Phase Offsets Research Articles

Coating film and asymmetric amplitude difference effects on measurement performance in phase grating interference-based metrology

In this study, the effects of the coating film thickness on a grating interferometry system were analyzed. The asymmetry-induced error of the deformed phase grating varied periodically with the coating film thickness, due to the spurious interference of multiple reflections in the film layer and asymmetry in the amplitudes of the diffraction orders, which led to phase offsets (and thus position errors). The average error obtained with a multiorder interferometer was compared to that simulated by atomic force microscopy. The simulation and measurement results were consistent with the theoretical analysis, which will facilitate accurate measurement error analysis.

Connected Vehicle-Based Traffic Signal Coordination

Abstract This study presents a connected vehicles (CVs)-based traffic signal optimization framework for a coordinated arterial corridor. The signal optimization and coordination problem are first formulated in a centralized scheme as a mixed-integer nonlinear program (MINLP). The optimal phase durations and offsets are solved together by minimizing fuel consumption and travel time considering an individual vehicle’s trajectories. Due to the complexity of the model, we decompose the problem into two levels: an intersection level to optimize phase durations using dynamic programming (DP), and a corridor level to optimize the offsets of all intersections. In order to solve the two-level model, a prediction-based solution technique is developed. The proposed models are tested using traffic simulation under various scenarios. Compared with the traditional actuated signal timing and coordination plan, the signal timing plans generated by solving the MINLP and the two-level model can reasonably improve the signal control performance. When considering varies vehicle types under high demand levels, the proposed two-level model reduced the total system cost by 3.8% comparing to baseline actuated plan. MINLP reduced the system cost by 5.9%. It also suggested that coordination scheme was beneficial to corridors with relatively high demand levels. For intersections with major and minor street, coordination conducted for major street had little impacts on the vehicles at the minor street.

Thermospheric gravity wave characteristics in the daytime over low-latitudes during geomagnetic quiet and disturbed conditions

The importance of gravity waves (GWs) in the redistribution of energy and momentum in the Earth's atmosphere is well known. It is extremely important to understand the nature of variation in the GW characteristics to address issues of coupling in the Earth's atmosphere. We have used digisonde measurements to investigate the variation in thermospheric wave dynamics in the daytime over low-latitudes (Ahmedabad, India) during geomagnetic quiet and disturbed times. Vertical phase speeds and wavelengths of GWs are calculated by monitoring the phase offsets in the digisonde derived height variations of constant electron densities throughout the day. During geomagnetic quiet times, the magnitudes of vertical propagation speeds of GWs and their vertical wavelengths show seasonal variation and vary in the range of 10–70 ms−1 and 70–520 km, respectively. The distributions of vertical propagation parameters of GWs for geomagnetic quiet days show two peaks annually and are anti-correlated with the thermospheric horizontal winds. As these winds over a given location flow in different directions in different seasons, this information can be used to infer the season dependent preferential direction of propagation of GWs. Changes in the vertical propagation characteristics of GWs during geomagnetic disturbed days as compared to their average quiet time values show good semblance with the integrated values of the AE index. A linear relation has been obtained between the changes in vertical phase speeds of GWs and the integrated values of AE index during geomagnetic disturbed days. This linear relation, in combination with empirical formulation arrived at for averaged values of vertical propagation speeds during geomagnetic quiet times, yields comprehensive information on propagation characteristics of GWs as a function of day of the year that includes geomagnetic disturbance conditions as well. Such systematic information on the GW characteristics in the daytime thermosphere and their quantification during geomagnetic quiet times and their changes due to enhancement in energy input at high-latitudes (as characterized by the AE index) also provides a strong and direct information on high-to low-latitude coupling.

Simultaneous thickness variation and surface profiling of glass plates using Fizeau interferometer with elimination of offset phase error

Fringe analysis using phase modulation through wavelength scanning is extensively utilized for profiling the surface and thickness variation of glass plates. However, the nonlinearity of the phase-modulation error during wavelength scanning causes an offset error in the calculated phase. Therefore, in this research, two types of 19-frame phase-extraction formulas were derived for simultaneous profiling of the thickness variation and surface of a glass plate, focusing on eliminating the offset phase error. These new formulas were visualized in the frequency domain using the Fourier description. The ability of these formulas to eliminate the offset error was confirmed through numerical error analysis. Finally, the surface and thickness variation of a glass plate were simultaneously profiled using the large-aperture Fizeau interferometer and the new 19-frame formulas. The standard deviations obtained for thickness variation and surface profiling were 4.687 and 14.421 nm, respectively, which were lower than those obtained when using other phase-extraction formulas.

