Radio Frequency Receiver Research Articles

Simulation of locating buried objects via fringe pattern-based measurements in an optical fiber sensor-integrated continuous-wave ground-penetrating radar system

A new ground-penetrating radar system that can determine the depth and position of a buried metal cylinder, which is the combination of an optical fiber sensor (OFS), continuous-wave transmitter, and optical communication link, is proposed and simulated in the MATLAB and Optiwave program packages. Employing an optical sensor instead of a standard radio frequency (RF) receiver offers the advantage of preventing electromagnetic interference along the sensor's main unit cable and lower amplification noise. In this paper, the electric field distribution at the OFS due to reflections from the soil surface and the buried object is obtained mathematically using Green's function. The OFS's detected electric signal is then simulated. The effects of the cylinder's depth and the RF transmitter frequency are observed. The plots of the OFS output voltages vs. the x-axis distance to the burial point display interference fringe patterns. The position and the depth of the cylinder are determined using the characteristics of these patterns. It is estimated that the burial depth can be ascertained with a maximum 5-cm error for a 1-GHz transmitter frequency.

Antarctic radio frequency albedo and implications for cosmic ray reconstruction

AbstractWe describe herein a measurement of the Antarctic surface “roughness” performed by the balloon‐borne ANITA (Antarctic Impulsive Transient Antenna) experiment. Originally purposed for cosmic ray astrophysics, the radio frequency (RF) receiver ANITA gondola, from its 38 km altitude vantage point, can scan a disk of snow surface 600 km in radius. The primary purpose of ANITA is to detect RF emissions from cosmic rays incident on Antarctica, such as neutrinos which penetrate through the atmosphere and interact within the ice, resulting in signal directed upward which then refracts at the ice‐air interface and up and out to ANITA, or high‐energy nuclei (most likely irons or protons), which interact in the upper atmosphere (at altitudes below ANITA) and produce a spray of down‐coming RF which reflects off the snow surface and back up to the gondola. The energy of such high‐energy nuclei can be inferred from the observed reflected signal only if the surface reflectivity is known. We describe herein an attempt to quantify the Antarctic surface reflectivity, using the Sun as a constant, unpolarized RF source. We find that the reflectivity of the surface generally follows the expectations from the Fresnel equations, lending support to the use of those equations to give an overall correction factor to calculate cosmic ray energies for all locations in Antarctica. The analysis described below is based on ANITA‐II data. After launching from McMurdo Station in December 2008, ANITA‐II was aloft for a period of 31 days with a typical instantaneous duty cycle exceeding 95%.

Low-power wireless ECG acquisition and classification system for body sensor networks.

A low-power biosignal acquisition and classification system for body sensor networks is proposed. The proposed system consists of three main parts: 1) a high-pass sigma delta modulator-based biosignal processor (BSP) for signal acquisition and digitization, 2) a low-power, super-regenerative on-off keying transceiver for short-range wireless transmission, and 3) a digital signal processor (DSP) for electrocardiogram (ECG) classification. The BSP and transmitter circuits, which are the body-end circuits, can be operated for over 80 days using two 605 mAH zinc-air batteries as the power supply; the power consumption is 586.5 μW. As for the radio frequency receiver and DSP, which are the receiving-end circuits that can be integrated in smartphones or personal computers, power consumption is less than 1 mW. With a wavelet transform-based digital signal processing circuit and a diagnosis control by cardiologists, the accuracy of beat detection and ECG classification are close to 99.44% and 97.25%, respectively. All chips are fabricated in TSMC 0.18-μm standard CMOS process.

Whole-Body Averaged Specific Absorption Rate Estimation Using a Personal, Distributed Exposimeter

