Phase Correction Scheme Research Articles

Diabatization around Conical Intersections with a New Phase-Corrected Valence-Bond-Based Compression Approach.

In the present work, the valence-bond-based compression approach for diabatization (VBCAD), previously presented in the literature [J. Phys. Chem. Lett. 2020, 11, 5295-5301] in the case of avoided crossings, is extended to the more general situation of conical intersections and their vicinity. A pointwise phase-correction scheme for diabatic states is proposed, based on the explicit use of the peculiarities of the nonorthogonality of ab initio valence bond (VB) theory. Rather than fitting or propagating nonadiabatic couplings, it allows us to determine the phase of diabatic states consistently and automatically at each geometry point. Moreover, it is shown that the undetermination of degenerate states around a conical intersection can be fixed naturally from a straightforward classical VB picture. These are illustrated with two prototypical symmetry-induced (Jahn-Teller) conical intersection models.

A Phase Calibration Method for Millimeter-Wave Up-Converter Using Electro-Optic Sampling

A phase measurement method using electro-optic sampling of millimeter-wave 4-tone signals is presented for phase calibration of frequency converters. Phase lock loop (PLL) circuits with high comparison frequencies of 2.6 and 15 GHz are used to synchronize the millimeter-wave 4-tone signal with a short optical pulse for low-jitter equivalent time sampling. A 4-tone separate detection method using a lock-in amplifier is applied for precise phase measurement with high frequency resolution. In addition, a phase-correction scheme using 4-tone signals with different frequency spacings is introduced for wideband phase calibration. The error vector magnitude (EVM) of the 10.2-Gbaud quadrature phase shift keying (QPSK) signal with 292.5-GHz carrier frequency is evaluated to confirm the effectiveness of the wideband phase calibration.

Stable RF transmission in dynamic phase correction with Rayleigh backscattering noise suppression

Abstract We propose a dynamic phase correction scheme for stable radio frequency (RF) transmission over fiber based on the phase-locked loop (PLL). Rayleigh backscattering noise is suppressed by separating the frequency of counter-propagating RF signals. We apply the optical carriers of the same central wavelength to the system. In this way, the effect of temperature induced group velocity dispersion (TIGVD) is effectively reduced and the symmetry of the round-trip transmission is improved. To eliminate fiber-induced phase variations during the round-trip transfer, the error signal drives the PLL structure to dynamically pre-compensate the phase of the outgoing 2.4 GHz signal. The stable frequency distribution over 80 km optical fiber with a relative frequency stability of 3.4E−17 at 10000 s is achieved, which is close to the noise floor.

CSR-Net: A Novel Complex-Valued Network for Fast and Precise 3-D Microwave Sparse Reconstruction

Since the compressed sensing (CS) theory broke through the limitation of the traditional Nyquist sampling theory, it has attracted extensive attention in the field of microwave imaging. However, in 3-D microwave sparse reconstruction application, conventional CS-based algorithms always suffer from huge computational cost. In this article, a novel 3-D microwave sparse reconstruction method based on a complex-valued sparse reconstruction network (CSR-Net) is proposed, which converts complex number operations into matrix operations for real and imaginary parts. Using the unfolding + network approximate scheme, each iteration process of CS-based iterative threshold optimization is designed as a block of CSR-Net, and a modified shrinkage term is introduced to improve the convergence performance of the approach. In addition, CSR-Net adopts a convolutional neural network module to replace a nonlinear sparse representation process, which dramatically reduces computational complexity and improves reconstruction performance over conventional CS-based iterative threshold optimization algorithms. Then, we divide the 3-D scene into a series of 2-D slices, and a phase correction scheme is adopted to ensure that the whole 3-D scene can be reconstructed with measurement matrix of a slice. Moreover, an efficient position–amplitude–random training method without additional real-measured data is employed for the proposed network, which effectively train the CSR-Net without enough real-measured data. Extensive experiment results demonstrate that CSR-Net outperforms both conventional iterative threshold optimization methods and deep network-based ISTA-NET-plus large margins. Its speed and reconstruction accuracy in 3-D imaging can achieve a state-of-the-art level.

