Research On Digital Predistortion Technology Of Power Amplifier In Wireless Communication

With the rapid development of modern mobile communication technology, the transmission channel and spectrum resources of the wireless communication become tense increasingly. At present, the communication system is developing towards broader bandwidth,multiple operating frequencies and lower power consumption. Wireless communication system is the focus of industrial energy saving, and the power consumption of microwave integrated device accounts for more than 60% of the primary device in base station. The performance of the core device RF power amplifier (RF PA) in wireless communication will affect the communication quality of the entire system directly. At present,the complex signals commonly used in communication systems have characteristics of non-constant envelope and high PAPR(Peak-to-Average Power Ratio, PAPR), which requires high linearity of the amplifier. With its programming flexibility, moderate cost and superior performance, digital predistortion (DPD) becomes an important linearization method in wireless communication transmitter technology.In view of the existing research situation at domestic and foreign,this paper conducts intensive research on the broadband and dual-band DPD modeling technology, how PA achieves high efficiency and high linearity excited by the signal with non-constant envelope and high PAPR,as well as the sampling rate reducing method in the feedback loop of broadband power amplifier predistortion system. The effective combination has been implemented of the crest factor reduction (CFR)and the broadband/dual-band predistortion systems. The algorithm based on the band-limited Volterra series has been optimized in broadband DPD system. The low sampling rate scheme based on compression sensing (CS)has been implemented in the feedback loop of broadband predistorted.The main work and innovation of this paper are described as follows:1, Based on the research of the intrinsic relationship between CFR and DPD, a combined structure of CFR-DPD-CM is proposed according to the reciprocal characteristics between them. The compensation module(CM) can be used to compensate for the clipping signal and the predistortion signal. The proposed structure can improve the transmission quality on the premise of ensuring linearity. When the 20MHz WCDMA 1001 signal with the PAPR value of 10.12dB is used to verify the F class amplifier, the signal PAPR is reduced by 3.27dB, the EVM is 2.98%lower than that without the predistortion and the ACPR value is below-53dBc.2, The 1D-CFR technique applied to the 2D-DPD has the shortcomings of under-reduction and over-reduction. In order to overcome the shortcomings, the method of combining 2D-CFR and 2D-DPD is proposed in this paper. The threshold is set according to the sum of the two band signals, and the comparison process of the instantaneous difference of the two bands is introduced. The proposed method is simulated and tested on the class F and Doherty amplifiers at center frequencies of 2.65GHz and 3.45GHz respectively. The drain efficiency of the amplifier is improved from 43.5% in the case of the original 1D-CFR to 46.3%, and the EVM performance is 1.47% and 1.53%lower than that of the traditional 2D-CFR technology at 2.61 GHz and 2.69GHz respectively. The ACPR is approximately -57 dBc at the 10 MHz frequency offset of the two bands. The test of Doherty power amplifier also get good effect.3, The implementation of DPD technology in broadband communication system is limited by the sampling rate of analog to digital converter (ADC) in feedback loop. And the traditional band-limited technique can not compensate the distortion outside the passband of the filter. A predictive compensation model algorithm based on band-limited Volterra series is proposed in this paper. The proposed method introduces the full-band model extraction and the coefficient iteration in the traditional band-limited DPD structure on the basis of Gauss-Seidel algorithm. The overall linearity of DPD system and the accuracy of the model can be improved meanwhile reducing the sampling rate of feedback loop. Through the simulation and the actual test, the performances of the traditional method and the proposed method are analyzed and the superiority of the proposed method is verified.4, Frequency-domain Random Demodulation (FRD) and broadband DPD combined technology based on CS theory is proposed. Applying the sparse characteristics of the transmitted signal, the method does not process the all N-dimensional signals but uses a M-dimensional observation matrix that is much smaller than N for linear measurement during sampling. It overcomes the shortcomings of the conventional ADC hardware sampling rate in the broadband power amplifier linearization system to achieve the practical application requirements. The proposed method is verified by simulation and test. When the input signal bandwidth is 40MHz, the linearization effect can be obtained with 20MHz low sampling rate. ACPR is improved by 21dB without no predistortion.