Digital Pre-distortion Function Of The Software Radio Platform Implementions

With the widespread use of the 3rd generation telecommunication technology and the 4th generation telecommunication technology, the demand for the bandwidth of the wireless communication network is increasing. Although the emergence of modulation techniques such as quadrature phase shift keying(QPSK), quadrature amplitude modulation(QAM) and orthogonal frequency division multiplexing(OFDM) makes the spectrum efficiency improved. However, the output signal of these techniques is greatly influenced by the nonlinear characteristics of power amplifier. The output signal through power amplifier leads to serious distortion in-band and out-of-band. So power amplifier linearization technique is particularly important. Among all the linearization technique, digital predistortion technology is widely studied with its better linearization performance, stronger adaptive capacity, higher stability, more flexible design and easier implementation. Then a kind of implementation scheme is proposed to improve the linearization performance of power amplifier based on the digital predistortion technology. The main contributions of this thesis are summarized as follows:Firstly, an overall design scheme of digital predistortion system is proposed based on the principle of digital predistortion. The aim of this scheme is that designing baseband predistortion device avoids the whole uplink signals affected by the nonlinear characteristics of power amplifier. Through theoretical analysis and simulation verification, indirect learning structure of adaptive digital predistortion learning structure which is easy engineering implementation is adopted. The least mean squares algorithm with low computational complexity, low resource consumption and easy implementation is suitable, and the step length factor of least mean squares algorithm is 0.125. Compairing the characteristic of different predistortion model, it is obviously that memory polynomial model is more proper, furtherly, the nonlinear order and memory depth of polynomial model is 3 to 5 respectively.Secondly, this thesis realizes the digital predistortion system of Field Programmable Gate Array(FPGA) based on the SystemGenerator tools. The implementation is divided into signal source module, digital predistorter module, data buffer module, digital predistortion training module and the digital frequency conversion module, furtherly, the the workflow and internal structure are introduced in detail. Adaptive algorithm implementation of digital predistortion system adopts MCode module of System Generator to develop the system efficiently, further more, the deficiency and corresponding solution of MCode module is given. Additionally, this thesis proves the rationality of digital predistortion system through the analysis of implementation resource utilization.Thirdly, the digital predistortion system function tests and performance analysis are completed based the software radio platform. Through the above-mentioned overall design scheme and FPGA implementation of digital predistortion system verify the digital predistortion system performance under different test environment on the strength of software radio platform. Compared the MATLAB/Simulink simulation with implementation system test prove that each module function is correct and work normally after cascade. The tests show that the nonlinear characteristics of output signal through joint digital predistortion device and Doherty power amplifier have obvious improvement, the adjacent power leakage ratios are reduced from-23.59 dB and-22.75 dB to-46.71 dB and-45.72 dB, which meets the requirements of the 3rd Generation Partnership Project(3GPP). So the digital predistortion system implementation of this thesis effectively restrains the nonlinear characteristics of the power amplifier.