For the first time, a body area network (BAN) is used to construct a personal, distributed exposimeter (PDE), which can measure the whole-body averaged specific absorption rate ( ${{\rm SAR}_{\rm wb}}$ ) in real life, together with the incident power density ( ${{\rm S}_{\rm inc}}$ ). The BAN consists of four textile antennas with integrated radio frequency receiver nodes tuned to the Global System for Mobile Communications (GSM) 900 downlink band. Calibration measurements at 942.5 MHz, using a human subject, are performed in an anechoic chamber. These are combined with numerical simulations to estimate both ${{\rm SAR}_{\rm wb}}$ and ${{\rm S}_{\rm inc}}$ from the averaged received power on the PDE. The PDE has 50% prediction intervals of 3 dB on ${{\rm S}_{\rm inc}}$ and 3.3 dB on the ${{\rm SAR}_{\rm wb}}$ , caused by the presence of the human body, whereas the best single textile antenna in our measurements exhibits ${{\rm PI}_{50}}$ ’s of 7.1 dB on ${\rm {S}}_{{\rm {inc}}}$ and 5 dB on ${{\rm SAR}_{\rm wb}}$ . Measurements using the PDE are carried out in Ghent, Belgium, during which a median ${{\rm S}_{\rm inc}} = 47~\mu\hbox{W}/\hbox{m}^{2}$ and ${{\rm SAR}_{\rm wb}} = 0.25~\mu\hbox{W}/{kg}$ are measured.

Methodology for RF receiver sensitivity analysis using electromagnetic field map

Accurate modelling of a chip is a very important factor when analysing and predicting radio-frequency (RF) receiver sensitivity. Chip modelling based on the tangential electromagnetic (EM) field from the near-field measurement just above the top surface of the chip is introduced, and a methodology utilising three-dimensional (3D) EM simulation, for predicting receiver sensitivity generated by the chip-noise of a mobile phone, is suggested. It is worth noting here that the calculation of absolute coupling values between a noise source and an antenna is possible by using the real measured data source to obtain reliable results.

Tissue electrical property mapping from zero echo-time magnetic resonance imaging.

The capability of magnetic resonance imaging (MRI) to produce spatially resolved estimation of tissue electrical properties (EPs) in vivo has been a subject of much recent interest. In this work we introduce a method to map tissue EPs from low-flip-angle, zero-echo-time (ZTE) imaging. It is based on a new theoretical formalism that allows calculation of EPs from the product of transmit and receive radio-frequency (RF) field maps. Compared to conventional methods requiring separation of the transmit RF field (B(1)(+)) from acquired MR images, the proposed method has such advantages as: 1) reduced theoretical error, 2) higher acquisition speed, and 3) flexibility in choice of different transmit and receive RF coils. The method is demonstrated in electrical conductivity and relative permittivity mapping in a salt water phantom, as well as in vivo measurement of brain conductivity in healthy volunteers. The phantom results show the validity and scan-time efficiency of the proposed method applied to a piece-wise homogeneous object. Quality of in vivo EP results was limited by reconstruction errors near tissue boundaries, which highlights need for image segmentation in EP mapping in a heterogeneous medium. Our results show the feasibility of rapid EP mapping from MRI without B(1)(+) mapping.

Time-domain response of the ARIANNA detector

The Antarctic Ross Ice Shelf Antenna Neutrino Array (ARIANNA) is a high-energy neutrino detector designed to record the Askaryan electric field signature of cosmogenic neutrino interactions in ice. To understand the inherent radio-frequency (RF) neutrino signature, the time-domain response of the ARIANNA RF receiver must be measured. ARIANNA uses Create CLP5130-2N log-periodic dipole arrays (LPDAs). The associated effective height operator converts incident electric fields to voltage waveforms at the LDPA terminals. The effective height versus time and incident angle was measured, along with the associated response of the ARIANNA RF amplifier. The results are verified by correlating to field measurements in air and ice, using oscilloscopes. Finally, theoretical models for the Askaryan electric field are combined with the detector response to predict the neutrino signature.

Feasibility of frequency-modulated wireless transmission for a multi-purpose MEMS-based accelerometer.