Phase-matched virtual coil reconstruction for highly accelerated diffusion echo-planar imaging

PurposeTo propose a virtual coil (VC) acquisition/reconstruction framework to improve highly accelerated single-shot EPI (SS-EPI) and generalized slice dithered enhanced resolution (gSlider) acquisition in high-resolution diffusion imaging (DI). MethodsFor robust VC-GRAPPA reconstruction, a background phase correction scheme was developed to match the image phase of the reference data with the corrupted phase of the accelerated diffusion-weighted data, where the corrupted phase of the diffusion data varies from shot to shot. A Gy prewinding-blip was also added to the EPI acquisition, to create a shifted-ky sampling strategy that allows for better exploitation of VC concept in the reconstruction. To evaluate the performance of the proposed methods, 1.5 mm isotropic whole-brain SS-EPI and 860 μm isotropic whole-brain gSlider-EPI diffusion data were acquired at an acceleration of 8–9 fold. Conventional and VC-GRAPPA reconstructions were performed and compared, and corresponding g-factors were calculated. ResultsThe proposed VC reconstruction substantially improves the image quality of both SS-EPI and gSlider-EPI, with reduced g-factor noise and reconstruction artifacts when compared to the conventional method. This has enabled high-quality low-noise diffusion imaging to be performed at 8–9 fold acceleration. ConclusionsThe proposed VC acquisition/reconstruction framework improves the reconstruction of DI at high accelerations. The ability to now employ such high accelerations will allow DI with EPI at reduced distortion and faster scan time, which should be beneficial for many clinical and neuroscience applications.

Stable radio frequency dissemination in a multi-access link based on passive phase fluctuation cancellation

We propose and experimentally demonstrate a novel scheme of stable radio frequency (RF) dissemination in a multi-access radio-over-fiber (ROF) link. Unlike the active compensation scheme based on a phase-locking loop, the proposed scheme provides fast responses and infinite dynamic ranges with a passive phase fluctuation cancellation (PPFC) approach. In our experiment, a 2.5-GHz signal is steadily distributed to a remote end through an arbitrary user site in a 25-km optical fiber link. The stability of the recovered signals at the user site and the remote end are evaluated. By using the phase correction scheme, the peak–peak phase excursion is reduced from 140 to 4 ps at the user site and 160 to 4 ps at the remote end, respectively. The achieved relative residual frequency instabilities are 1.4117×10−16∕104s and 1.6503×10−16∕104s at the user site and the remote end, respectively.

Correction of phase errors in quantitative water-fat imaging using a monopolar time-interleaved multi-echo gradient echo sequence.

To propose a phase error correction scheme for monopolar time-interleaved multi-echo gradient echo water-fat imaging that allows accurate and robust complex-based quantification of the proton density fat fraction (PDFF). A three-step phase correction scheme is proposed to address a) a phase term induced by echo misalignments that can be measured with a reference scan using reversed readout polarity, b) a phase term induced by the concomitant gradient field that can be predicted from the gradient waveforms, and c) a phase offset between time-interleaved echo trains. Simulations were carried out to characterize the concomitant gradient field-induced PDFF bias and the performance estimating the phase offset between time-interleaved echo trains. Phantom experiments and in vivo liver and thigh imaging were performed to study the relevance of each of the three phase correction steps on PDFF accuracy and robustness. The simulation, phantom, and in vivo results showed in agreement with the theory an echo time-dependent PDFF bias introduced by the three phase error sources. The proposed phase correction scheme was found to provide accurate PDFF estimation independent of the employed echo time combination. Complex-based time-interleaved water-fat imaging was found to give accurate and robust PDFF measurements after applying the proposed phase error correction scheme. Magn Reson Med 78:984-996, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Optical parametric mixer-based passive phase correction for stable transfer of millimeter waves.

We propose to use a parametric mixer in a passive phase correction scheme for stable distribution of a millimeter-wave signal over fiber links. The parametric mixer is realized via ultrafast four-wave mixing processes, thereby overcoming the bandwidth limit of conventional electronic devices. The mixing of round-trip probe and local pumps generates a signal immune to transmission delay fluctuations at the remote end. This scheme is validated by an experiment that distributes a 36-GHz millimeter wave over 20-km optical fiber. The remote frequency stability achieves 4.0×10-16 at 2000 s, indicating that this approach is promising for phase-stabilized delivery of high-frequency signals.

