Design Of Digital Predistortion Using Orthogonal Polynomials

With the rapid development of modern wireless communication, highbandwidth and high PAPR (peak-to-average ratio) signal communication system results in more and more attention focus on Power Amplifier(PA)Linearization Techniques. In this paper, a Digital Pre-distortion(DPD) using OrthogonalPolynomials is proposed, which is based on PC,DSP and FPGA. In this design, PC is used to extract the algorithmmodelparameters, DSP is used to send data, FPGA is used to implement DPD structure.Firstly, based on Memory Polynomial(MP) and OrthogonalPolynomials (OP)model, Simplified Generalized OrthogonalPolynomials (SGOP) model is proposed which has more wide applicability. The Analysis on model parameters shows that OrthogonalPolynomials and Simplified Generalized OrthogonalPolynomialsmodel has better convergence and smaller dynamic range.Secondly, OP model and SGOP model are implemented on FPGA, whosememory depth is 2 and nonlinear order is 7.DPD module is based on Look-Up Table(LUT), in which OP model has 3 LUTs and SGOP has 9 LUTs. The LUTs reduce the FPGA implementation complexity and computational effectively.Finally, DPD test is carried out on a 2.55GHz high-efficiency class-J PA withoutput powerof over 40dBm driven by aLTE-Advanced signal with bandwidth of 100MHz and PAPR of7.5dB. The results show that after 3 times power calibrations Adjacent Channel Leakage Ratio (ACLR) of MP,OP and SGOP model improve to -46dBc,-47dBc and -48dBc respectively. The ACLR improves 11dB with MP model, whosenonlinear order is 7 and memory depth is 2. While the ACLR reachesl2dB improvement with OP model and 13dB improvement with SGOP.For the verification of a DohertyPower Amplifier with 6 times power calibration, there are only 14dB improvement of ACLR with MP model and OP model.However, the ACLR improves more than 20dB with SGOP model.Beside, the validation of DPD with differentmemory depth and nonlinear order has beencarried out. For the high-efficiency class-J Power Amplifier, there are notobvious differencesbetween MP, OP and SGOP model which has differentmemory depth and nonlinear order. OrthogonalPolynomials (OP)model has simple structure in FPGA design compare with Simplified Generalized OrthogonalPolynomials (SGOP) model. For the broadband DohertyPower Amplifier, the pre-distortion performance of SGOP model is much better than the MP and OP model. When the memory depth is greater than 2, the nonlinear order is greater than 7, the ACLR of power amplifier improvement has become stable.