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Conventional synthetic aperture radar (SAR) processing can be adapted to obtain 2-D high-resolution images of moving targets. The resulting images, however, suffer from layover effects, therefore making their interpretation more challenging. This problem can be dealt with by combining radar imaging with interferometry, which allows for 3-D target reconstruction. This article presents a 3-D imaging technique built upon the well-known global backprojection (GBP) algorithm, which has been adapted to focus moving targets rather than the stationary scene. The proposed algorithm is tested on real datasets acquired with a circular acquisition geometry. Using a nonlinear flight-path allows to monitor the target from multiple viewing angles, therefore overcoming the limitations that are common to stripmap SAR (e.g., shadowing effects, geometrically adverse cases, and so on). The obtained results, which were compared with independent ground measurements, demonstrate the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

Time-domain processing of seismic reflection data has always been an important engine that is routinely utilized to produce seismic images and to expeditiously construct subsurface models. The conventional procedure involves analysing parameters related to the derivatives of reflection traveltime with respect to offset including normal moveout (NMO) velocities (second-order derivatives) and quartic coefficients (fourth-order derivatives). In this study, we propose to go beyond the typical assumption of 1-D laterally homogeneous medium when relating those 'processing' parameters to the subsurface medium parameters and take into account the additional influences from lateral heterogeneity including curved interfaces and smoothly variable velocities. We fill in the theoretical gap from previous studies and develop a general framework for such connection in layered anisotropic media. We show that in general, the influences of lateral heterogeneity get accumulated from all layers via a recursive relationship according to the Fermat's principle and can be approximately quantified in terms of the lateral derivatives of the layer interface surfaces and velocities. Based on the same general principle, we show that our approach can also be used to study the lateral heterogeneity effects on diffraction traveltime and its second-order derivative related to time-migration velocity. In this paper, we explicitly specify expressions for NMO and time-migration velocities with the influences from both types of heterogeneity suitable for 2-D data sets and also discuss possible extensions of the proposed theory to 3-D data sets and to parameters related to higher-order traveltime derivatives. Using numerical examples, we demonstrate that the proposed theory can lead to more accurate reflection and diffraction traveltime predictions in comparison with those obtained based on the 1-D assumption. Both the proposed theoretical framework and its numerical testing for forward traveltime computation presented in this study aid in understanding the effects from lateral heterogeneity on time-processing parameters and also serve as an important basis for designing an efficient technique to separate those influences in important processes such as Dix inversion for a more accurate subsurface model in the future. [ABSTRACT FROM AUTHOR]

Future wireless communication systems are demanding a more flexible physical layer. GFDM is a block filtered multicarrier modulation scheme proposed to add multiple degrees of freedom and to cover other waveforms in a single framework. In this paper, GFDM modulation and demodulation is presented as a frequency-domain circular convolution, allowing for a reduction of the implementation complexity when MF, ZF and MMSE filters are employed as linear demodulators. The frequency-domain circular convolution shows that the DFT used in the GFDM signal generation can be seen as a precoding operation. This new point-of-view opens the possibility to use other unitary transforms, further increasing the GFDM flexibility and covering a wider set of applications. The following three precoding transforms are considered in this paper to illustrate the benefits of precoded GFDM: (i) Walsh Hadamard Transform; (ii) CAZAC transform and; (iii) Discrete Hartley Transform. The PAPR and symbol error rate of these three unitary transform combined with GFDM are analyzed as well. [ABSTRACT FROM AUTHOR]

A number of central auditory neurons exhibit paradoxical latency shift (PLS), a response characterized by longer response latencies at higher sound levels. PLS neurons are known to play a role in target ranging for echolocating bats that emit frequency-modulated sounds. We recently reported that early inhibition of unit's oscillatory discharges is critical for PLS in the inferior colliculus (IC) of little brown bats. The goal of this study was to determine in echolocating bats and in nonecholocating animals (frogs): 1) the detailed characteristics of PLS and whether PLS was dependent on sound level, frequency, and duration; 2) the time course of inhibition underlying PLS using a paired-pulse paradigm. We found that 22% of IC neurons in bats and 15% in frogs exhibited periodic discharge patterns in response to tone pulses at high sound levels. The firing periodicity was unit specific and independent of sound level and duration. Other IC neurons (28% in bats; 14% in frogs) exhibited PLS. These PLS neurons shared several response characteristics: 1) PLS was largely independent of sound frequency and 2) the magnitude of shift in first-spike latency was either duration dependent or duration tolerant. For PLS neurons, application of bicuculline abolished PLS and unmasked the unit's periodical firing pattern that served as the building block for PLS. In response to paired sound pulses, PLS neurons exhibited delay-dependent response suppression, confirming that high-threshold leading inhibition was responsible for PLS. Results also revealed the timing of excitatory and inhibitory inputs underlying PLS and its role in time-domain processing. [ABSTRACT FROM AUTHOR]