Modeling and suppression of respiration induced B0-fluctuations in non-balanced steady-state free precession sequences at 7 Tesla

To develop and evaluate a model for describing the S1 (S+) and S2 (S-) phase in the presence of off-resonance frequency fluctuations, and to evaluate the performance of a novel interleaved navigator echo scheme. Using the extended phase graph model, a linear phase term was added to the evolution of transverse states. An approximation for the total S2 phase was derived with one fit parameter τl, which serves as an effective lifetime of the S2 signal. The model was evaluated using synthetic and in vivo phase evolution data. In addition, a novel interleaved phase correction scheme for the nb-SSFP sequence was applied to BOLD-fMRI data, and the number of activated voxels before and after phase correction was determined. The phases of S1 and S2 signals are significantly different from each other. The proposed nb-SSFP phase model provided a good description of the measured phase evolution data, and the approximate model for the S2 phase provided both at good fit to the data, as well as an effective lifetime of the S2 signal. In some subjects the phase contribution from older pathways was underestimated. In the BOLD-fMRI data, a twofold increase of the number of activated voxels for the S2 signal was observed, compared to no correction and a conventional navigator echo method. The different phase evolution of S1 and S2 signals can be qualitatively described by the proposed model, and detrimental phase history effects are significant at 7 Tesla when not appropriately corrected.

ART의 위상 정보를 이용한 형태기반 영상 검색 방법

영상에 포함된 객체의 형태는 내용 기반 영상검색에 있어서 중요한 정보를 가지고 있기 때문에, 이를 이용하여 영상을 검색하는 방법들이 활발히 연구되어 왔다. 그중에서도 최근에 제안된 저니키 모멘트의 위상과 크기를 이용하는 회전불변 서술자(IZMD: Invariant Zernike moment descriptor)을 이용한 영상 검색 방법은 기존의 크기 정보만을 이용한 저니키 모멘트 서술자보다 높은 영상 검색 성능을 보인다. 본 논문에서는 IZMD를 이용한 방법 보다 향상된 영상 검색 성능을 얻기 위해서 ART(Angular Radial Transform)의 크기와 위상을 이용한 회전 불변 특징 서술자(IARTD: Invariant Angular Radial Transform Descriptor)와 이를 이용해서 영상을 정합하는 방법을 제안한다. IARTD는 ART 기저함수의 특성을 이용해서 정렬된 ART 계수의 위상과 크기로 구성된 특징벡터이다. 영상의 검색은 두 IARTD의 크기차이와 위상차이의 곱을 이용하여 정의된 거리 계산 방법을 이용해서 수행한다. MPEG-7 데이터셋을 이용한 실험 결과, 제안하는 방법의 평균 BEP(Bull's Eye performance)는 0.5806으로서, ARTD나 IZMD를 이용한 영상 검색 결과의 평균 BEP 0.3574, 0.4234보다 우수한 검색 성능을 제공하는 것을 확인하였다. Since shape of an object in an image carries important information in contents based image retrieval (CBIR), many shape description methods have been proposed to retrieve images using shape information. Among the existing shape based image retrieval methods, the method which employs invariant Zernike moment desciptor (IZMD) showed better performance compared to other methods which employ traditional Zernike moments descriptor in CBIR. In this paper, we propose a new image retrieval method which applies invariant angular radial transform descriptor (IARTD) to obtain higher performance than the method which employs IZMD in CBIR. IARTD is a rotationally invariant feature which consists of magnitudes and alligned phases of angular radial transform coefficients. To produce rotationally invariant phase coefficients, a phase correction scheme is performed while extracting the IARTD. The distance between two IARTDs is defined by combining the differences of the magnitudes and the aligned phases. Through the experiment using MPEG-7 shape dataset, the average bull's eye performance (BEP) of the proposed method is 0.5806 while the average BEPs of the exsiting methods which employ IZMD and traditional ART are 0.4234 and 0.3574, respectively.