Recent advances in the Micro Electro-Mechanical System (MEMS) technology have made wireless MEMS accelerometers an attractive tool for Structural Health Monitoring (SHM) of civil engineering structures. To date, sensors' low sensitivity and accuracy—especially at very low frequencies—have imposed serious limitations for their application in monitoring large-sized structures. Conventionally, the MEMS sensor's analog signals are converted to digital signals before radio-frequency (RF) wireless transmission. The conversion can cause a low sensitivity to the important low-frequency and low-amplitude signals. To overcome this difficulty, the authors have developed a MEMS accelerometer system, which converts the sensor output voltage to a frequency-modulated signal before RF transmission. This is achieved by using a Voltage to Frequency Conversion (V/F) instead of the conventional Analog to Digital Conversion (ADC). In this paper, a prototype MEMS accelerometer system is presented, which consists of a transmitter and receiver circuit boards. The former is equipped with a MEMS accelerometer, a V/F converter and a wireless RF transmitter, while the latter contains an RF receiver and a F/V converter for demodulating the signal. The efficacy of the MEMS accelerometer system in measuring low-frequency and low-amplitude dynamic responses is demonstrated through extensive laboratory tests and experiments on a flow-loop pipeline.

Phase stabilized downlink transmission for wideband radio frequency signal via optical fiber link.

In this paper, we propose and demonstrate a phase stabilized wideband downlink transmission scheme, which directly transmits the received radio frequency (RF) signals from remote antennas to central station. A reference RF tone is round-trip transferred between the central station and remote end to obtain the delay variation caused by the fiber link. The delay variation is then used to alter a tunable laser. Since optical carriers with different wavelengths propagate at different velocities in fiber, a tunable optical delay line is realized to cancel the delay variation of the fiber link. The tunable delay range is in proportion to the length of the fiber link, which means a very long delivery distance can be expected. Experimentally, a RF signal at frequency of 2.50 GHz has been downlink transferred through a 45 km fiber link, with stability of 3.3 x 10⁻¹³ at 1 s and 7.5 x 10⁻¹⁷ at 10⁴s.

Comparing like with like: the power of knowing where you are.

Magnetic resonance imaging can now provide human brain images of structure, function, and connectivity with isotropic voxels smaller than one millimeter, and thus much smaller than the cortical thickness. This resolution, achievable in a scan time of less than 1 h, enables visualization of myeloarchitectural layer structure, intracortical variations in functional activity--recorded in changes in blood oxygenation level dependent signal or cerebral blood volume CBV--and intracortical axonal orientational structure via diffusion-weighted magnetic resonance imaging. While recent improvements in radiofrequency receiver coils now enable excellent image data to be obtained at 3T, scanning at the ultra-high field of 7 T offers further gains in signal-to-noise ratio and speed of image acquisition, with a structural image resolution of about 300 μm. These improvements throw into sharp question the strategies that have become conventional for the analysis of functional imaging data, especially the practice of spatial smoothing of raw functional data before further analysis. Creation of a native cortical map for each human subject that provides a reliable individual parcellation into cortical areas related to Brodmann Areas enables a strikingly different approach to functional image analysis. This proposed approach involves surface registration of the cortices of groups of subjects using maps of the longitudinal relaxation time T1 as an index of myelination, and methods for inferring statistical significance that do not entail spatial smoothing. The outcome should be a far more precise comparison of like-with-like cortical areas across subjects, with the potential to greatly increase experimental power, to discriminate activity in neighboring cortical areas, and to enable correlation of function and connectivity with specific cytoarchitecture. Such analyses should enable a far more convincing modeling of brain mechanisms than current graph-based methods that require gross over-simplification of brain activity patterns in order to be computationally tractable.

Optimization of magnetic flux density measurement using multiple RF receiver coils and multi-echo in MREIT

Magnetic Resonance Electrical Impedance Tomography (MREIT) is an MRI method that enables mapping of internal conductivity and/or current density via measurements of magnetic flux density signals. The MREIT measures only the z-component of the induced magnetic flux density B = (Bx, By, Bz) by external current injection. The measured noise of Bz complicates recovery of magnetic flux density maps, resulting in lower quality conductivity and current-density maps. We present a new method for more accurate measurement of the spatial gradient of the magnetic flux density gradient (∇ Bz). The method relies on the use of multiple radio-frequency receiver coils and an interleaved multi-echo pulse sequence that acquires multiple sampling points within each repetition time. The noise level of the measured magnetic flux density Bz depends on the decay rate of the signal magnitude, the injection current duration, and the coil sensitivity map. The proposed method uses three key steps. The first step is to determine a representative magnetic flux density gradient from multiple receiver coils by using a weighted combination and by denoising the measured noisy data. The second step is to optimize the magnetic flux density gradient by using multi-echo magnetic flux densities at each pixel in order to reduce the noise level of ∇ Bz and the third step is to remove a random noise component from the recovered ∇ Bz by solving an elliptic partial differential equation in a region of interest. Numerical simulation experiments using a cylindrical phantom model with included regions of low MRI signal to noise (‘defects’) verified the proposed method. Experimental results using a real phantom experiment, that included three different kinds of anomalies, demonstrated that the proposed method reduced the noise level of the measured magnetic flux density. The quality of the recovered conductivity maps using denoised ∇ Bz data showed that the proposed method reduced the conductivity noise level up to 3–4 times at each anomaly region in comparison to the conventional method.

