Now showing 1 - 8 of 8
  • Publication
    Low-Complexity Digital Predistortion for 5G Massive MIMO and Handset Transmitters
    (University College Dublin. School of Electrical and Electronic Engineering, 2022)
    The demand for new wireless communication systems to support high mobility and low latency necessitates a rethink of the architecture of wireless communication systems as well as the design of their key components. This thesis presents several novel techniques to solve the major challenges in digital predistortion (DPD) for millimeter wave multi-input multi-output (MIMO) and handset transmitters to lower the hardware cost and computational complexity of the fifth generation (5G) communication systems. The first part of the thesis focuses on the architecture of the MIMO DPD solution for 5G transmitters. To extract DPD model coefficients, a feedback data acquisition path is required. In conventional single-input single-output (SISO) systems, the output is usually acquired directly from the power amplifier (PA) with a coupler. In massive MIMO systems, the number of RF chains is large. Using dedicated feedback paths for each PA separately is not feasible. To lower the hardware cost, a novel data acquisition scheme is proposed to obtain the output signals in far field over the air (OTA) using a single antenna and feedback loop, and then reconstruct the output of each PA. Simulation and experimental results demonstrate that the proposed OTA data acquisition can accurately reconstruct the output of each PA in the MIMO systems and the DPD solutions derived from the reconstructed data can successfully linearize the nonlinear MIMO transmitters. In the multi-user scenario, the nonlinearity of the transmitters varies with the movement of user equipments (UEs), and the DPD model coefficients need to be updated accordingly. To meet the requirement of high mobility, the complexity of the system update must be low. In the second part of the thesis, we present a new DPD system, where DPD model can be updated fast and accurately without capturing PA output or applying costly model extraction algorithms. In the proposed method, nonlinear characteristics of the PA are encoded into low-dimensional PA features using feature extraction algorithms. To identify DPD model coefficients, PA features are extracted first and the DPD model coefficients are then generated directly by DPD generator with PA features. Experimental results show that the proposed DPD solution can linearize PA with very low complexity compared to that using the conventional solutions. Finally, the focus shifts to handset transmitters. Conventionally, DPD is usually deployed for high power base stations. With the continuously increasing bandwidth, DPD may also be required for handset PAs in 5G communication systems. Different from the models used for base stations, DPD model for handset PAs must have very low complexity because of the stringent power budget limit. At the same time, the tolerance for load mismatch must also be considered. The third part of the thesis analyzes the characteristics of handset PAs with load mismatch and introduces a low-complexity DPD model based on magnitude-selective affine (MSA) function. Experimental results demonstrate that the extended MSA (EMSA) model shows better linearization performance while keeping much lower complexity than the conventional DPD models.
  • Publication
    Sampling Rate Reduction for Digital Predistortion of Broadband RF Power Amplifiers
    In this article, we present a novel technique to build digital predistorters that can linearize broadband power amplifiers (PAs) using reduced sampling rates. In contrast to conventional digital predistortion (DPD) where oversampling is necessary to avoid aliasing effect, the proposed method cancels the aliasing distortion using a sliced multistage cancellation scheme. A large reduction of sampling rate can be achieved in digital implementation of DPD, significantly reducing power consumption and implementation cost. Experimental results show that a DPD with a sampling rate of merely 1.5x, instead of 5x, signal bandwidth, can still produce satisfactory performance within the linearization bandwidth but consume only one-third of power, compared with that using the conventional approaches. The proposed technique provides a promising solution for the next-generation 5G systems, where large signal bandwidths are required.
      546Scopus© Citations 19
  • Publication
    OTA-Based Data Acquisition and Signal Separation for Digital Predistortion of Multi-User MIMO Transmitters in 5G
    In this paper, a non-interruptive over-the-air (OTA) data acquisition method is presented to characterize system nonlinearity and calibrate digital predistortion (DPD) in multi-user (MU) multiple-input multiple-output (MIMO) transmitters with low hardware complexity. Based on a unified system model for fully-digital and hybrid MU-MIMO transmitters, the requirements and difficulties of data acquisition for MU-MIMO DPD are outlined first. A versatile OTA data acquisition technique, featuring a multi-observation forward modelling procedure, is thereby developed to obtain power amplifier (PA) output without interrupting the operation of the MIMO transmitters. By using this method, the output of each PA can be reconstructed instead of directly measuring each PA's output, even when the output signals of PAs are correlated. DPD can therefore be effectively constructed to mitigate the distortion. Experimental results demonstrate that the proposed approach can accurately estimate each PA output and linearize all PAs in the array.