Testing the Atacama Compact Array Phase-Correction Scheme Using the Submillimeter Array

Abstract We conducted observational tests of a phase-correction scheme for the Atacama Compact Array (ACA) of the Atacama Large Millimeter and submillimeter Array (ALMA) using the Submillimeter Array (SMA) on Mauna Kea in Hawaii. Interferometers at millimeter and submillimeter waves are highly affected by refraction induced by water vapor in the troposphere, which results in phase fluctuations. The ACA is planning to compensate for the atmospheric phase fluctuation using phase information of the outermost antennas with interpolating into the inner antennas by creating a phase screen. The interpolation and extrapolation phase-correction schemes using phase screens have been tested by the SMA to study how effective they are. We produce a plane of wavefront (phase screen) from the phase information of three antennas for each integration, and this phase screen is used for interpolating the phase of the inner antenna and extrapolating that of the outer one. The interpolation scheme apparently obtains improved results, suggesting that the ACA phase-correction scheme works well. On the other hand, the extrapolation one often does not improve the results. After extrapolation, unexpectedly large phase fluctuations occur in the phases of the antennas at a distance of $\sim\ $140 m away from the center of the three reference antennas. The directions of these antennas are almost perpendicular to that of the wind, suggesting that the phase fluctuations can be well explained by the frozen phase screen in the atmosphere.

A digital phase corrector with a duty cycle detector and transmitter for a Quad Data Rate I/O scheme

A digital phase corrector is proposed to reduce the time jitter noise in a high speed parallel chip-to-chip interface system with a quad data rate (QDR) input/output (I/O) scheme. The proposed digital phase corrector for the 4-phase clock utilizes only one duty cycle detector by using a digitally controlled phase correction method and multiplexing function of transmitter for a QDR I/O scheme. Also, it reduces the static phase error generated in the transmitter, because of the inclusion of a replica of the transmitter in the feedback loop. To verify the proposed digital phase correction scheme, a digital phase corrector for the QDR I/O scheme with a 1.25GHz 4-phase clock was designed by using a 70nm 3-metal CMOS process with a 1.35V supply. The current consumption and phase correction range were 2.73mA and ±8%, respectively.

Frequency-domain channel estimation and equalization for shallow-water acoustic communications

A new frequency-domain channel estimation and equalization (FDE) scheme, combined with a new group-wise phase correction scheme, is proposed for single-carrier (SC) underwater acoustic communications systems employing single transducer and multiple hydrophones. The proposed SC-FDE scheme employs a 2 N -point Fast Fourier Transform (FFT) to estimate and equalize the channel in frequency domain, where N is the number of symbols in a data block. Both the frequency-domain channel estimation and equalization are designed by the linear minimum mean square error criterion. Initial channel estimation is performed by a pilot signal block and later updates are achieved using the detected data blocks. The proposed phase correction scheme utilizes a few pilot symbols in each data block to estimate the initial phase shift and then correct it for the block to combat the large phase rotation due to the instantaneous Doppler drifts in the acoustic channels. Time-varying instantaneous phase drifts are re-estimated and compensated adaptively by averaging the phase variation across a group of symbols. The proposed SC-FDE and phase correction method is applied to the AUVFest’07 experimental data measured off the coast of Panama City, Florida, USA, June 2007. With the Quadrature Phase Shift Keying (QPSK) modulation and a symbol rate of 4 ksps, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10 −4 for fixed-to-fixed channels with the source–receiver range of 5.06 km. For the moving-to-fixed source–receiver channels where the source–receiver range is 1–3 km, the multipath delay spread is 5 ms, the average Doppler shifts are ±20 Hz, and the maximum instantaneous Doppler drifts from the mean is ±4 Hz, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10 −3 .

All-optical temporal phase correction scheme for few-cycle optical pulses using diffractive optics

All-optical phase correction scheme for few-cycle optical pulses has been proposed. The temporal phase structure of a phase-modulated pulse is recorded as spatial index modulation in a two-photon recording medium. This scheme drastically relaxes complicacy in stretcher design for few-cycle chirped pulse amplification (CPA) systems.

Rapid imaging of free radicals in vivo using hybrid FISP field-cycled PEDRI

A new pulse sequence for rapid imaging of free radicals is presented which combines snapshot imaging methods and conventional field-cycled proton electron double resonance imaging (FC-PEDRI). The new sequence allows the number of EPR irradiation periods to be optimized to obtain an acceptable SNR and spatial resolution of free radical distribution in the final image while reducing the RF power deposition and increasing the temporal resolution. Centric reordered phase encoding has been employed to counter the problem of rapid decay of the Overhauser-enhanced signal. A phase-correction scheme has also been used to correct problems arising from instability of the magnetic field following field-cycling. In vivo experiments were carried out using triaryl methyl free radical contrast agent, injected at a dose of 0.214 mmol kg−1 body weight in anaesthetized adult male Sprague–Dawley rats. Transaxial images through the abdomen were collected using 1, 2, 4 and 8 EPR irradiation periods. Using 4 EPR irradiation periods it was possible to generate free radical distributions of acceptable SNR and resolution. The EPR power deposition is reduced by a factor of 16 and the acquisition time is reduced by a factor of 4 compared to an acquisition using the conventional FC-PEDRI pulse sequence.

