Reduced Radio Frequency Research Articles

Precise analysis of potassium isotopic composition in plant materials by multi-collector inductively coupled plasma mass spectrometry

Potassium (K) is a crucial macronutrient for plants and is the predominant inorganic cation in plants, constituting up to 10% of plant biomass on a dry weight basis and originating from the soil. The utilization of potassium isotopes enables the examination of potassium cycling in the soil-plant. In this investigation, the potassium isotopic composition (δ41K) of five certified plant reference materials with varying potassium concentrations (BCR 679, GSV-2, GSB-2a, GSB-3 and GSB-6a) was assessed using multi-collector inductively coupled plasma mass spectrometry technique, employing a reduced Radio Frequency (RF) forward power and low-resolution mode without collision cell. The δ41K values of the plant reference materials BCR 679, GSV-2, GSB-2a, GSB-3 and GSB-6a are reported as −2.44 ± 0.03‰, −0.37 ± 0.04‰, −0.37 ± 0.05‰, −1.27 ± 0.04‰ and − 0.62 ± 0.01‰, respectively. This study not only presents the δ41K values of plants but also establishes a methodology for the pre-treatment and determination of δ41K in plant. Additionally, the observed variations in δ41K values among the plant reference materials suggest that δ41K has the potential to serve as a significant tracer in “soil-plant systems” within ecosystems.

Sparse Hybrid Precoding for Power Minimization With an Adaptive Antenna Structure in Massive MIMO Systems

In massive multiple-input multiple-output (MIMO) systems employing hybrid analog-digital precoder, there are two commonly used antenna structures that have their own pros and cons, namely, the fully-connected antenna structure (FCAS) and the partially-connected antenna structure (PCAS). The FCAS achieves better spectrum efficiency (SE) even with reduced radio frequency (RF) chains, but the hardware cost and power consumption are still grievous. Contrarily, the PCAS has less hardware cost and power consumption, but suffers from severe performance loss. In this paper, by combining the advantages of both structures, we first propose a sparse adaptive antenna structure (SAAS) for the implementation of the hybrid precoder, which can jointly control the on/off state of all phase shifters (PS) and RF chains through a switch network. Then, a sparse hybrid precoding (SHP) optimization problem based on the proposed SAAS is established aiming at minimizing the total power consumption under individual average data rate requirements. To tackle the challenging non-smooth non-convex stochastic optimization (NSO) emerged with the SHP design and reduce the power consumption of PSs and RF chains, we propose a sparse smooth approximation based an online algorithm to find a stationary point of the NSO problem and establish its convergence. Simulations verify that the proposed antenna structure and algorithm achieve a better balance between power consumption and system throughput than the existing schemes.

Rover-mounted Hydrated Mineral Detector for Mars Exploration: A Preliminary Report

The rover-mounted hydrated mineral detector (HMD) is based on a dielectric spectrometer (tuned frequency range of 0.8–4 MHz) that will detect bulk subsurface hydrated minerals in local scale. The HMD is a compact, low-mass (1.5 kg), and low-power instrument (5 W) suitable for Mars application. Compared with orbiter-based instruments with low horizontal resolution (5–10 km), the HMD will offer high resolution (2 m) that can identify optimal sampling sites for the presence of hydrated minerals. From field testing with reduced radio frequency (RF) power (P rf = 10 mW), the HMD demonstrated detection of shallow buried gypsum samples. When developed with full RF power, the HMD could potentially detect hydrates down to depths of 2 m (P rf = 1 W) or 4 m (P rf = 36 W) in Martian soil. Unlike conventional dielectric spectrometers, which require small amounts of the sample (5–10 g) to be placed inside sample cells, the HMD will detect hydrated minerals by scanning over the Mars surface mounted on a rover platform (<1 m above ground) without any sample preparation. This capability will allow future rovers to rapidly assess the scientific potential of field sites. Compared with optical techniques that are limited to surface exposed mineralogical features that could be obscured by dust and weathering products, the HMD will detect bulk subsurface hydrated minerals along the rover traverse irrespective of surface obscuration. The HMD (TRL 4) is a proof-of-concept instrument that can be developed and matured for future Mars rover missions.

