Digital Predistortion Principle Based on RS Spectrum and Signal Analyzer

Although the Doherty power amplifier is known for its high efficiency, it typically suffers from poor linearity, which necessitates the use of digital predistortion (DPD) techniques to enhance its performance. As communication systems continue to evolve toward higher data rates and broader bandwidths, the operating bandwidth of Doherty amplifiers has also expanded significantly. To effectively evaluate and improve the linearization performance of these broadband Doherty amplifiers, it is essential to establish a robust and flexible digital predistortion platform. In this study, we employed a broadband digital predistortion system based on the R&S signal and spectrum analyzer (FSW26) to linearize our laboratory's broadband Doherty power amplifier. Experimental results demonstrate that with digital predistortion, the Doherty amplifier achieves improved linearity across various operational modes, including single-tone, dual-tone, and multi-tone scenarios.

With the increasing demand for faster communication speeds, signal bandwidths have become wider, particularly with the widespread adoption of carrier aggregation technology. This advancement has led to a significant increase in the bandwidth requirements for transmitters. To meet these challenges, a more cost-effective solution is to develop concurrent multi-frequency transmitters that can handle both traditional single-carrier signals and multi-carrier aggregated signals simultaneously. Consequently, the bandwidth of the transmission channel has also increased substantially. Among the key components of the transmit chain, the power amplifier is often the limiting factor in terms of bandwidth capability. The Doherty power amplifier, despite its superior efficiency compared to traditional class AB amplifiers, typically has a relatively narrow bandwidth—usually around 10%. Therefore, designing broadband Doherty amplifiers has become a major focus in recent power amplifier research to support modern communication systems.

Due to its inherent nonlinearity, the Doherty power amplifier usually requires digital predistortion (DPD) to be used in base station applications. Hence, besides efficiency and bandwidth, the effectiveness of linearization through DPD is a critical concern in the industry. Once a Doherty amplifier design is completed, it is crucial to test the linearity performance of the DPD system to ensure the overall performance of the amplifier meets the required specifications.

Digital predistortion is a technique designed to counteract the nonlinear behavior of power amplifiers. Normally, power amplifiers operate most efficiently when saturated. For example, in a Doherty amplifier, the main power amplifier operates in saturation at the back-off power level, while both the main and auxiliary amplifiers are in saturation at peak power. This allows the Doherty amplifier to achieve high efficiency at both low and high power levels. However, when an amplifier is in saturation, it introduces nonlinear distortion, causing the output signal to spread out in bandwidth beyond the original input signal.

Figure 1: Illustration of the digital predistortion principle

To mitigate this distortion, digital predistortion involves pre-processing the input signal in the digital domain. This process essentially adds components that are equal in magnitude but opposite in phase to the distortion introduced by the power amplifier. By doing so, the distortion generated by the amplifier is canceled out, resulting in a more linear output. Figure 1 illustrates this concept from a gain perspective. It shows that as the input signal increases, the gain of the amplifier decreases, while the predistortion module compensates by increasing the gain. When these two effects are combined, the overall gain becomes flat, leading to a more linear response between the input and output signals.

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