Evaluation of algorithms for analysis of NMR relaxation decay curves

Quantitative processing of NMR relaxation images depends on the characteristics of the used fitting algorithm. Therefore several common fitting algorithms are compared for decay curves with low signal-to-noise ratios. The use of magnitude data yields a non-zero base line, and is shown to result in an overestimation of the decay time. A simple base line correction is no solution since this yields an equally large underestimation due to overcorrection of the first part of the curve. The use of squared data does yield reliable results, but only in the case of monoexponential decays. The best fitting algorithm under all experimentally occurring conditions turns out to be using real data after phase correction. A phase correction scheme is proposed, which applies to all imaging experiments for which the phase of the pixels is constant over the echo train. This scheme is validated for a phantom and for a tulip bulb.

Navigated Diffusion Imaging of Normal and Ischemic Human Brain

The principal barrier to clinical application of diffusion-weighted MR imaging is the severe image degradation caused by patient motion. One way to compensate for motion effects is the use of a "navigator echo" phase correction scheme. In this work, a modification of this technique is introduced, in which the phase correction step is performed in the frequency domain (i.e., after the readout Fourier transform). This significantly improves the robustness of the navigator echo approach and, when combined with cardiac gating, allows diagnostic quality diffusion-weighted images of the brain to be routinely obtained on standard clinical scanner hardware. The technique was evaluated in phantom studies and in 23 humans (3 normal volunteers and 20 patients). Diffusion anisotropy and apparent diffusion coefficient maps were generated from the image data and showed decreased apparent diffusion in acute stroke lesions and, in several cases, increased apparent diffusion in chronic stroke lesions.

Doppler correction for scattering matrix measurements of random targets

A central topic in active microwave remote sensing is the measurement of polarimet-ric scattering matrix time series. At least two successive measurements, employing orthogonal transmitter polarisations, are necessary to determine the full S-matrix of an object. In many remote sensing applications, however, the observed targets experience small but non-negligible variations even in the small time interval which is required to perform the S-matrix measurement. In order to cope with this problem, a Doppler phase correction scheme can be applied which compensates for the deterministic mean target motion. The possible internal variation of a composite target due to the random motion of the target's individual scattering centres of particles, however, results in decorrelation of the consecutively measured S-matrix columns and leaves significant statistical errors uncorrected. In the present paper the mean Doppler phase correction method and the related estimation errors are studied in greater detail, employing a quite general model of time-variant random targets. It turns out that the critical quantity which determines the statistical accuracy of the Doppler correction is given by the ratio of the pulse repetition interval and the decorrelation time of the observed random target. The pulse repetition frequency of the radar measurement, hence, has to be chosen carefully in order to avoid large estimation errors in the S-matrix determination. Matching the pulse repetition frequency properly to the decorrelation time of the observed target, on the other hand, allows the measurement and processing of polarimetric S-matrix series with maximum information content on the target. The present statistical studies, this way, are of fundamental interest in many polarimetric remote sensing applications.

A simple method for the restoration of signal polarity in multi‐image inversion recovery sequences for measuring T1

A simple scheme for the polarity correction of absolute images from inversion recovery multi-image T1 measurement sequences is described and demonstrated. This method is less sensitive to errors from eddy currents than the conventional full phase correction schemes and does not involve an increase in measurement time as it requires no additional scans.

The removal of azimuth distortion in synthetic aperture radar images

Abstract The sources of distortion in Synthetic Aperture Radar (SAR) imagery are discussed. The Royal Signals and Radar Establishment airborne SAR system is described briefly and a phase correction scheme, based on autofocus measurements, to remove azimuth distortions is derived with the requirements for accuracy and update frequency of autofocus measurements. Examples of the application of the techniques to real SAR data are given.