A channel estimation method based on frequency-domain pilots and time-domain processing (FPTP) is proposed for OFDM systems in time-varying channels. Through Discrete Fourier Transform (DFT), the channel impulse response (CIR) is estimated by using the frequency-domain pilots, which is equivalent to the CIR estimated by cyclic correlation using time-domain pilot sequence. Hence, the channel estimation error, caused by the spectral leakage of the DFT, can be reduced by canceling the non-idea auto-correlation property of the pilot sequence in time-domain. At the same time, the complicated cyclic correlation computation in time-domain is evitable. The theoretical analysis and simulation results show that the proposed method outperforms the traditional DFT-based method and Wiener filter algorithm in time-varying multi-path fading channels. Moreover, the uncoded bit error rate (BER) performance of proposed algorithm is very close to that with perfect channel knowledge, especially in fast fading channels. [ABSTRACT FROM AUTHOR]

Presents a spread spectrum system with a time-domain processing device which can be used in a wireless communication system. Analysis of the interference rejection capability of this system; Description of the implementation of this system.

Presnts the results of a study of the cortical neurons used by the Jamaican mustached bat in interpreting time-delayed echoes from an emitted biosonar pulse. The conclusion that these neurons improve information processing in the inferior colliculus (IC) by way of the corticocollicular projection; The use of electronic stimulation to the bat's cortical neurons; Adjustments in the long-term changes in the overall function of the IC, medial geniculate body, and auditory cortex.

Time-lapse seismic imaging, used to detect changes in strata and physical properties beneath the seafloor, plays a crucial role in traditional resource development for reservoir monitoring. It can also be used for carbon capture and storage (CCS) monitoring in the field of carbon reduction. Continuous research and development are underway in this domain. However, the application of time-lapse seismic imaging techniques to shallow strata in coastal waters near the land remains underexplored. Despite its potential in various fields, there is a lack of sufficient demonstrations and reviews of monitoring technology using downsized data acquisition techniques. This paper introduces a portable ultra-high-resolution (UHR) 3D seismic survey system designed to monitor shallow strata in coastal waters. The field applicability of this system is examined, particularly in terms of its seismic repeatability. In this study, we developed a 3D seismic survey system suitable for the operation of ships weighing 40 tons or less. The survey was conducted with a one-year time lag in waters near Pohang, Korea, close to the shore (minimum distance 1.3 km) and with low water depths (9.5 to 25.2 m). This study employed traditional time-domain processing workflows and 4D processing techniques to generate baseline and 4D processed monitoring cube. Repeatability analyses are conducted from various perspectives. Our findings demonstrate the efficient application of the proposed UHR 3D seismic survey technique for monitoring shallow media beneath the seafloor in coastal areas where diverse engineering activities and marine geology research are conducted. [ABSTRACT FROM AUTHOR]

Delay-bandwidth product (DBP) is a key metric in slow light waveguides, requiring a balance between a large group index and broad bandwidth—two parameters that often involve a trade-off. Here, we propose and demonstrate a slow light waveguide with large DBP using a pseudospin-polarized transverse electromagnetic mode. This waveguide features a folded edge configuration that supports a 200% relative bandwidth from quasistatic limit (zero frequency) and an arbitrarily large group index. Owing to the pseudospin-polarized design, the dense folding would not introduce backscattering and the associated group velocity dispersion (GVD). The resulting gapless linear dispersion and pulse transmission behavior in folded edge waveguide are observed in microwave experiments. Our scheme provides a way to overcome the trade-off between group index and working bandwidth in slow light waveguide, which has potential applications in broadband optical buffering, light-matter interaction enhancement, terahertz radiation source and time domain processing. Delay-bandwidth product (DBP), which require a large group index and a wide bandwidth, is an important indicator in slow light waveguides. This work relaxes the trade-off between group velocity and working bandwidth in 200% relative bandwidth, and realizes a pseudospin-polarized slow-light waveguide with large DBP and low group velocity dispersion. [ABSTRACT FROM AUTHOR]

