Compensation of Impairments in Millimeter-Wave/Sub-THz Power Amplifier and Frequency Multiplier Based Transmitters

Abstract

The ever-growing demand for increased network capacity and higher data rates has instigated, in recent years, a shift towards harnessing millimeter-wave (MMW) and sub-THz frequency bands, offering abundant and untapped spectrum resources. Nevertheless, transmitting wideband signals at these frequencies faces formidable challenges due to significant propagation path loss and the limitations of semiconductor devices. This doctoral thesis adopts a comprehensive approach to tackle these challenges by integrating advanced modeling and analysis methodologies with the development of innovative system-level architectures and leveraging advanced signal processing techniques and algorithms. To mitigate the significant path loss challenge inherent to MMW/sub-THz wireless links, beamforming arrays are commonly employed, where a high count of closely spaced antenna elements are used to achieve high effective radiated power. These arrays are deliberately operated within their optimal operational range to maximize radiated power while concurrently promoting environmentally sustainable communication links. Yet, the deployment of closely spaced antenna elements brings forth new challenges e.g., antenna mutual coupling, which precipitates a degradation in array linearity. This thesis introduces a seminal closed-form analysis elucidating the nuanced interplay between array nonlinearity and array beam steering. Building upon this theoretical foundation, a novel optimization algorithm is formulated to meticulously design tapering coefficients aimed at minimizing variation in the array nonlinearity, due to antenna mutual coupling, across a wide range of array beam steering angles. Comprehensive numerical and experimental validation are provided, substantiating the capacity of the proposed scheme to enhance the linearizability of beamforming arrays when beam steering. The thesis further addresses the challenge of implementing the transmitter observation receiver (TOR) in beamforming arrays needed for array calibration and linearization training. Traditionally, the TOR feedback mechanisms in single-antenna systems have been reliant on couplers situated at the output of power amplifiers (PAs). However, the integration of such couplers within MMW/sub-THz beamforming transmitters presents formidable technical hurdles. This thesis propounds a pioneering paradigm shift, advocating for the utilization of near-field (NF) probes intricately embedded within the array architecture itself. Different algorithms are presented in this thesis that enable harnessing the information received from these NF probes, enabling accurate estimation and compensation of the array's linear and nonlinear impairments. The proposed TOR and enabling algorithms, facilitate in-situ calibration and/or digital-predistortion training, thereby engendering a marked enhancement in system performance. Furthermore, the thesis proposed an active array calibration scheme that leverages the signals received from the NF probes to enable simultaneous calibration of the array without resorting to element-wise sequential calibration or a far-field receiver. Extensive experimental validation on two beamforming arrays operating at 28 GHz and 39 GHz are presented showcasing the capacity of the proposed schemes.

The generation of wideband vector-modulated signals in the MMW/sub-THz bands poses a formidable technical challenge, especially at frequencies where PA based transmitters have limited performance. This thesis endeavors to surmount this obstacle through the exploration of frequency multipliers (FXs), which are characterized by the capacity to generate signals that can surpass the transistor unity-power-gain frequency (fmax). The thesis presents linearization algorithms that can mitigate the inherent nonlinearity associated with FXs thereby enabling the generation of wideband vector-modulated signals with significantly increased bandwidths compared to existing methods. Furthermore, the thesis delves into the scalability of these linearization algorithms for FX-based array systems, thereby underscored by their inherent resilience to antenna mutual coupling vis-a-vis PA-based arrays. These algorithms and findings are validated in simulation and experimentally on different single channel FXs and FX-based arrays with different multiplication orders and operation frequencies. In tandem with these innovations, this thesis introduces a pioneering novel measurement system tailored for comprehensive testing of components and systems operating at sub-THz frequencies. Leveraging frequency extenders traditionally reserved for continuous-wave (CW) characterization, the proposed measurement system enables components and systems under both CW and modulated signal excitation conditions. The enabling algorithms used by the proposed measurement system are also developed and presented. Specifically, an iterative learning control linearization algorithm and a receiver signal stitching algorithm that enable the linearization and capture of wideband signals using narrowband receivers, are presented. The efficacy of the proposed measurement system and associated algorithms are rigorously substantiated through experimental validation. In summary, this thesis represents a seminal contribution toward mitigating hardware limitations endemic to MMW/sub-THz radio frequency systems. By espousing a holistic approach that seamlessly integrates advanced modeling and analysis techniques, novel system-level architectures, and state-of-the-art signal processing techniques, this work lays the groundwork for the development of future-proof, high-capacity, and high-data-rate radio systems poised to thrive within these transformative frequency bands.