Photonics-Based Microwave Phase Detector With Reduced DC Offset and Phase Offset

A cascaded optical modulator topology for detecting the phase difference of two microwave signals is presented. It operates based on mixing of two microwave signals. The phase difference of two microwave signals can be obtained from the system output DC voltage. The phase detector is designed to eliminate the problems of DC offset and phase offset, which are present in reported structures. Experimental results demonstrate phase detection of two microwave signals with a 0° to 180° phase difference, less than ±3° phase detection errors and less than 1 mV DC offset. Results also demonstrate the phase detector can be operated over a wide frequency range.

Hydrodynamics of tandem flapping pectoral fins with varying stroke phase offsets

We show how phasing between tandem bioinspired fins flapping at high-stroke amplitudes modulates rear fin thrust production and wake characteristics. Load cell thrust measurements show that the rear fin generates 25% more thrust than the front fin when it lags the latter by a quarter cycle, and performs 8% worse when it leads the front fin by the same amount. The flow interactions between the fins responsible for these observations are analyzed using two-dimensional particle image velocimetry measurements and three-dimensional computational fluid dynamics simulations. Distributions of velocity elucidate variations in the effective flow induced on the rear fin for different phase offsets. Vortex structure interactions and particle rakes reveal the contributions of the leading- and trailing-edge vortices shed by the front fin in modulating the suction at the rear fin leading edge. Furthermore, the wake structure far downstream of the fins changes in its coherence, axial and radial extents for the different phase offsets. These findings are relevant for the design and performance optimization of various unmanned underwater vehicles that utilize such tandem systems.

RF-SML: A SAR-Based Multi-Granular and Real-Time Localization Method for RFID Tags

With the rapid development of the Internet of Things (IoT) technology, location based service in context awareness has received increasing attention. As one of the main localization technologies, UHF RFID technology has been widely used in many fields of life and industry due to its advantages. In this article, we introduce a RFID-based system RF-SML, which is a method for quickly and accurately locating static objects via the tag and mobile reader. Specifically, the method utilizes the idea of multi-granularity in order to find the high-probability region of the target position by reconstructing the reflection coefficient of the scene in the coarse-grained localization stage. Subsequently, in the fine-grained localization stage, the grid is traversed in this area to calculate the corresponding evaluation factor to determine the final position result, thereby reducing the time-consuming of localization calculation. At the same time, it uses phase calibration to remove the phase offsets that are caused by the hardware device and the antenna phase center, thereby obtaining higher localization accuracy. We conduct experiments to verify the performance of RF-SML with commercial-off-the-shelf (COTS) RFID equipment. The results show that the proposed method can efficiently achieve the centimeter-level positioning of objects.

Experimental Demonstration of Thrust Vectoring with a Vertical Axis Wind Turbine using Normal Load Measurements

The aerodynamic loading of a vertical axis wind turbine varies with the azimuth position of the blades. The thrust of the VAWT can be computed as a decomposition of the normal force on each of the blades. By varying the blade loading as a function of turbine azimuth, it is possible to vary the direction of the average thrust of the turbine. An experiment is performed using an active pitch controlled H-VAWT turbine in the Open Jet Facility at TU Delft demonstrating the ability to actively vary the rotor aerodynamic loading and as a result the average thrust vector. By applying a sinusoidal pitch actuation with phase offsets, a directional change in the average thrust vector of over 78° was demonstrated.

Study of the effect of the offset phase in time-delay electro-optical systems.

We show that the effect of the offset phase on the dynamics of the time-delay optoelectronic oscillators that is observed experimentally can be explained in terms of switching between the subcritical and supercritical Hopf bifurcations. The domains of the offset phase for which the system functions are determined analytically. We also show that the width of these domains exceptionally depends on the interplay between the three time scales of the system. Our theoretical results fit with the experimental measurements.