Online Detuning Computation and Quench Detection for Superconducting Resonators

Superconducting cavities are responsible for beam acceleration in superconducting linear accelerators. Challenging cavity control specifications are necessary to reduce radio frequency (RF) costs and to maximize the availability of the accelerator. Cavity detuning and bandwidth are two critical parameters to monitor when operating particle accelerators. Cavity detuning is strongly related to the power required to generate the desired accelerating gradient. Cavity bandwidth is related to the cavity RF losses. A sudden increase in bandwidth can indicate the presence of a quench or multipacting event. Therefore, calculating these parameters in real time in the low-level RF (LLRF) system is highly desirable. A real-time estimation of the bandwidth allows for a faster response of the machine protection system in the case of quench events, whereas the estimation of cavity detuning can be used to drive piezoelectric tuner-based resonance control algorithms. In this article, a new field programmable gate array (FPGA)-based estimation component is presented. Such a component is designed to be used either in continuous wave (CW) or pulsed operation mode with loaded quality factors between $10^{6}$ and $10^{8}$ . Results of this component with free-electron LASer in Hamburg (FLASH), European X-ray free electron laser (EuXFEL), cryo module test bench (CMTB), and electron linac for beams with high brilliance and low emittance (ELBE) are presented.

Real‐time device tracking under MRI using an acousto‐optic active marker

This work aims to demonstrate the use of an "active" acousto-optic marker with enhanced visibility and reduced radiofrequency (RF) -induced heating for interventional MRI. The acousto-optic marker was fabricated using bulk piezoelectric crystal and π-phase shifted fiber Bragg grating (FBGs) and coupled to a distal receiver coil on an 8F catheter. The received MR signal is transmitted over an optical fiber to mitigate RF-induced heating. A photodetector converts the optical signal into electrical signal, which is used as the input signal to the MRI receiver plug. Acousto-optic markers were characterized in phantom studies. RF-induced heating risk was evaluated according to ASTM 2182 standard. In vivo real-time tracking capability was tested in an animal model under a 0.55T scanner. Signal-to-noise ratio (SNR) levels suitable for real-time tracking were obtained by using high sensitivity FBG and piezoelectric transducer with resonance matched to Larmor frequency. Single and multiple marker coils integrated to 8F catheters were readout for position and orientation tracking by a single acousto-optic sensor. RF-induced heating was significantly reduced compared to a coax cable connected reference marker. Real-time distal tip tracking of an active device was demonstrated in an animal model with a standard real-time cardiac MR sequence. Acousto-optic markers provide sufficient SNR with a simple structure for real-time device tracking. RF-induced heating is significantly reduced compared to conventional active markers. Also, multiple RF receiver coils connected on an acousto-optic modulator can be used on a single catheter for determining catheter orientation and shape.

A counterpoise design for RF-induced heating reduction.

This paper presents a novel lead body design for active implantable medical devices (AIMD) to reduce Radio-frequency (RF) induced heating during magnetic resonance imaging (MRI) scanning. By introducing a counterpoise electrode to the original lead construct, part of the RF-induced energy can be decoyed into the surrounding tissues while the therapy signal is intact. The numerical simulation studies of three leads with different configurations are presented to demonstrate the effectiveness of this technique. From simulation results at 1.5 T, the peak 1g average SAR value can be reduced by a factor of 3 when the length of the counterpoise electrode is properly designed.

