Research Of Digital Predistortion For Dualband Envelope Tracking Power Amplifier

With the rapid development of wireless communication technology,more and more transmitter structures appeared.With the characteristics of high frequency efficiency and simple structure,rf multi-band transmitter has been widely used.In a two-band system,the power amplifier of the nonlinear device will cause the input spectrum extension,which will make the signals of the two frequency bands interact with each other,resulting in a new nonlinear distortion different from that of the single-band system.The fifth generation wireless communication systems will provide high data rates up to 1gigabit per second(Gb/s).This requires transceivers to support multi-band and multi-standards with improved spectral efficiency,and data throughput,like OFDM used in LTE-A.However,this kind of complex modulation will bring high peak average ratio(PAPR),which will have a great impact on the efficiency and linearity of RF power amplifier.In this thesis,the envelope tracking power amplifier is used to improve the efficiency of dual-band transmitter and digital predistortion is used to improve the linearity of dual-band transmitter.The main work and innovations of this thesis are as follows:1.This thesis expounds the advantages of Envelope Tracking(ET)structure over other structures that improve the efficiency of power amplifier,as well as some disadvantages of its own.Then,the power modulator applied to ET structure is studied emphatically and the characteristics of various power modulator structures are analyzed.After comparison,the multilevel switching structure is finally selected,and improved on the basis of the idea of DAC.2.Considering that the digital control system is easier to control the delay than the analog control system,and has a simple structure and can be combined with digital pre-distortion,so FPGA is used as the control system.The generation of envelope signal and the expansion of signal bandwidth are realized in FPGA.In this thesis,the slew-rate limited algorithm is used to reduce the envelope bandwidth,and the switch switching rate of multilevel structure is reduced.Finally,the control signal to control multilevel switching is generated in FPGA.3.In this thesis,Principal component Analysis(PCA)algorithm is used to reduce the number of model coefficients of digital predistortion,thus reducing the calculation amount and complexity of digital predistortion.According to the characteristics of double-band digital predistortion system,the structure of double-band digital predistortion is analyzed.On the basis of the existing double-band digital predistortion technology,the dimension reduction is carried out by PCA algorithm.Through the experimental test,without the iterative algorithm,the ACPR in the case of single band and double band excitation is reduced by 18 dBc to-52 dBc,which is far better than the requirements of the communication protocol.At the same time,it also proves that PCA algorithm can reduce the coefficient by more than 20% on the premise of maintaining the same performance,but not too much,otherwise it will affect the performance of DPD.4.The amplitude of the envelopment signal changes dynamically,which will bring new nonlinear distortion to the double-band transmitter.In this thesis,the amplitude of the envelopment signal is introduced into the digital pre-distortion module as a parameter to compensate the nonlinear distortion.Through experimental tests,without iteration,3D-DPD can make the single-channel output signal reach-51dBc.