Adaptive Digital Predistortion Technology For RF Power Amplifiers

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 formats, such as wide band code division multiple access(WCDMA) or orthogonal frequency division multiplexing(OFDM), are especially vulnerable to the nonlinear distortions due to their high peak-to-average power ratios (PAPRs). This kind of input signal causes memory effects of power amplifier, which are with phenomenon of PA nonlinear characteristic variation and asymmetries in lower and upper side bands of inter-modulation products, which will also result in distortion. 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.In order to achieve high linear and reasonably efficient Power amplifier, we can use linearization techniques in general. Digital predistortion is one of the most effective ways among all linearization techniques. So the main goal of our research is that implements digital predistortion system. In order to study digital predistortion system and simulate it, we need to master and understand nonlinear characteristics and memory effects of power amplifier, and mean while, to build exact behavioral model of power amplifier. This thesis discusses and studies the following contents step by step, including PA nonlinear characteristic and distortion, PA memory effect and cancellation, PA behavioral model and digital predistortion system and simulating realization.In the section of PA nonlinear characteristic, introduces representation of nonlinear characteristic, nonlinear characteristic parameters, distortion products, nonlinear analytical methods, characteristic simulation, and so on. In the section of PA memory effects, analyzes identification of memory effects, reason causing memory effects, verifies existence of memory effects, and gives some methods canceling memory effects. In the section of PA behavioral model, analyzes and compares a few typical PA behavioral models. In the sectionof digital predistortion system and simulating implementation, analyzes and compares a few main linearization techniques, builds and simulates digital-base band predistortion system inMATLAB circumstance.The main tasks and contributions in this thesis can be summarized as follows1. Analyzes PA nonlinear characteristics and resulting distortion, concludes that amplitude of inter modulation products will increase and output spectrum will be deteriorate when input power increases. Verifies existence of memory effects through theoretical analysis. The principles of predistorter are described. The look-up-table based predistorters are discussed in detail.2. A predistortion method using look-up-table based on simplified Volterra series is proposed to linearize power amplifiers. The general Volterra model can hardly be utilized to design the higher order predistorters with memory because of its high computational complexity. In this paper, a predistortion method using look-up-table based on simplified Volterra series is developed. The general Volterra model is simplified according to the PA characteristics and then the predistorter is designed using look-up-table based on the simplified model. The indirect learning architechture is applied to implement the linearization system and the NLMS algorithm is utilized to identify the predistorter parameters. Therefore, the predistortion system can trace the variation of the PA characteristics effectively and the data storage space and computational complexity are also decreased. Finally, the performance of the proposed predistortion method is validated through computer simulations.