107 research outputs found

    Linear Predistortion-less MIMO Transmitters

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    Low Complexity DPD for Multi-Band Radio over Fiber Transmission Systems

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    The increasing demand for broadband wireless transmission in the modern internet has led to the proposal and standardization of the fifth-generation (5G) mobile communication system, which offers massive device connectivity, high bit rates, low latency, and cost sustainability. However, maintaining a high transmission rate as well as low latency is difficult to achieve simultaneously, which requires some state-of-art fronthaul transmission techniques. Therefore, radio over fiber (RoF) with different approaches like digital RoF (D-RoF), analog RoF (A-RoF), and delta-sigma modulation based RoF (DSM-RoF) for 5G fronthaul transmission has been introduced. Those RoF techniques may significantly reduce complexity and power consumption at base stations, but the extra electric to optic (E/O), optic to electric (O/E) converters and power amplifiers could introduce extra nonlinearity into the system. Moreover, ultra-broadband or multi-band ultra-broadband signal is introduced in 5G to further increase the transmission rate, which further increases the impact of the nonlinearity. Therefore, broadband linearization techniques are necessary for RoF fronthaul transmission systems due to the fragile of the signal and the inherent nonlinear distortions introduced by RoF link. To reduce the degradation of nonlinearity for RoF link, digital predistortion (DPD) techniques have been extensively researched to address these challenges. In a multi-band or multi-dimensional RoF system, multi-band DPD is required. Multi-dimensional DPD should be able to suppress the internal distortion within each band/dimension but also inter-distortion between different bands/dimensions. Unfortunately, the dimension higher than 3 causes a high calculation complexity to get the DPD function coefficients. There have been lots of efforts that have been made to obtain less-complexity DPD with better accuracy for multi-band or multidimensional signals. However, very limited DPD techniques have been proposed in simplifying the fundamental linearization function for bands exceeding four. Thus, the multi-band/multidimensional DPD has not been really got in used in commercial products because of the high complexity, high cost and high-power consumption. Thus, a simplified linearization approach for multi-band DPD is still needed. In this thesis, a new low-complexity multidimensional DPD is introduced. This proposed DPD introduces a simplified DPD function, which evolves from the conventional memory polynomial function. Compared with the conventional multi-dimensional DPD, this proposed approach has lower complexity increased with the increase of signal bands or dimensions, nonlinearity orders, and memory effect depth. For example, the conventional DPD function needs a total of 40040 coefficients for the 6-band signals with a nonlinearity order of 10 and a memory depth of 5. However, this proposed low-complexity DPD function needs 640 coefficients. A substantial reduction in complexity is clearly observed. The performance of the proposed DPD is evaluated by both simulation and experiments. An up to 6-band 64-QAM orthogonal frequency division multiplexing (OFDM) signal with each band of 200 MHz in simulations and an up to 5-band 20 MHz 64-QAM OFDM signal in experiments are used. The performance is evaluated in the means of error vector magnitude (EVM) of the received signal. The average improvement of EVM in simulation for 3-band, 4-band, 5-band and 6-band signals is 19.97 dB, 18.65 dB, 16.64 dB and 15.44 dB, respectively. The average improvement of EVM in experiments for 4-band and 5-band signals is 5.67 dB and 8.1 dB, respectively. The above results prove that the proposed DPD can significantly reduce the complexity and provide good linearization

    Advanced Techniques for Ground Penetrating Radar Imaging

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    Ground penetrating radar (GPR) has become one of the key technologies in subsurface sensing and, in general, in non-destructive testing (NDT), since it is able to detect both metallic and nonmetallic targets. GPR for NDT has been successfully introduced in a wide range of sectors, such as mining and geology, glaciology, civil engineering and civil works, archaeology, and security and defense. In recent decades, improvements in georeferencing and positioning systems have enabled the introduction of synthetic aperture radar (SAR) techniques in GPR systems, yielding GPR–SAR systems capable of providing high-resolution microwave images. In parallel, the radiofrequency front-end of GPR systems has been optimized in terms of compactness (e.g., smaller Tx/Rx antennas) and cost. These advances, combined with improvements in autonomous platforms, such as unmanned terrestrial and aerial vehicles, have fostered new fields of application for GPR, where fast and reliable detection capabilities are demanded. In addition, processing techniques have been improved, taking advantage of the research conducted in related fields like inverse scattering and imaging. As a result, novel and robust algorithms have been developed for clutter reduction, automatic target recognition, and efficient processing of large sets of measurements to enable real-time imaging, among others. This Special Issue provides an overview of the state of the art in GPR imaging, focusing on the latest advances from both hardware and software perspectives

    Broadband Class-J GaN Doherty Power Amplifier

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    This paper presents a broadband 3 GHz–3.7GHz class-J Doherty power amplifier exploiting second harmonic tuning in the output network. Furthermore, the output impedance inverter is eliminated and its effect is embedded in the main device’s output matching network, thus trading off among bandwidth, efficiency, and gain. The proposed amplifier adopts two 10W packaged GaN transistors, and it achieves in measurement 60–74%, and 46–50% drain efficiency at saturation and 6 dB output back-off, respectively, with a saturated output power of 43 dBm–44.2dBm and a small-signal gain of 10 dB–13 dB. The proposed DPA exhibits a simulated adjacent channel power ratio less than 30 dBc at 36dBm average output power when a 16-QAM modulation with 5 MHz bandwidth is applied to the 3.5 GHz carrier

