Linearization of RF Power Amplifiers: Chapter 3

Wideband Linearization

The Need for Advanced Digital Pre-Distortion (DPD)

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This knowledge series delves into the challenges and solutions related to linearising RF power amplifiers with wide instantaneous bandwidth. A large bandwidth is essential for modern communication systems, with 5G networks being the most notable example today. Because of this, the increasing bandwidth demands and complex modulation schemes introduce new constraints to ensure signal integrity and energy efficiency in amplifiers.

First chapter: We introduce modulated signals and their behaviour when passing through a power amplifier.
Second chapter: We explore the challenges of wide instantaneous bandwidth in RF systems.
This chapter: Finally, we discuss advanced linearization techniques, such as Digital Pre-Distortion (DPD), to handle bandwidths up to 600 MHz, highlighting our expertise with RFSoC platforms.Second chapter

In modern RF communication systems, the demand for high data rates continues to push the boundaries of instantaneous bandwidth (IBW) in power amplifiers (PAs) as seen in the previous chapter. As systems evolve from handling narrowband signals to increasingly wideband modulated signals, challenges arise in maintaining signal integrity, especially when aiming to linearize broad frequency ranges, such as 400 MHz or even 600 MHz. To meet these challenges, Digital Pre-Distortion (DPD) techniques must adapt accordingly, capable of linearizing ever-wider bandwidths while reducing signal distortion and maintaining efficiency.

This article delves into the challenges and solutions involved in linearizing wideband signals and the role of advanced DPD in enabling efficient amplification for high-bandwidth applications.

Introduction to Digital Pre-Distortion (DPD)

Digital Pre-Distortion (DPD) is a powerful technique used to correct the inherent nonlinearities of power amplifiers (PAs). By applying a pre-emptive « inverse distortion » to the input signal, DPD ensures that the output of the amplifier closely matches the desired linear response. This process helps maintain signal integrity even when amplifiers operate near their saturation point.

To illustrate, the schematic below shows how DPD works. The first block represents the transfer function of the DPD, which introduces an inverse distortion to the input signal. This pre-distorted signal is then passed through the power amplifier, represented by the second block. By combining the effects of these two transfer functions, the overall system achieves a linear output response, ensuring high signal fidelity and minimal distortion.

When dealing with modulated signals that span a large spectral bandwidth, non-linearities manifest as elevated power levels in adjacent channels. The role of DPD is to mitigate these distortions by reducing the power in these adjacent channels, ensuring compliance with spectral requirements, and preventing interference with other applications operating outside the intended bandwidth. This process is demonstrated in the video below, where successive DPD iterations progressively lower the adjacent channel power.

The Challenge of Linearizing Wideband Signals

To effectively linearize a wideband signal, the Digital Pre-Distortion (DPD) system must be capable of analyzing a frequency range that is typically at least three times the bandwidth of the signal. This requirement arises because the non-linearities introduced by the Power Amplifier (PA) generate out-of-band distortions, such as intermodulation products, which can significantly affect the Adjacent Channel Power Ratio (ACPR) and compliance with spectral mask requirements. By extending the analysis bandwidth, the DPD system can accurately measure and mitigate these distortions.

In traditional narrowband systems, linearizing a signal involves analyzing a relatively small portion of the spectrum. However, as the instantaneous bandwidth of PAs increases (see previous article [insert link]), maintaining linearity over the entire bandwidth becomes significantly more complex. Amplifying signals over 400 or even 600 MHz presents unique challenges due to the higher potential for distortions and the broader range of intermodulation products that need to be addressed.

Two main challenges arise when extending the analysis bandwidth for DPD:

Increased Memory Effect Complexity: Wideband signals require more sophisticated DPD models to account for the memory effects, which in turn demand efficient usage of FPGA resources. AMD’s DPD solution strikes a balance between model complexity and resource consumption, making it an effective choice for such scenarios.
Limitations of the Measurement Test Bench: The test bench must be capable of generating and analyzing wideband signals accurately. If the test bench introduces its own limitations, it becomes difficult to isolate and correct the PA’s non-linearities. This is where mastering the RFSoC solution becomes critical. By leveraging the advanced capabilities of RFSoC, we ensure that the test bench itself does not constrain the measurement process.

For example, to linearize a 400 MHz modulated signal, as illustrated below, the DPD system must analyze and process at least 1.2 GHz of bandwidth (x 3 the original signal, as mentioned before). This extended bandwidth enables the DPD algorithm to pre-distort the input signal accurately, thereby canceling out the PA’s non-linear behavior across the entire spectrum. The figure below demonstrates this process.

Advanced RFSoC Platforms for High-Bandwidth Measurement and Linearization

At Wupatec, we leverage advanced RFSoC (Radio Frequency System-on-Chip) technology to handle the large bandwidths required for modern Digital Pre-Distortion (DPD) applications. RFSoC platforms integrate high-speed ADCs (Analog-to-Digital Converters) and DACs (Digital-to-Analog Converters) directly into the chip, enabling real-time signal analysis and high-fidelity linearization across wideband signals.

Our approach focuses on direct hardware control, allowing us to fully exploit the RFSoC’s capabilities for wideband performance. By precisely managing the DACs and ADCs, we can efficiently analyze and linearize signals over extremely wide frequency ranges, making this technology highly suitable for demanding applications like 5G and beyond.

A concrete example of this is our use of AMD’s ZCU670 evaluation board, which is specifically designed for Digital Front End (DFE) applications. Featuring the latest generation of AMD’s RFSoC DFE FPGA, this platform integrates high-speed RF-DACs and RF-ADCs with impressive sampling rates. Additionally, the built-in DPD block allows for efficient, autonomous correction of nonlinearities in wideband signals, making it ideal for next-generation communication systems.

Below is a an image illustrating AMD’s ZCU670 RFSoC , which we use towards PA linearization. For more detailed technical information, please refer to our white paper on the subject.

Partner with Wupatec for Wideband Amplifier Solutions

As the RF industry continues to push the limits of instantaneous bandwidth, advanced DPD systems and high-performance measurement platforms are becoming more critical than ever. At Wupatec, we specialize in delivering solutions tailored to the demands of wideband RF systems, offering expertise in both hardware and software to ensure the highest levels of performance.

Whether you are working on satellite communication systems, 5G infrastructure, or radar applications, our RFSoC-based solutions can provide the real-time signal processing and control needed to linearize even the most challenging wideband signals.

Contact Wupatec today to discuss how we can help you achieve optimal performance in your wideband RF amplifier designs.