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Digital Predistortion Linearization of Power Amplifier for X-band Radar System

机译：X波段雷达系统功率放大器的数字预失真线性化

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摘要

Modern communication systems with advanced modulation schemes have increased the linearity demands of power amplifiers. This thesis investigates the use of behavioral modeling of power amplifiers to perform digital predistortion on a GaN PA from Cree intended as the transmitter PA in an X-band MIMO-OFDM radar system currently in development by the Norwegian Defence Research Establishment (FFI). Predistortion is a technique where the non-linear distortion of the PA is ideally removed by introducing a unit with the inverse distortion characteristics of the PA. The cascade of the predistorter and the PA thus achieves a linear input/output relationship. By linearizing the PA there is potential for higher power outputs without suffering too much distortion which also leads to an efficiency benefit.In order to perform digital predistortion linearization, a number of behavioral models based on the Volterra series have been used. Behavioral models are mathematical models describing the relationship between the PA input and output. They require no knowledge about the physical structure of the PA, which is advantageous as this information rarely is available. Three models have been evaluated; the memoryless complex power series (CPS) model, the Wiener model and the memory polynomial (MP) model. The models are implemented in MATLAB and used to describe the behavior of two radio frequency power amplifiers with different operating frequencies, the first being a PA designed at NTNU based on the Cree 10W GaN transistor, and the second being the Cree CMPA5585025F GaN PA intended as the transmitter PA in the radar system. Characterization measurements are performed using a 16-QAM signal and by using the indirect learning architecture, the digital predistortion coefficients are calculated and applied to the input signal. Linearization is then performed on both a 16-QAM signal and an OFDM signal. The results show that the adjacent-channel-power ratio (ACPR) and the error-vector magnitude (EVM) of the 16-QAM signal can be reduced by $15$ dB and $5%$ respectively for the 10W PA at the 1-dB compression point, but only an ACPR reduction of $5$ dB was achieved for the OFDM signal. Linearization of the 25W PA only received an ACPR and EVM reduction of $6$ dB and $3%$ respectively for the 16-QAM signal, however the ACPR reduction for the OFDM signal surpassed this, achieving more than $10$ dB reduction. The reason for the poorer linearziation performance of the 25W PA was found to be large memory effects leading to less accurate behavioral modeling, even for models incorporating memory. For both of the PAs and signals the linearization performance was quickly reduced as the output power was increased. Thus for higher power outputs, there are less linearization benefits. The results lead to a conclusion that there are not very clear benefits of implementing digital predistortion in the radar system.
机译：具有高级调制方案的现代通信系统已经增加了功率放大器的线性要求。本文研究使用功率放大器的行为模型对Cree上的GaN PA进行数字预失真，该Cree打算由挪威国防研究机构（FFI）正在开发的X波段MIMO-OFDM雷达系统中用作发射机PA。预失真是一种通过引入具有PA反向失真特性的单元来理想地消除PA非线性失真的技术。因此，预失真器和功率放大器的级联实现了线性输入/输出关系。通过使功率放大器线性化，有可能获得更高的功率输出而不会遭受太大的失真，这也带来了效率优势。为了执行数字预失真线性化，已经使用了许多基于Volterra系列的行为模型。行为模型是描述PA输入和输出之间关系的数学模型。他们不需要有关PA物理结构的知识，这非常有用，因为该信息很少可用。评估了三个模型；无记忆复数幂系列（CPS）模型，维纳模型和记忆多项式（MP）模型。这些模型是在MATLAB中实现的，用于描述两个工作频率不同的射频功率放大器的性能，第一个是在NTNU上基于Cree 10W GaN晶体管设计的功率放大器，第二个是打算用于以下用途的Cree CMPA5585025F GaN PA：雷达系统中的发射机PA。使用16-QAM信号进行特性测量，并通过使用间接学习体系结构，计算数字预失真系数并将其应用于输入信号。然后对16-QAM信号和OFDM信号都执行线性化。结果表明，对于10W PA处的16-QAM信号，相邻信道功率比（ACPR）和误差矢量幅度（EVM）分别可以降低$ 15 $ dB和$ 5 ％$。 dB压缩点，但OFDM信号的ACPR仅降低了$ 5 $ dB。 25W PA的线性化仅使16-QAM信号的ACPR和EVM降低了$ 6 $ dB和$ 3 ％$，但是OFDM信号的ACPR降低超过了此值，实现了$ 10 $ dB的降低。发现25W PA的线性化性能较差的原因是较大的记忆效应，即使对于包含记忆的模型，也会导致行为建模的准确性降低。对于两个PA和信号，线性化性能随输出功率的增加而迅速降低。因此，对于更高的功率输出，线性化的好处较少。结果得出的结论是，在雷达系统中实施数字预失真并没有非常明显的好处。

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    Bache Magnus;

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  • 年度 2015
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  • 正文语种 eng
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