      186Scopus© Citations 4
  • Publication
    Real-Time Single Channel Over-the-Air Data Acquisition for Digital Predistortion of 5G Massive MIMO Wireless Transmitters
    In this paper, a single channel over-the-air (OTA) data acquisition approach for real-time calibration of digital predistorter in multiple-input multiple-output transmitters is proposed. By using the data acquired from the far-field OTA tests, the output of each power amplifier (PA) can be virtually reconstructed and thus the linearization reference at the main beam direction can be accurately estimated. Digital predistortion (DPD) can therefore be effectively constructed without direct measurement at PA output. Experimental results demonstrate that the proposed scheme can accurately estimate far-field main beam data and the proposed DPD can achieve excellent linearization performance.
      300Scopus© Citations 12
  • Publication
    Design of Broadband Continuous Mode MMIC Power Amplifiers With Bandwidth Improvement
    This paper presents the design of continuous mode monolithic microwave integrated circuit (MMIC) power amplifiers (PAs) with bandwidth improvement. The output matching network (OMN) satisfying the continuous mode in broadband can be efficiently constructed with LC resonant circuits. The bandwidth can be further improved through generating the new LC unit in OMN with a minimum number of components added possible. Two prototype MMIC PAs are demonstrated with 0.25-µm Gallium Nitride (GaN) MMIC process. The measured drain efficiency is over 39.2% and 38.7% from 4.5 GHz to 6.5 GHz, respectively. When driven by 100-MHz modulated signals, two PAs exhibit great linearity performance with adjacent channel leakage ratio lower than −51.2 dBc and −53.4 dBc with digital predistortion.
  • Publication
    Complexity-Reduced Model Adaptation for Digital Predistortion of RF Power Amplifiers With Pretraining-Based Feature Extraction
    In this article, we present a new method to reduce the model adaptation complexity for digital predistortion (DPD) of radio frequency (RF) power amplifiers (PAs) under varying operating conditions, using pretrained transformation of model coefficients. Experimental studies show that the PA behavior variations can be effectively tracked using a small number of ``transformed'' coefficients, even with large deviations in its output characteristics. Based on this discovery, to avoid reextracting all the original coefficients every time when the operating condition changes, we propose to conduct a one-time off-line pretraining stage to extract the common features of PA behaviors under different operating conditions first. The online model adaptation process will then only need to identify a small number of transformed coefficients, which can result in a drastic reduction in the computational complexity of the model adaptation process. The proposed solution is validated by experimental results considering varying signal bandwidth and output power levels on a high-efficiency gallium-nitride Doherty PA, where the computational complexity is significantly reduced and the system performance is not compromised.
      308Scopus© Citations 8
  • Publication
    Enhancing Bandwidth and Back-Off Range of Doherty Power Amplifier With Modified Load Modulation Network
    This article presents a novel methodology for designing a broadband Doherty power amplifier (DPA) with extended output power back-off (OBO) range. A modified load modulation network (LMN) is proposed to enhance the OBO range and the bandwidth of the DPA simultaneously. Analysis is conducted to explore the relationship between the proposed LMN parameters and the broadband performance under various OBO levels. Generalized design formulas of the LMN parameters are then introduced to offer the broadband solution for arbitrary current ratios and OBO levels. An asymmetric DPA is demonstrated and implemented with gallium nitride (GaN) transistors using the proposed approach. The fabricated DPA operates from 1.4 to 2.5 GHz with 9-dB OBO range. The measured drain efficiency reaches 61%-75.5% at saturation and 44.6%-54.6% at 9-dB OBO within the operating bandwidth. When driven by a 60-MHz modulated signal with 9-dB peak-to-average power ratio (PAPR), the fabricated DPA attains 47.4%-53.5% average drain efficiency and better than -45.5-dBc adjacent channel leakage ratio (ACLR) after digital predistortion.
      286Scopus© Citations 12
  • Publication
    Three-Stage Load Modulated Power Amplifier With Efficiency Enhancement at Power Back-Off
    This article presents the analysis and design of a three-stage load modulated power amplifier (PA), in which three amplifiers, each with different biasing, are connected to a four-port coupler. It is illustrated that, by properly configuring current relationships between the three amplifiers, this new load modulated PA can provide flexible output power back-off (OBO) and achieve high efficiency within a large OBO range. A detailed theoretical analysis and design methodology are given. In this architecture, the OBO level can be adjusted by simply setting bias conditions of the relevant amplifiers that correspond to the current relationships. Therefore, after circuit fabrication, the OBO range can still be reconfigured without redesigning the circuit. To validate the proposed approach, a prototype operating at 3.45 GHz is demonstrated and implemented with gallium nitride (GaN) transistors. The measured saturated output power reaches 45 dBm with 70.1% drain efficiency. At 6-/8-/10-dB OBO, the fabricated PA can provide up to 62.1%/53.8%/47.3% drain efficiency, respectively. When driven by a 60-MHz 9-dB peak-to-average power ratio (PAPR) long-term evolution (LTE) signal, the PA provides 34-dBm average output power with 44.3% average efficiency. Moreover, measurement results prove that the PA can offer efficiency enhancement when the OBO is reconfigured to 8 or 12 dB after fabrication.