Metalorganic Chemical Vapor Phase Epitaxy Growth of Buffer Layers on 3C‐SiC/Si(111) Templates for AlGaN/GaN High Electron Mobility Transistors with Low RF Losses

Herein, the interest of cubic silicon carbide as a template for the growth of AlGaN/GaN high electron mobility transistor (HEMT) heterostructures on silicon substrates for high‐frequency operation is shown. On the one hand, 0.6–0.8 μm‐thick 3C‐SiC grown by chemical vapor deposition on intrinsic silicon substrate having initial resistivity superior to 5 kΩ cm enables the metalorganic vapor phase epitaxy of GaN buffer layers with propagation losses below 0.4 dB mm−1 at 40 GHz and 0.5 dB mm−1 at 67 GHz. On the other hand, an HEMT heterostructure is grown on 1.5 μm‐thick 3C‐SiC on 4° off‐axis silicon substrate having an initial resistivity superior to 200 Ω cm that allows to keep a sufficiently resistive epilayer stack limiting the loss up to 0.78 dB mm−1 at 40 GHz. Device process developed on a piece of the 100 mm diameter wafer leads to the demonstration of DC transistor operation with low leakage currents. Compared with direct growth on silicon, these templates enable reduced radio frequency (RF) propagation losses that are very interesting for high‐frequency transistors and circuits operation.

Generalized Beamspace Modulation Using Multiplexing: A Breakthrough in mmWave MIMO

This paper proposes a generalized beamspace modulation using multiplexing (GBMM) scheme for millimeter wave (mmWave) multiple-input multiple-output (MIMO) communications with reduced radio frequency (RF) chains. Besides achieving a multiplexing gain over the selected beamspace set, GBMM additionally makes use of the index of the beamspace set to carry information. In the proposed GBMM, the beamspace sets are non-uniformly activated. We investigate the spectral efficiency (SE) of the proposed GBMM and the SE-oriented beamspace set activation probability optimization as well as the hybrid precoder design. In the hybrid precoder design procedure, we first design the fully-digital precoders and then adopt the optimized fully-digital precoders to design the hybrid precoders. A gradient ascent algorithm is developed to find the optimal fully-digital precoders and precoder activation probabilities. In the high signal-to-noise-ratio (SNR) regime, closed-form solutions of the fully-digital precoders and the precoder activation probabilities are derived. Moreover, we investigate the impact of the hybrid receiver structure on the performance of GBMM, propose a coding method to realize the optimized precoder activation, and discuss the extension to orthogonal frequency division multiplexing (OFDM)-based mmWave broadband communications. Both analytical and numerical results show that GBMM outperforms the spatial multiplexing over the best beamspace set in terms of SE, which has been well recognized as the best transmission solution in mmWave MIMO communications.

Joint Precoding and Combining Design for Hybrid Beamforming Systems With Subconnected Structure

Hybrid analog and digital structure with reduced radio frequency (RF) chains is a key enabler to reach a compromise between system complexity and performance in future fifth-generation high-frequency communications. A fully connected architecture that each antenna connects to all RF chains has been proven to achieve similar performance compared to a traditional full-digital system where the number of RF chains is equal to the number of antennas. However, such a structure is difficult for practical implementation owing to the problem of circuit layout for high-frequency antenna array structures. Therefore, a subconnected alternative where each RF chain only connects to a disjoint subset of antennas becomes more attractive. In this paper, we study the system with the subconnected subarray architecture employed by both the transmitter and the receiver, and aim to maximize the sum-rate by taking joint precoding and combining into consideration. We propose two schemes, named joint-SIC and MU-SIC, for single-user and multi-user communications, respectively. Moreover, we provide an upper bound of the sum-rate achieved by such a subconnected subarray structure. The simulation results show the fast convergence and effectiveness of the proposed schemes.

Reconstruction of randomly under-sampled spectra for in vivo13C magnetic resonance spectroscopy