    Novel Specialty Optical Fibers and Applications

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    Novel Specialty Optical Fibers and Applications focuses on the latest developments in specialty fiber technology and its applications. The aim of this reprint is to provide an overview of specialty optical fibers in terms of their technological developments and applications. Contributions include:1. Specialty fibers composed of special materials for new functionalities and applications in new spectral windows.2. Hollow-core fiber-based applications.3. Functionalized fibers.4. Structurally engineered fibers.5. Specialty fibers for distributed fiber sensors.6. Specialty fibers for communications

    Fiber Optic Sensors and Fiber Lasers

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    The optical fiber industry is emerging from the market for selling simple accessories using optical fiber to the new optical-IT convergence sensor market combined with high value-added smart industries such as the bio industry. Among them, fiber optic sensors and fiber lasers are growing faster and more accurately by utilizing fiber optics in various fields such as shipbuilding, construction, energy, military, railway, security, and medical.This Special Issue aims to present novel and innovative applications of sensors and devices based on fiber optic sensors and fiber lasers, and covers a wide range of applications of optical sensors. In this Special Issue, original research articles, as well as reviews, have been published

    Field resolving spectrometer for mid-infrared molecular spectroscopy

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    Field resolving spectrometer for mid-infrared molecular spectroscopy

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    The interrogation of molecular samples with broadband mid-infrared (MIR) radiation results in highly specific “vibrational fingerprints,” containing a wealth of information on molecular structure and composition. This renders vibrational spectroscopy a powerful and versatile tool for applications ranging from fundamental science to the life sciences and to industrial applications. Conventional MIR spectroscopic techniques face severe limitations in detection sensitivity, in particular due to the poor coherence properties of common MIR sources as well as to the moderate detectivity and dynamic range of broadband MIR detectors. The research reported in this thesis has addressed the quest for novel routes towards tapping the potential of MIR spectral fingerprinting, harnessing modern, high-power femtosecond laser technology. The first part of the work reports the construction of octave-spanning, coherent femtosecond MIR sources, employing state-of-the-art 100-W-average-power-level thin-disk Yb:YAG modelocked oscillators. We demonstrated ultrabroadband coherent MIR sources with a brilliance exceeding that of MIR beamlines at 3rd-generation synchrotrons, and found that pulses emerging via intra-pulse difference frequency generation offer superior (and unparalleled) optical-waveform stability as compared to standard optical-parametric amplification. The temporal confinement of broadband MIR radiation to trains of sub-100-femtosecond pulses, together with field-resolved detection via electro-optic sampling (EOS) affords detection of the molecular fingerprint signal in the near-infrared region, where highly-efficient, high-dynamic-range detectors exist. Optimized EOS detection enabled a linear response over an intensity dynamic range of 150 dB at a central wavelength of 8.6 µm. This exceeds the previous state of the art by a large margin and has paved the way to high-signal-to-noise-ratio transmission measurements of aqueous biological samples like living cells and tissue. The waveform stability of the mid-infrared pulses plays a crucial role for real-life field-resolved spectroscopy measurements, and is of paramount importance for precision-metrological applications. In the second part of this thesis, high-quantum-efficiency EOS was employed for precision measurements of waveform jitter, evaluated for millions of pulses. This study demonstrated few-attosecond temporal jitter in the 1-Hz-to-0.625-MHz band, between the centre of mass of the driving near-infrared pulses, and individual field zero-crossings of the emerging, broadband mid-infrared field. This confirms the outstanding waveform stability achievable with second-order parametric processes with an order-of-magnitude improved accuracy compared to previous measurements. Furthermore, chirping the MIR pulse revealed attosecond-level optical-frequency-dependent waveform jitter, whose dynamics were quantitatively traced back to excessive intensity noise of the mode-locked oscillator. Thus, this study validated EOS as a broadband (both in the radio-frequency and in the optical domain), high-sensitivity measurement technique for the dynamics of optical waveforms beyond the standard, optical-spectrum-integrating carrier-envelope phase model. The instrument developed during this thesis was utilized for the first highly sensitive field-resolved measurements in the MIR molecular fingerprint region. It enabled the detection of molecular concentrations spanning 5 orders of magnitude down to 200-ng/mL in aqueous solutions and the examination of living biological systems with a thickness of up to 0.2 mm. Currently, the instrument is being used for the first large-scale studies on disease recognition based on vibrational fingerprinting of human blood serum. The implementation of intra-scan referencing, successfully carried out in the last weeks of this doctoral work, together with fast-scanning techniques and the extension of the MIR spectral bandwidth which are underway at our laboratory, promise to extend the technology pioneered in this thesis to new levels of sensitivity and reproducibility in vibrational spectroscopy. In addition to directly benefitting analytical applications, these developments are likely to afford novel insights into light-matter interactions
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