Digital Predistortion In WCDMA Power Amplifier Using Embedded Processor

Abstract

Power amplifiers are essential components in communication systems and are inherently nonlinear. The nonlinearity creates spectral growth (broadening) beyond the signal bandwidth, which interferes with adjacent channels. It also causes distortions within the signal bandwidth, which decreases the bit error rate at the receiver. Newer transmission format, such as wideband code division multiple access (WCDMA) is especially open to the nonlinear distortions due to their high peak-to-average power ratios (PAPRs). If we simply back-off the input signal to achieve the linearity required for the power amplifier, the power amplifier efficiency will be very low for high PAPR signals. Another choice is to linearize a nonlinear power amplifier so that overall we have a linear and reasonably efficient device. Digital predistortion is one of the most cost effective ways among all linearization techniques. However, most of the existing designs treat the power amplifier as a memoryless device. For wideband or high power applications, the power amplifier exhibits memory effects, for which memoryless predistorters can achieve only limited linearization performance. The memory polynomial predistorter can correct both the nonlinear distortions and the linear frequency response that may exist in the power amplifier. It is a robust predistorter, which has demonstrated good performance on several nonlinear system models. The predistorter models considered in this dissertation include both even- and odd order nonlinear terms. Here, the benefits to include even-order nonlinear terms in both the baseband power amplifier and predistorter models are described. By including these even-order nonlinear terms, a richer basis set us obtained, which offers appreciable improvement. The ideal performance of digital predistortion certainly relies on robust predistorters that can completely compensate for the nonlinearities of the power amplifier. Then there developed a method to construct compensators for the imbalance and dc offset. This compensation technique helps to correct for the analog imperfections, which in turn improve the overall predistortion performance.