Multi-channel energy detection under phase noise: analysis and mitigation

For the development of highly integrated, flexible and low-cost cognitive radio (CR) devices, simple transceiver architectures, like direct-conversion receiver, are expected to be deployed and provide viable radio frequency (RF) spectrum sensing solutions for practical implementation. Yet, this can be very challenging task especially if spectrum sensing and down-conversion are conducted over multiple RF channels simultaneously for improved efficiency in channel scans. Then, the so-called dirty RF problem that degrades link performance of traditional transmission systems starts to be influential from spectrum sensing perspective as well. The unavoidable RF impairments, e.g., oscillator phase noise in direct-conversion receiver, could generate crosstalk between multiple channels that are down-converted simultaneously, and thus considerably limit the spectrum sensing capabilities. Most of the existing spectrum sensing studies in literature assume an ideal RF receiver and have not considered such practical RF hardware problem. In this article, we study the impact of oscillator phase noise on energy detection (ED) based spectrum sensing in multi-channel direct-conversion receiver scenario. With complex Gaussian primary user (PU) signal models, we first derive the detection and false alarm probabilities in closed-form expression. The analytical results, verified through extensive simulations, show that the wideband multi-channel sensing receiver is very sensitive to the neighboring channel crosstalk induced by oscillator phase noise. More specifically, it is shown that the false alarm probability of multi-channel energy detection increases significantly, compared to the ideal RF receiver case. The exact performance degradation depends on the power of neighboring channels as well as statistical characteristics of the phase noise in the deployed receiver. In order to prevent such performance degradation in spectrum identification, an enhanced energy detection technique is proposed. The proposed technique calculates the leakage power from neighboring channels for each channel and improves the sample energy statistics by subtracting this leakage power from the raw values. An analytical expression is derived for the leakage power which is shown to be a function of power spectral levels of neighboring channels and 3-dB bandwidth of phase noise process. Practical schemes for estimating these two quantities are discussed. Extensive computer simulations show that the proposed enhanced detection yields false alarm rates that are very close to those of an ideal RF receiver and hence clearly outperforms classical energy detection.

Optical Signal Processor for Millimeter-Wave Interferometric Radiometry

In interferometric radiometry, the correlations between all pairs of radio-frequency (RF) receivers must be performed in order to obtain the scene visibilities. This represents a cumbersome problem in passive imaging where large signal bandwidths are typically required to achieve fast-acquisition times and improve the radiometric resolution of the image. In this case, the signal distribution and the correlation at intermediate frequencies require very fast signal acquisition and processing subsystems. This paper presents a technique to perform correlations of millimeter-wave signals produced by thermal emission in the optical domain as a solution to the aforementioned problem. The proposed method is based on converting the RF signal to the optical domain by modulating a laser beam with the RF signal using a LiNbO3 phase modulator. This conversion allows to perform the signal distribution in the optical domain to obtain the combination of the receiver pairs. The correlation is obtained by measuring the power of the photocurrent produced by photodetecting the combined signal. The results of an experimental validation consisting on the acquisition of passive images using a linear interferometric array are presented to support the feasibility of the method. In addition, performance considerations of the system have been developed and validated by calculating the standard deviation of a visibility measurement.

