Study Of Nonlinearity, Behavioral Models And Digital Predistortion System For Broadband Power Amplifier

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 bandwitdth, which decreases the bit error rate at the receiver. Newer transmissionformats, such as wideband code division multiple access (WCDMA) ororthogonal 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 sidebands 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, main goal of our research is that implements digital predistortion system, including actual system and simulation 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 meanwhile, 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 model proposed, proposes a memory polynomial model algorithm with sparse delay taps and compare it with memory polynomial model with unit delay taps, derives algorithm of Volterra model based on Laguerre orthonomal function and verifies performance of this model. In the section of digital predistortion system and simulating implementation, analyzes and compares a few main linearization techniques, builds and simulates digital/RF predistortion and digital-baseband predistortion system in ADS circumstance.The study of PA is based on ADS and MATLAB software. The PA used in the thesis comes from freescale semiconductor Inc.The main tasks and contributions in this thesis can be summarized as follows1. Analyzes PA nonlinear characteristics and resulting distortion with ADS software, concludes that amplitude of intermodulation products will increase and output spectrum will be deteriorate when input power increases.2. Verifies existence of memory effects through theoretical analysis and simulation, proposes an idea reducing thermal memory effects, that is, temperature compensation method of predistorter, and simulate the idea and validate its validity.3. On basis of comparison of some PA behavioral models, proposes a kind of memory polynomial behavioral model with sparse delay taps and a kind of Volterra-series expansion model based on Laguerre orthonomal function, and verifies accuracy of these two kinds of models.4. Builds digital predistortion system in ADS circumstance and simulate results show its validity. Creates PA behavioral model with memory effects and simulate predistortion system with it, verifies adaptation of built digital predistortion system.According to above tasks and achievements, some technical papers have been published in relative conference and periodical.