基于Landweber迭代算法的欠采样恢复数字预失真技术

传统宽带数字预失真(DPD)为了更好地矫正功率放大器(PA)非线性特性，通常要求反馈通道带宽达到发送信号带宽的5倍，相应地要求更高采样率的模数转换器(ADC)，这将导致数字预失真系统面临着硬件成本和能耗问题。针对这一问题，该文提出一种基于Landweber迭代算法的欠采样恢复(USR)数字预失真(Landweber-USR DPD)技术。这种以内外循环的方式进行处理，可将反馈通道带宽从理论要求的5倍降低至2倍，以良好的质量从欠采样的功放输出信号中恢复全频带的输出信号，使还原出的数据更接近真实的功放输出信号，以实现更好的预失真效果。实验选用基于单管氮化镓(GaN)器件的宽带F类功率放大器，在1.8 GHz工作频点下用5 MHz的长期演进(LTE)信号激励，反馈ADC速率分别设置为全采样速率(40 Msps)和欠采样速率(10 Msps)。实验结果充分证明了Landweber迭代算法恢复功放数据的可靠性以及Landweber-USR DPD技术的有效性，为宽带通信系统中数字预失真技术的工程实现提供了有效降低ADC采样率的思路和方法。

An under-Sampling Restoration Digital Predistortion Technique Based on Landweber Iteration Algorithm

In order to better correct the nonlinear characteristics of Power Amplifier (PA), conventional broadband Digital PreDistortion (DPD) requires usually the feedback channel bandwidth to be 5 times of the transmitting signal bandwidth, and Analog to Digital Converter (ADC) with higher sampling rate is required accordingly, which will lead to the hardware cost and energy consumption problems of digital predistortion system. To solve this problem, an Landweber iterative algorithm-based UnderSampled Restoration (USR) Digital PreDistortion method (Landweber-USR DPD) is propsed. It is processed in an internal and external loop, which can recover the full-band output signals from the undersampled PA output signals under the requirements that the sampling rate requirement of ADC is reduced from 5 times to twice. And the restored data is closer to the real PA output signals to achieve better pre-distortion effect. In experiment, a single-device Gallium Nitride (GaN) broadband class-F PA is excited by 5 MHz-LTE signal in 1.8 GHz frequency. The feedback ADC rate is set as full sampling rate (40 Msps) and undersampling rate (10 Msps), respectively. The experimental results fully demonstrate the reliability of Landweber iterative algorithm and the effectiveness of Landweber-USR DPD technology, which can provide an idea and method for effectively reducing ADC sampling rate in the engineering implementation of digital predistortion technology in broadband communication systems.