Wireless Subnanosecond RF Synchronization for Distributed Ultrawideband Software-Defined Radar Networks

In this article, we present a distributed and decentralized synchronization algorithm for wireless sensor networks (WSNs). The proposed method achieves subnanosecond synchronization using low-cost commercial-off-the-shelf (COTS) Universal Software Radio Peripheral (USRP) software-defined radios (SDRs) and is implemented entirely in software without the need for custom hardware or atomic clocks. In an N sensor network, the proposed protocol results in each sensor having full knowledge of baseband clock offsets, RF carrier phase offsets, and pairwise RF time of flight to subnanosecond precision for the entire network after 2N total transmissions, making this method efficiently extendible to larger sensor networks. The method is decentralized and does not rely on a hierarchical master-slave structure, making it robust to sensor dropout in contested or harsh environments. The proposed methodology is validated in simulation and tested in field experiments using a three-sensor network. This work has a wide range of applications, including transmit (TX) beamforming, distributed sensor localization, and coherent multistatic/multiple-input-multiple-output (MIMO) radar imaging for autonomous sensor swarms.

Changes in the daytime thermospheric gravity wave propagation characteristics over low-latitudes in response to the variation in solar flux

Gravity waves play an important role in the propagation of energy and momentum from the lower atmospheric regions into the upper atmosphere. Depending on the varying dissipation conditions, these waves generated in the lower atmosphere can propagate higher into the atmosphere. As the neutrals and plasma share the same volume in the upper atmosphere, these waves affect both these densities as they propagate. In our analysis, using the information on the variations in phase offsets at different heights corresponding to isoelectron density contours at several defined transmission frequencies of digisonde, vertical phase propagation speeds, and vertical scale sizes of gravity waves in the daytime thermosphere have been estimated. Investigation of gravity waves by radio measurements, unlike the conventional optical techniques, which are limited to clear sky conditions alone, enable an unhindered study of gravity wave dynamics in all weather conditions. We have found a very good semblance between the variations in derived vertical phase speeds of gravity waves in the thermosphere and Solar F10.7cm flux. Further, gravity wave activity, i.e., the number of gravity waves present in the thermosphere in daytime increases with increasing Solar flux. We quantify these inter-relationships and present linear relations of gravity wave activity and their propagation speeds as a function of variation in Solar flux.

In-Flight Multichannel Calibration for Along-Track Interferometric Airborne Radar

Multichannel calibration is essential for detecting moving targets and for estimating their positions and velocities accurately. This article presents a fast and efficient calibration algorithm for the along-track multichannel systems, in particular for space-time adaptive processing (STAP) techniques. The proposed algorithm corrects the phase and magnitude offsets of the receive channels and also takes into account the Doppler centroid variation (e.g., caused by atmospheric turbulences) along the slant range and the azimuth time. The knowledge of the Doppler centroid variation is especially important for an accurate clutter covariance matrix estimation, which is required by STAP for efficient clutter suppression. Important calibration parameters and offsets are estimated directly from the range-compressed training data. The proposed algorithm is evaluated based on real multichannel X-band radar data acquired with DLR's airborne system F-SAR and compared with the state-of-the-art digital channel balancing technique. The experimental results show the potential of the proposed calibration algorithm toward real-time applications.

I/Q Imbalance Compensation in Wideband Millimeter-Wave Transmitters Using a Single Undersampling ADC

In this paper, a new method to identify and compensate for the I/Q imbalance in wideband millimeter-wave transmitters using a single undersampling analog-to-digital converter (ADC) is proposed. The proposed method alleviates the need for an I/Q demodulator; hence, simplifying the structure and calibration procedure of the required transmitter-observation receiver (TOR). In addition, an algorithm for estimating the delay and phase offsets between the transmitter and the single-ADC TOR is devised. The presented algorithm improves upon the exhaustive search methods employed in the literature. Measurement results using an off-the-shelf I/Q modulator, driven by an 800 MHz test signal, show about a 5% reduction in the root normalized mean-square error (RNMSE) of its output signal when the proposed method is utilized. Further measurements were conducted to assess the performance of this method, in conjunction with digital predistortion (DPD), when a power amplifier (PA) is added to the transmitter chain. When compared with the application of DPD only, the I/Q imbalance correction allowed additional reductions of about 4% in the RNMSE and 1.5 dB in the adjacent-channel power ratio of the PA output signal. These improvements were obtained with ADC sampling rates as low as 20 times below the Nyquist rate.

