| TD-LTE system adopts OFDM and multi-carrier modulation technologies to improve the spectrumefficiency, which makes LTE signal have high peak-to-average power ratio (PAPR), non-constant envelopeand broadband characteristics. The amplified broadband signal through the power amplifier will generatevery serious nonlinear distortion. Meanwhile, in order to improve the output efficiency of the poweramplifier, Doherty technology is used in modern wireless communication systems. However, Dohertypower amplifier makes nonlinear distortion more seriously. So how to improve the power amplifierlinearity and maintain the efficiency at the same time becomes the key technology in TD-LTE system.This paper researches on the digital pre-distortion technique and the nonlinearity of Dohertyamplifier in TD-LTE system. Firstly, the paper analyzes the problem of high PAPR existing in the OFDMsystem to points out the necessity of power amplifier linearization. The nonlinear characteristics of poweramplifier are analyzed in theory. The memoryless and memory models of power amplifier are introduced,and the nonlinear effects on LTE signals are analyzed. Secondly, the paper researches on the principles ofadaptive pre-distortion and its key technology, and focuses on three methods of pre-distortion technology:Lookup Table model (LUT), Memory Polynomial model (MP) and Augmented Hammerstein model. Thelinear effect of three methods of pre-distortion is verified by simulation and analysis with ADS software.Then the paper focuses on two digital pre-distortion technologies based on LUT model and MemoryPolynomial model. For the LUT based pre-distortion, memoryless polynomial model is adopted to fitAM/AM and AM/PM characteristics of power amplifier. For the memory polynomial based pre-distortion,Memory Polynomial pre-distortion technique is implemented based on QRD_RLS adaptive algorithm. Inorder to implement adaptive algorithm on FPGA easily, CORDIC technology and systolic array areintroducted to reduce the complexity of QRD_RLS algorithm. A modified CORDIC algorithm is proposedfor solving the problem of resources and computing speed. It improves the speed of parameter extractionalgorithm of predistorter and reduces hardware consumption.For the accuracy of pre-distortion, the paper also focuses on the loop delay estimation algorithm ofpre-distortion, and proposes a segmented interpolation method to achieve delay estimation algorithm. Thesimulation results show that proposed algorithm can estimate the loop delay time correctly.Finally, with the Quartus II the two digital pre-distortion systems are implemented, and experimentalplatform based on FGPA for power amplifier linearization has been designed. With Doherty poweramplifier a pre-distortion test has been carried out for the designed pre-distortion system in this paper. Thetested power amplifier works at1.90GHz frequency, and its peak output power can reach48.8dBm. Thepaper uses LTE signal with different bandwidths for the test of two pre-distortion solutions. The final testresults show that: For narrowband LTE signal (5MHz), the ACPR of LUT and MP both improve about16dB. While for20MHz broadband LTE signal, MP models gains advantage over LUT models on itsmemory effect improvement. The ACPR by MP models reaches about-47dBc，while it is only-38dBc byLUT. The test results verify that pre-distortion system can improve linearity effectively... |
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