Research On Nonlinear Characteristics Of RF Power Amplifier And Pre-distortion Linearization In LTE System

The power amplifier (PA) is the principal source of non-linearity in communication system,non-linearity of PA directly affects the signal transmission quality, brings spectral re-growth,furthermore results in interfering with the adjacent channel. In order to obtain good linearity, themethod of power back-off is the most convenient way, but it caused low efficiency of PA as well asresource waste. The linearization technology is widely used to improve the linearity of PA atpresent. There are different linearization techniques, including feed-forward, LINC, thepre-distortion. Among them, the pre-distortion technique is the most promising method.Linearization techniques for Doherty RF power amplifiers in a Long Time Evolution (LTE)system has been mainly studied in this paper. In the first part of this thesis, the standard of LTE andthe prospects in the wireless communication are introduced. At the same time, the necessity of PAlinearization has been analyzed in the LTE system.In the second part, the PA and its non-linearity specification have been discussed in detail.There are P1dB, third-order intercept point and inter-modulation distortion for the PA specification.Adjacent Channel Power Ratio (ACPR) and Error Vector Magnitude (EVM) have been used toreflect the non-linearity measurement. Then, according to the non-linearity of PA exhibitingdifferent characteristics from the conventional narrowband signal with the increase of thebandwidth of the input signal, several nonlinear model of PA are given in detail in this thesis. Suchas memoryless models, which including Lookup table model (LUT) and memoryless polynomialmodel, memory model which including memory polynomial, Hammerstein, Wiener andaugmented Hammerstein models and so on. The methods of extracting parameters for LUT,memory polynomial and augmented Hammerstein models are explained at the end of this part. Atthe same time, the pre-distorters with different model have been built in Advanced Design System(ADS) and parameters of model are imported to obtain pre-distortion signal.In the third part, the design for experiment platform of digital pre-distortion has beenelaborated in detail, which includes the design of data acquisition board and FPGA developmentboard, connection of entire system hardware, digital down-conversion and VHDL code for model training, the communication between the proposed platform and host computer has been achieved,the calculation of pre-distortion signal and connection between Digital Analogy Converter (DAC)and mixer.Finally, linearization experiments for the PA have been completed based on the test platform,and linearity ability of different models has been compared. With the20MHz LTE signal as inputsignal and a Doherty PA as the test PA, the results of experiment demonstrated that ACPR has beenimproved26dB by polynomial model and24dB by augmented Hammerstein model.