The Influence of the Nonideal Phase Offset on SAR-Based Localization in Passive UHF RFID

The existing radio frequency identification (RFID) localization methods typically regard the initial phase of the signal as constant. This is feasible when the antenna and the tag are stationary. However, in synthetic aperture radar (SAR) RFID localization, the relative motion of the reader antenna and tag changes the antenna phase center and tag orientation, thus resulting in a continuous change of the initial phase. This change, which will cause the nonideal phase offset (NPO) and, thus, have an adverse effect on positioning, has been neglected in most previous SAR RFID studies. In this article, we investigate the antenna phase uncertainty and the tag orientation-dependent phase offset and analyze their influence on localization accuracy. To the best of our knowledge, this is the first time that the initial phase is regarded as a variable in SAR RFID localization. In addition, a novel motion model-based localization (MoLoc) method is proposed, which exploits the relative motion model and visualizes the influence of NPO more intuitively. Compared with the conventional grid-matching methods, MoLoc shows higher accuracy and much lower computation time. Both the influence of NPO and the performance of MoLoc are verified by experiments implemented in real scenario. The experiments also evaluate the influence of NPO on the grid-matching method. The experimental results show that NPO has a significant impact on the accuracy of SAR-based localization methods.

Correcting frequency and phase offsets in MRS data using robust spectral registration.

An algorithm for retrospective correction of frequency and phase offsets in MRS data is presented. The algorithm, termed robust spectral registration (rSR), contains a set of subroutines designed to robustly align individual transients in a given dataset even in cases of significant frequency and phase offsets or unstable lipid contamination and residual water signals. Data acquired by complex multiplexed editing approaches with distinct subspectral profiles are also accurately aligned. Automated removal of unstable lipid contamination and residual water signals is applied first, when needed. Frequency and phase offsets are corrected in the time domain by aligning each transient to a weighted average reference in a statistically optimal order using nonlinear least-squares optimization. The alignment of subspectra in edited datasets is performed using an approach that specifically targets subtraction artifacts in the frequency domain. Weighted averaging is then used for signal averaging to down-weight poorer-quality transients. Algorithm performance was assessed on one simulated and 67 in vivo pediatric GABA-/GSH-edited HERMES datasets and compared with the performance of a multistep correction method previously developed for aligning HERMES data. The performance of the novel approach was quantitatively assessed by comparing the estimated frequency/phase offsets against the known values for the simulated dataset or by examining the presence of subtraction artifacts in the in vivo data. Spectral quality was improved following robust alignment, especially in cases of significant spectral distortion. rSR reduced more subtraction artifacts than the multistep method in 64% of the GABA difference spectra and 75% of the GSH difference spectra. rSR overcomes the major challenges of frequency and phase correction.

Blind Modulation Classification for Asynchronous OFDM Systems Over Unknown Signal Parameters and Channel Statistics

Blind modulation classification (MC) is an integral part of designing an adaptive or intelligent transceiver for future wireless communications. However, till date, only a few works have been reported in the literature for blind MC of orthogonal frequency division multiplexing (OFDM) system over frequency-selective fading environment. In this paper, a blind MC algorithm has been proposed and implemented over National Instruments (NI) testbed setup for linearly modulated signals of OFDM system by using discrete Fourier transform (DFT) and normalized fourth-order cumulant. The proposed MC algorithm works in the presence of synchronization errors, i.e., frequency, timing, and phase offsets and without the prior information about the signal parameters and channel statistics. To nullify the effect of timing offset in the feature extraction process, a statistical average has been taken over OFDM symbols after introducing uniformly distributed random timing offsets in each of the OFDM symbols. In this work, we have classified a more extensive pool of modulation formats for OFDM signal, i.e., binary phase-shift keying (BPSK), quadrature PSK (QPSK), offset QPSK (OQPSK), minimum shift keying (MSK), and 16 quadrature amplitude modulation (16-QAM). Classification is performed in two stages. At the first stage, a normalized fourth-order cumulant is used on the DFT of the received OFDM signal to classify OQPSK, MSK, and 16-QAM modulation formats. At the second stage, first we compute the DFT of the square of the received OFDM signal and then a normalized fourth-order cumulant is used to classify BPSK and QPSK modulation formats. The success rate of the proposed MC algorithm is evaluated through analytical and Monte Carlo simulations and compared with existing methods. Finally, the work is validated by providing an experimental setup on NI hardware over an indoor propagation environment.

