show that, among the models studied, the enhanced NARMA model has the best tradeoff for accuracy versus complexity in mimicking the dynamic behaviors of a wideband PA.4. The thesis presents an efficient adaptive digital predistortion method compensating for nonlinear distortions caused by power amplifiers with memory in wideband communication system. The generalized nonlinear auto-regressive moving average (NARMA) model and the indirect learning architecture are employed to construct the adaptive predistortion system in this approach, and the parameters of the predistorter can be directly estimated from a real-time online system identification process by using the Gauss-Newton algorithm. The result of computer simulation shows that the generalized NARMA model predistorter can compensate both dynamic nonlinearity and memory of wideband power amplifier very well. Meanwhile, this method owns the advantage of converges fast and robust and suite for hardware implementation of digital circuit.