| In order to obtain greater output signal power,the power amplifier generally works in a range close to the saturation point,and meanwhile produces nonlinear distortion.Predistortion is an effective technique for compensating the nonlinear distortion of power amplifiers.Predistortion techniques can be divided into digital predistortion and analog predistortion.They are usually implemented separately in the digital domain and the analog domain.In digital predistortion,the mean square error of the baseband digital signal is the main design goal of the predistorter,but it will optimize some distortions that can be directly filtered out,so there is a waste of overhead.In the analog predistortion,the out-of-band leakage and intermodulation distortion of the analog signal are the key metrics to be optimized.However,there are problems such as difficulty in implementation,low precision,and poor real-time performance.According to the different characteristics of digital/analog predistortion,this paper proposes a joint optimization architecture for digital predistortion based on a hybrid digital-analog performance metric to simultaneously optimize the linearity performance in the above two different fields.To make joint optimization possible,the analog characteristics of the signal need to be modeled at the baseband.This paper defines a new performance metric to represent the out-of-band intermodulation distortion of the signal,and its explicit expression can be derived through the mathematical polynomial model of the power amplifier and the predistorter.Based on this expression,this paper further constructs a new digital predistortion optimization objective based on baseband-radio frequency joint optimization.For the new joint optimization objective,the classical recursive least-squares algorithm is modified,so that the baseband mean square error and the analog intermodulation distortion metrics can convergesimultaneously without increasing the complexity.The simulation results show that,for the two-tone signal,the proposed digital predistortion can reduce the third-order intermodulation distortion by 8.6 d B compared with the traditional scheme.In addition,the linearization performance of the proposed digital predistortion is further verified in actual power amplifier links and communication scenarios.A measured link platform is built with test instruments and power amplifiers,and the solution is applied to actual RF.The corresponding performance metrics are obtained through the link platform.Then the proposed scheme is tested in the real communication link based on the Open Air Interface open source code,which proves that the scheme can eliminate the nonlinear distortion of the power amplifier and improve the system linearity performance in practice. |