Compensation of Phase Errors for Spotlight SAR With Discrete Azimuth Beam Steering Based on Entropy Minimization

Spotlight synthetic aperture radar (SAR) achieves very high-resolution (VHR) images by steering the azimuth beam during the formation of the synthetic aperture. In practice, the steering is implemented through discrete azimuth beam switching. Then, phase shifts can occur between the adjacent beams due to the error of the antenna pattern. In addition, the troposphere introduces a beam-angle-dependent delay to the echo. Those undesired phase shifts and delays cause phase errors in the received echo and result in image quality deterioration. In this letter, an algorithm, called the Newton entropy minimization (N-EM), is proposed to estimate and compensate the phase errors caused by the discrete azimuth beam steering for the spotlight SAR data. Combining with the subaperture imaging approach of the spotlight SAR, the algorithm estimates the phase offsets between each couple of the adjacent beams based on the minimum entropy criterion. The analytic expression, which is a nonlinear equation, is developed for the optimal estimation. Then, the Newton's method is employed to solve the equation. The real spaceborne SAR (both the staring and sliding spotlight modes) data processing results demonstrate the efficiency and accuracy of the proposed algorithm.

The Multiple Mechanisms of Spatially Discordant Alternans in the Heart

Cardiac alternans, referring to beat-to-beat alternations to the action potential (AP) duration (APD) of cardiomyocytes (Fig. 1), are associated with contractile dysfunction and arrhythmogenesis in multiple disease conditions. In tissue, these cellular phenomena can manifest as spatially concordant alternans (SCA), wherein all regions alternate with the same phase, or spatially discordant alternans (SDA), wherein different regions alternate with opposite (period-2; Fig. 1) or otherwise offset (higher period) phase. Whereas SCA can directly affect cardiac output (mechanical force can alternate strong/weak associated with long/short APD), SDA present also the possibility for transition to potentially fatal arrhythmias (1).

A Soft-Output STBC Decoder for Aeronautical Telemetry

Alamouti encoding is a well-known space time block encoding technique used to improve the received signal quality in Rayleigh fading channels. In aeronautical telemetry, this encoding technique is applied to shaped offset quadrature phase shift keying tier generation (SOQPSK-TG) modulation in order to handle the two-antenna issue. It is provided for in telemetry-related IRIG standards. In this paper, we propose a unique decoding architecture for Alamouti-encoded SOQPSK-TG signals, taking advantage of pulse amplitude modulation decomposition with soft and hard outputs. We exploit this result to obtain a Viterbi algorithm (VA) for hard decoding and a soft output Viterbi algorithm (SOVA) for soft and hard decoding, with a twofold benefit: operation using one trellis structure, unlike decoders that are based on the 8-waveforms cross-correlated trellis-coded quadrature modulation (XTCQM) approximation, and very attractive bit error rate performance, as well as a complexity trade-off

Independent Component Analysis for Phase and Residual Frequency Offset Compensation in OQAM Multicarrier Systems

Phase noise in the offset quadrature amplitude modulation (OQAM) multicarrier system results in not only constellation rotation but also crosstalk from the unique intrinsic imaginary interference (IMI). Therefore, the method for phase and residual frequency offset (RFO) compensation should be designed specifically to address this. In this article, we exploit the statistical difference of the OQAM signal and the IMI, and propose a novel independent component analysis (ICA) based method for phase and RFO compensation. It is proved that the signal exhibits the minimal entropy with the probability distribution deviating from the Gaussian one the most when the phase is correctly compensated. Several metrics and a recursive algorithm are proposed to separate the signal and the IMI. Simulations and experiments are performed to verify the proposed theory and to compare the ICA method with modified blind phase search (M-BPS), constellation classification (CC), and Kalman filtering (KL). It is shown that the ICA method exhibits significantly better tolerance to the laser linewidth and RFO than CC and KL, and greatly reduces the complexity compared to M-BPS. Therefore, the proposed ICA method can be the most promising solution for phase and RFO compensation in OQAM multicarrier systems.