Passive Radio-Frequency Repeater for Enhancing Signal Reception and Transmission in a Wireless Charging Platform

This paper reports new results of an emerging field that combines radio-frequency (RF) repeater design and wireless power technologies for charging applications. Power electronics-based wireless charging pads have recently been developed for charging portable electronic devices such as mobile phones. However, the electromagnetic shield underneath the pad could reduce transmission and reception signals. In this paper, a novel passive RF repeater for a wireless charging platform is proposed and investigated for mitigating the signal blocking problem. Since the effective shielding structure for the low-frequency charging signal is not RF friendly, the received RF signal of a wireless mobile device can be reduced when the phone is placed on a wireless charging platform. A dual-polarization free-positioning passive RF signal repeater has been successfully designed and implemented. It can improve the RF reception signal strength by at least 3.6 dB without noticeable reduction in energy efficiency in the charging process.

Precision Agriculture: RTK Base-to-Tractor Range Limitations Using RF Communication

<abstract> <sc><bold>Abstract.</bold> </sc>Precision agriculture is heavily dependent upon Real-Time Kinematic (RTK) technology for real-time field positioning. To reduce RTK equipment costs, producers can access a shared reference station rather than purchasing portable units. However, those tractor operators that utilize a shared station may experience positioning failures while operating in remote outlying fields. These failures may be due to sporadic radio frequency (RF) communication links with the station. To examine possible RF communication issues impacting precision agriculture, a test was conducted employing a tractor-based RTK system mounted on a road vehicle. The instrumented vehicle was used to traverse the highways around two commercial RTK reference towers. A GIS viewshed analysis (line-of-sight, LOS) based upon tower distance and terrain blockage from landforms and woodlands was performed. Percentage results were compared to a comprehensive radio frequency path design tool (Pathloss Ver. 4), which is a commercial simulation model for radio links that operate in the frequency range of 30 MHz to 100 GHz. Highway tests of RTK base station reception show the Pathloss (PL) model predicted by percent the data fit of both high and low probability areas. The PL model prediction percentages were found slightly more robust in its prediction of RF reception than the LOS model. Both models performed better at predicting areas of reception than of non-reception. The PL model had higher inaccurate prediction percentages of reception regions, whereas the LOS model had higher inaccurate prediction percentages of non-reception regions. Base-to-tractor separation distances, forestation, and terrain blockages were the primary factors for maintaining the tractor RF link with the base station. As a low-cost tool, the site-specific success in communicating with a reference base station can be determined by using the positioning NMEA output from the RTK receiver plotted within Google Earth.

Energy Detection under IQ Imbalance with Single- and Multi-Channel Direct-Conversion Receiver: Analysis and Mitigation

Direct-conversion radio receivers can offer highly integrated low-cost hardware solutions for cognitive radio (CR) devices. Such receivers are, however, also very sensitive to various radio frequency (RF) impairments such as IQ imbalance, which can considerably limit the spectrum sensing capabilities. Most of the existing spectrum sensing studies in literature assume an ideal RF receiver and hence neglect the impacts of such practical RF hardware limitations. In this article, we study energy detection (ED) based spectrum sensing in both single-channel and multi-channel direct-conversion receiver scenarios impaired by IQ imbalance. With complex Gaussian primary user (PU) signal models, we first derive the detection and false alarm probabilities in closed-form for both receiver scenarios. The analytical results, confirmed through extensive simulations, show that while the single-channel receiver scenario is fairly robust to IQ imbalance, the wideband multi-channel sensing receiver is very sensitive to the image channel crosstalk induced by IQ imbalance. More specifically, it is shown that the false alarm probability of multi-channel energy detection increases significantly, compared to ideal RF receiver case, and the exact performance depends on the image channel power level and IQ imbalance values. In order to prevent such degradation in the ability to identify free spectrum, a waveform level interference cancellation method is then proposed to mitigate the image channel crosstalk. The optimum cancellation coefficient yielding interference-free signal is first derived, being also complemented with a practical coefficient sample estimator. An explicit condition is also derived for which the proposed cancellation scheme deploying the practical coefficient sample estimator provides performance gain in the sensing decisions compared to uncompensated energy detection. Extensive computer simulations with various signal and imbalance conditions are provided which demonstrate that the proposed enhanced energy detection can suppress the image channel crosstalk efficiently, yielding detection and false alarm probabilities essentially identical to those of an ideal RF receiver.