PurposeOver the past decade, many techniques have been developed to reduce radiofrequency (RF) power deposition associated with proton decoupling in in vivo Carbon-13 (13C) magnetic resonance spectroscopy (MRS). In this work we propose a new strategy that uses data under-sampling to achieve reduction in RF power deposition. Materials and methodsEssentially, proton decoupling is required only during randomly selected segments of data acquisition. By taking advantage of the sparse spectral pattern of the carboxylic/amide region of in vivo13C spectra of brain, we developed an iterative algorithm to reconstruct spectra from randomly under-sampled data. Fully sampled data were used as references. Reconstructed spectra were compared with the fully sampled references and evaluated using residuals and relative signal intensity errors. ResultsNumerical simulations and in vivo experiments at 7Tesla demonstrated that this novel decoupling and data processing strategy can effectively reduce decoupling power deposition by greater than 30%. ConclusionThis study proposes and evaluates a novel approach to acquire 13C data with reduced proton decoupling power deposition and reconstruct in vivo13C spectra of carboxylic/amide metabolite signals using randomly under-sampled data. Because proton decoupling is not needed over a significant portion of data acquisition, this novel approach can effectively reduce the required decoupling power and thus SAR. It opens the possibility of performing in vivo13C experiments of human brain at very high magnetic fields.

Adaptive Hybrid Precoding for Multiuser Massive MIMO

Hybrid precoding (HP) is widely utilized in millimeter wave-based massive MIMO systems with significantly reduced radio frequency (RF) chains, but it requires a large number of analog phase shifters (APSs) and RF adders to realize the connection between RF chains and antenna elements. In this letter, an adaptive hybrid precoding (AHP) is proposed to approach the performance of the conventional HP with reduced complexity. Different from the conventional HP where each antenna is connected to all RF chains through APSs and RF adders, the proposed AHP connects each antenna with only one RF chain through an adaptive connection network. This adaptive connection network and the phases of APSs are jointly designed, which is formulated as an optimization problem to maximize the users' average downlink achievable rate. Moreover, a multiuser adaptive analog precoding (MU-AAP) algorithm is proposed to provide a near-optimal solution to this joint-design problem. Simulation results verify the performance gain of the proposed AHP in typical multiuser massive MIMO scenarios.

Study on Cu/Ni Nano Superlattice Conductors for Reduced RF Loss

We report on Cu/Ni paired superlattice conductors featuring reduced radio frequency (RF) loss based on eddy current cancelling (ECC). Also, the effects of the width and the individual layer thickness of the superlattice conductors have been studied for improved RF performance. The usage of Ni as the ferromagnetic material in the non-ferromagnetic/ferromagnetic superlattice structure is advantageous as it has a high contrast between the in-plane and out-of-plane magnetic coercivity fields making it suitable for effective thin film superlattice ECC, is abundant, and does not require a stoichiometric control of composition as other alloy magnetic materials do. Ni's negative permeability is also able to effectively cancel out the positive permeability of copper in the frequency of interest. A transmission line consisting of 10 superlattice Cu/Ni layers, with each layer being of 150 nm/25 nm totaling 1.75 $\mu\text{m}$ shows the same resistance value as one consisting of 10 Cu/Ni layers with each being 600 nm/100 nm totaling 7 $\mu\text{m}$ thick at 13 GHz, revealing 75% conductor volume reduction. Experimental results show more than three times improvement in the figure of merit defined as frequency/effective resistivity, compared with other state-of-the-art devices.

Low-Loss Air-Isolated Through-Silicon Vias for Silicon Interposers

An air-isolated through-silicon via (TSV) technique is proposed to reduce radio-frequency (RF) losses in silicon interposers. A testbed containing air-isolated and conventional TSVs is fabricated and characterized from 10 MHz to 20 GHz with an L-2L de-embedding technique. The proposed air-isolated TSV technique yields 46.7% lower insertion loss compared to conventional TSVs at 20 GHz from 3-D full-wave simulations and measurements. Moreover, the impact of the air-isolation region width between TSVs on capacitance and conductance is quantified.

MR fingerprinting using the quick echo splitting NMR imaging technique.

