A Research On Nonlinear Identification Of Power Amplifier

In order to meet the explosive growth of data rates in wireless communications,the signal bandwidth supported by future 5G base stations will reach hundreds of MHz.For broadband zero-IF transmitters,I/Q imbalance and power amplifier nonlinearity are the two main sources of distortion that affect its transmission performance.Through the existing testing equipments in the laboratory,the writer built an experimental platform that can be used for test the power amplifier with broadband signals.And then based on this platform,studied the digital predistortion technology related to I/Q imbalance and power amplifier nonlinearity in the transmitter.In this paper,the writer will introduce his work including experiments,methods and arguments in detail mainly from four parts as follows:Firstly,by dividing the test platform into two part,channel and power amplifier,the structure of every part of the system is stydied,and the distortion and its causation in the test plotform is clarified.The result indicates that the I/Q imbalance of signal source and the nonlinearity of power amplifier are the most important part of the distortion in the platform.And then the three parts of the paper content of I/Q imbalance,PA nonlinearity and distortion joint correction are determined.Secondly,for I/Q imbalance,the limitations of the I/Q correction method of time domain filtering are analyzed,and then an I/Q compensation method of frequency domain interception is proposed,which can be used for any bandwidth signal at a specific sampling rate.In addition,with an in-depth analysis of calibration results from frequency domain,the reasons for the gradual degradation of the I/Q calibration effect as the bandwidth increases are analyzed.Thirdly,Regarding the nonlinearity of the power amplifier,the characteristics of the respective models of the polynomial class and the twin-box class are analyzed,and then an extended memory polynomial twin-box(TB-EMP)model is proposed.On the one hand,the proposed model adopts a twin-box structure,then it has fewer coefficients;on the other hand,the dynamic nonlinearity module of the model considers the influence of the signal and the signal envelope cross term,hence it can pursuit a high modeling accuracy.The actual measurement results show that for a single carrier signal with 80 MHz bandwidth,the TB-EMP model proposed in this paper is 3d B better than the MP model for NMSE and ACPR improved by 1d B;compared with the GMP model and the EH model,the number of coefficients is reduced by 69% and 35%,respectively while the predistortion performance is not greatly degraded.Finally,in order to jointly correct the two distortions of I/Q imbalance and power amplifier nonlinearity,a joint extended memory polynomial twin-box model is derived.Experiments show that comparied with the method of first correcting I/Q imbalance and then performing predistortion,the joint model has a comparable linearization capability.