Filling the Spectral Holes: Novel\/Future Wireless Communications and Radar Receiver Architectures

The requirement of wireless access by novel telecommunications and remote-sensing applications, such as Internet-in-mobile services and ultrawideband (UWB) radar systems, is continuously growing in western society but also in countries with big and emerging economies such as China, Brazil, India, and even some African nations. As a consequence, the radiofrequency (RF) spectrum is becoming a very valuable but scarce natural resource. In relation to this, it is well known that some portions of the RF spectrum, such as those assigned to military and emergency services, remain underutilized. These spectral holes, usually referred as white spaces, then become excellent opportunities for new wireless communications and radar systems to operate. Nevertheless, the necessary dynamic access to properly exploit these free spectral holes can only be performed through very sophisticated and reliable receiver architectures. They must be capable of sensing very broad spectrum ranges while assuring a minimum quality for the received signals. This article addresses modern RF receiver schemes for sensing widebands in communications and radar scenarios.

Growing Graphene Receivers

Engineering![Figure][1] CREDIT: S. HAN ET AL., NATURE COMMUNICATIONS 5 , 3086 (30 JANUARY 2014) © 2014 NATURE PUBLISHING GROUP One use of the extremely high conductivity of graphene is in radio-frequency (RF) applications. However, the devices they use require specialized fabrication methods to avoid damaging the active graphene channel layer. Conventional fabrication of practical RF devices starts by placing the channel material on a silicon substrate and then fabricating other passive device components on top of it, using deposition steps that can involve high temperatures that can degrade the device performance. Han et al. report on a fabrication scheme for a graphene RF receiver that first assembles the passive elements on a silicon substrate. After metal lines were fabricated, atomic layer deposition was used to deposit a thin alumina gate dielectric layer, and the active graphene was grown through a chemical vapor deposition process. None of the processing steps required temperatures in excess of 400°C. The final device, which contains three GFETs transistors, four inductors, two capacitors, and two resistors in a 0.6-mm-square area, operates at 4.3 gigahertz and could receive and restore digital text transmitted with that carrier frequency. Nat. Commun. 10.1038/ncomms4086 (2014). [1]: pending:yes

Observability Analysis of Collaborative Opportunistic Navigation With Pseudorange Measurements

The observability analysis of a collaborative opportunistic navigation (COpNav) environment whose states may be partially known is considered. A COpNav environment can be thought of as a radio frequency (RF) signal landscape within which one or more RF receivers locate themselves in space and time by extracting and, possibly, sharing information from ambient signals of opportunity (SOPs). These receivers, whether vehicle mounted or integrated into handheld devices, exploit signal diversity to improve navigation and timing robustness compared with stand-alone Global Positioning System (GPS) receivers in deep urban, indoor, or, otherwise, GPS-hostile environments. Available SOPs may have a fully known, partially known, or unknown characterization. In this paper, the receivers are assumed to draw only pseudorange-type measurements from the SOPs. Separate observations are fused to produce an estimate of each receiver's position, velocity, and time (PVT). Since not all SOP states in the COpNav environment may be known a priori, the receivers must estimate the unknown SOP states of interest simultaneously with their own PVT. This paper establishes the minimal conditions under which a COpNav environment consisting of multiple receivers and multiple SOPs is completely observable. Moreover, in scenarios where the COpNav environment is unobservable, the unobservable directions in the state space are specified. Simulation and experimental results are presented to confirm the theoretical observability conditions.

Graphene radio frequency receiver integrated circuit

Graphene has attracted much interest as a future channel material in radio frequency electronics because of its superior electrical properties. Fabrication of a graphene integrated circuit without significantly degrading transistor performance has proven to be challenging, posing one of the major bottlenecks to compete with existing technologies. Here we present a fabrication method fully preserving graphene transistor quality, demonstrated with the implementation of a high-performance three-stage graphene integrated circuit. The circuit operates as a radio frequency receiver performing signal amplification, filtering and downconversion mixing. All circuit components are integrated into 0.6 mm(2) area and fabricated on 200 mm silicon wafers, showing the unprecedented graphene circuit complexity and silicon complementary metal-oxide-semiconductor process compatibility. The demonstrated circuit performance allow us to use graphene integrated circuit to perform practical wireless communication functions, receiving and restoring digital text transmitted on a 4.3-GHz carrier signal.