The purpose of the study is to develop a quantitative method for the relaxation properties with a reduced radio frequency (RF) power deposition by combining magnetic resonance fingerprinting (MRF) technique with quick echo splitting NMR imaging technique (QUEST). A QUEST-based MRF sequence was implemented to acquire high-order echoes by increasing the gaps between RF pulses. Bloch simulations were used to calculate a dictionary containing the range of physically plausible signal evolutions using a range of T1 and T2 values based on the pulse sequence. MRF-QUEST was evaluated by comparing to the results of spin-echo methods. The specific absorption rate (SAR) of MRF-QUEST was compared with the clinically available methods. MRF-QUEST quantifies the relaxation properties with good accuracy at the estimated head SAR of 0.03 W/kg. T1 and T2 values estimated by MRF-QUEST are in good agreement with the traditional methods. The combination of the MRF and the QUEST provides an accurate quantification of T1 and T2 simultaneously with reduced RF power deposition. The resulting lower SAR may provide a new acquisition strategy for MRF when RF energy deposition is problematic. Magn Reson Med 77:979-988, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Passive multistatic radar experiment using WiMAX signals of opportunity. Part 2: Multistatic velocity backprojection

Passive radar can reduce radio frequency (RF) spectrum crowding and facilitate RF system interoperability by eliminating the necessity to transmit. Part 1 of this study discussed a passive multistatic radar experiment using Worldwide Interoperability for Microwave Access (WiMAX) signals of opportunity and the accompanying signal processing techniques that were employed. Part 2 discusses the multistatic velocity backprojection technique for visualising multistatic radar data and applies this technique to simulated data and passive multistatic data from the experiment discussed in Part 1.

Simultaneous multislice (SMS) imaging techniques.

Simultaneous multislice imaging (SMS) using parallel image reconstruction has rapidly advanced to become a major imaging technique. The primary benefit is an acceleration in data acquisition that is equal to the number of simultaneously excited slices. Unlike in‐plane parallel imaging this can have only a marginal intrinsic signal‐to‐noise ratio penalty, and the full acceleration is attainable at fixed echo time, as is required for many echo planar imaging applications. Furthermore, for some implementations SMS techniques can reduce radiofrequency (RF) power deposition. In this review the current state of the art of SMS imaging is presented. In the Introduction, a historical overview is given of the history of SMS excitation in MRI. The following section on RF pulses gives both the theoretical background and practical application. The section on encoding and reconstruction shows how the collapsed multislice images can be disentangled by means of the transmitter pulse phase, gradient pulses, and most importantly using multichannel receiver coils. The relationship between classic parallel imaging techniques and SMS reconstruction methods is explored. The subsequent section describes the practical implementation, including the acquisition of reference data, and slice cross‐talk. Published applications of SMS imaging are then reviewed, and the article concludes with an outlook and perspective of SMS imaging. Magn Reson Med 75:63–81, 2016. © 2015 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.

Low RF-Complexity Millimeter-Wave Beamspace-MIMO Systems by Beam Selection

Communications in millimeter-wave (mm-wave) spectrum (30-300 GHz) have experienced a continuous increase in relevance for short-range, high-capacity wireless links, because of the wider bandwidths they are able to provide. In this work, we introduce a new mm-wave frequency transmission scheme that exploits a combination of the concepts of beamspace multi-input multi-output (B-MIMO) communications and beam selection to provide near-optimal performances with a low hardware-complexity transceiver. While large-scale MIMO approaches in mm-wave are affected by high dimensional signal space that increases considerably both complexity and costs of the system, the proposed scheme is able to achieve near-optimal performances with a reduced radio-frequency (RF) complexity thanks to beam selection. We evaluate the advantages of the proposed scheme via capacity computations, comparisons of numbers of RF chains required and by studying the trade-off between spectral and power efficiency. Our analytical and simulation results show that the proposed scheme is capable of offering a significant reduction in RF complexity with a realistic low-cost approach, for a given performance. In particular, we show that the proposed beam selection algorithms achieve higher power efficiencies than a full system where all beams are utilized.

