Research Of Linearization Technology On Doherty Power Amplifiers

With the increasing demand for baseband bandwidth of wireless communication systems,the peak-to-average power ratio of the signal will also increase dramatically.Therefore,as the basis of wireless communication systesm,RF power amplifiers(PAs)have to ensure a high back-off area and high efficiency.In order to solve these problems,there have been heterogeneous techniques,envelope tracking and Doherty power amplifier structures and so on.Among them,Doherty is the mainstream of current power amplifier design method because it has simple structure,low cost,and it is easy to implement and improve.Digital pre-distortion technology,which is closely related to the quality of communication,has drawn more and more attention.The main content of this paper is study on the digital predistortion technology of Doherty power amplifier.In this paper,a dual-band Doherty power amplifier with a 1.8-2.6GHz operating frequency is chosen as the object of research.Firstly,the nonlinear characteristic is analyzed.Then the behavior model of PA and the digital pre-distortion algorithm are analyzed and discussed.At the same time,an improved model,weighted memory polynomial(WMP),which is based on memory polynomial(MP),is proposed.The model can be applied to the modeling of RF power amplifier(PA).Weighted memory polynomial model introduces weight function depends on the immediate power into the static and dynamic terms of memory polynomial model.The weight function used in this article is hyperbolic tangent function.At the same time,based on WMP model,this paper realised multi-step iterative WMP pre-distortion,which can obtain better linearity.In order to verify the validity and superiority of the multi-step iterative digital predistortion structure based on the WMP model proposed in this paper.We used the existing instruments to build a experimental platform and analyzed and calibrated the possible non-linear distortion of the platform.In this paper,20 MHz LTE(Long Term Evolution)signal is used to validate the predistortion and the amplifier model.The ACPR(Adjacent Channel Power Ratio)of the output signal is used as a measure of the linearity of the power amplifier standard.The experimental results are compared with the traditional memory polynomial model and the generalized memory polynomial model to evaluate the new model.The results show that the ACPR of the dual-band Doherty power amplifier used in this paper is-26.08dBc/-28.67 dBc,-28.88dBc/-28.87 dBc respectively at 1.9GHz and 2.6GHz without predistortion processing.After the digital pre-distortion processing,the ACPR of the PA can be improved at least 26.19 dBc,to achieve the desired target-52 dBc.