Shielded Design of Screen-Printed EEG Electrode Set Reduces Interference Pick-Up

Thick-film technologies, such as screen printing, are becoming more popular in the manufacturing of bioelectrodes due to their simplicity, low cost, high reproducibility, and efficiency in large-scale production. Since electroencephalography (EEG) is a method for measuring potential differences in the microvolt scale, it is important to minimize all electromagnetic interferences. However, protecting screen-printed electrodes and their conduction traces from electromagnetic interferences has not been adequately investigated. We hypothesized that interference pick-up could be effectively reduced by an optimized silver or graphite shielded construction. The interference pick-up of the electrodes was technically evaluated in an electromagnetic compatibility laboratory and with EEG recordings of healthy volunteers. The grounding layer significantly (p<; 0.001) reduced radio frequency (RF) interference in the standardized laboratory tests and the shielded electrodes exhibited significantly better power-line interference immunity in the EEG recordings (p<; 0.05). The silver layer achieved better shielding in the RF tests than the graphite layer (p= 0.029). Since the silver layer is straightforward and cheap to produce with the screen printing technique, it could be advantageously used to shield, e.g., in emergency rooms where many interfering electronic devices are close to patient.

Fabrication and Characterization of 150-mm Silicon-on-Polycrystalline Silicon Carbide Substrates

Silicon-on-insulator (SOI) substrates can reduce radiofrequency (RF) substrate losses due to their buried oxide (BOX). On the other hand, the BOX causes problems since it acts as a thermal barrier. Oxide has low thermal conductivity and traps heat generated by devices on the SOI. This paper presents a hybrid substrate which uses a thin layer of polycrystalline silicon and polycrystalline silicon carbide (Si-on-poly-SiC) to replace the thermally unfavorable BOX and the silicon substrate. Substrates of 150 mm were fabricated by wafer bonding and shown to be stress and strain free. Various electronic devices and test structures were processed on the hybrid substrate as well as on a low-resistivity SOI reference wafer. The substrates were characterized electrically and thermally and compared with each other. Results showed that the Si-on-poly-SiC wafer had 2.5 times lower thermal resistance and exhibited equal or better electrical performance compared with the SOI reference wafer.

Dynamic Contrast-Enhanced Renal MRI at 7 Tesla

With the successful implementation of ultra-high-field imaging in neuro- and musculoskeletal imaging, the interest of scientific research expanded toward whole-body applications. The aim of this study was to assess the feasibility of dynamic contrast-enhanced renal magnetic resonance imaging (MRI) at 7 Tesla (T), with optimization and implementation of a dedicated examination protocol. In vivo dynamic contrast-enhanced high-field examinations were obtained in 10 healthy subjects on a 7 T whole-body MR scanner. A custom-built body transmit/receive reduced radiofrequency (RF) coil suitable for RF shimming was used for image acquisition. The examination protocol included (1) true fast imaging with steady-state precession imaging, (2) T2-weighted turbo spine echo imaging, (3) T1-weighted (T1w) in- and opposed-phase imaging, and (4) a fat-saturated 2D FLASH sequence. For dynamic imaging, gadobutrol was injected intravenously and T1w 3D FLASH images were obtained precontrast and at 20, 70, and 120 seconds delay. Qualitative image analysis was performed by 2 senior radiologists using a 3-point scale (1 = poor, 2 = moderate, 3 = good quality). Signal-to-noise ratio and contrast-to-noise ratio (CNR) of the renal cortex/medulla were measured for all sequences. For statistical analysis, a Wilcoxon Rank Test was used. All examinations were performed successfully and were well tolerated by all subjects without any side effects. Best overall image quality was rated for the T1w 2D FLASH sequence with an average score of 2.57, followed by the contrast-enhanced 3D FLASH sequence in the equilibrium phase (mean, 2.22). T2-weighted turbo spine echo imaging provided the weakest overall image quality score (1.30) and was most impaired by artifacts. Quantitative analysis showed highest CNR between cortex and medulla for arterial phase 3D FLASH imaging (CNR = 12.2), providing a statistically significant difference to all other sequences, except for the in- and opposed-phase and the fat-saturated 2D FLASH sequence. Conversely, equilibrium phase FLASH imaging yielded the weakest CNR score of 3.6. This feasibility study reveals the diagnostic potential and current constraints of ultra-high-field abdominal MRI. Our initial results demonstrate the potential of dedicated dynamic-contrast 7 T renal MRI and the need for further optimization of imaging sequences and RF coil concepts.