Real-time source localization is crucial for high-end automation and artificial intelligence (AI) products. However, a low signal-to-noise ratio (SNR) and limited processing time can reduce localization accuracy. This work proposes a new architecture for a time-domain feedback-based beamformer that meets real-time processing demands. The main objective of this design is to locate reflective sources by estimating their direction of arrival (DOA) and signal range. Incorporating a feedback mechanism in this architecture refines localization precision, a unique aspect of this approach. We conducted an in-depth analysis to compare the effectiveness of time-domain feedback beamforming against conventional time-domain methods, highlighting their benefits and limitations. Our evaluation of the proposed architecture, based on critical performance indicators such as peak-to-sidelobe ratio, mainlobe width, and directivity factor, demonstrates its ability to improve beamformer effectiveness significantly. [ABSTRACT FROM AUTHOR]

Parameter estimation is important in weather radar signal processing. Time-domain processing (TDP) and frequency-domain processing (FDP) are two basic parameter estimation methods used in the weather radar field. TDP is widely used in operational weather radars because of its high efficiency and robustness; however, it must be assumed that the received signal has a symmetrical or Gaussian power spectrum, which limits its performance. FDP does not require assumptions about its power spectrum model and has a seamless connection to spectrum analysis; however, its application performance has not been fully validated to ensure its robustness in an operational environment. In this study, we introduce several technical details in FDP, including window function selection, aliasing correction, and noise correction. Additionally, we evaluate the performance of FDP and compare the performance of FDP and TDP based on simulated and measured weather in-phase/quadrature (I/Q) data. The results show that FDP has potential for operational applications; however, further improvements are required, e.g., windowing processing for signals mixed with severe clutter. [ABSTRACT FROM AUTHOR]

Experimental demonstration of light propagation with ultralow group velocity, i.e., slow light, allows for revolutionary solutions for time-domain processing and buffering of optical signals. It can spatially compress optical energy, which lessens the device footprint and enhances linear and nonlinear optical effects. Photonic crystal waveguides (PCWs) are appealing for producing slow light since they can be on-chip integrated and operated under room temperature. However, most PCW slow-light devices are restricted to the narrow spectral range of material resonance, leading to a small delay-bandwidth product, which restricts the maximum data rate, operation frequency, and storage capacity. Furthermore, the lack of broadly tunable slow light hinders practical applications in tunable photonic devices. We propose a reconfigurable slow-light device using a PCW based on a prototypical chalcogenide glass, Ge2Sb2Te5 (GST225) to solve the problems. We find that the operating wavelength of the slow light within the structure can be reversibly switched between 3575 and 4905 nm by changing the structural state of GST225 between amorphous and crystalline ones. The corresponding average group indices are 40.8 and 54.4, respectively. We experimentally illustrate that the reversible phase transition of GST225 between amorphous and crystalline ones can be realized in nanoseconds. Our proof of concept may provide a platform for actively engineering slow light that might otherwise be difficult to obtain in photonic systems. We expect it to improve the device performance in the fields of nonlinearity and sensing. [ABSTRACT FROM AUTHOR]

We present the NPIPE processing pipeline, which produces calibrated frequency maps in temperature and polarization from data from the Planck Low Frequency Instrument (LFI) and High Frequency Instrument (HFI) using high-performance computers. NPIPE represents a natural evolution of previous Planck analysis efforts, and combines some of the most powerful features of the separate LFI and HFI analysis pipelines. For example, following the LFI 2018 processing procedure, NPIPE uses foreground polarization priors during the calibration stage in order to break scanning-induced degeneracies. Similarly, NPIPE employs the HFI 2018 time-domain processing methodology to correct for bandpass mismatch at all frequencies. In addition, NPIPE introduces several improvements, including, but not limited to: inclusion of the 8% of data collected during repointing manoeuvres; smoothing of the LFI reference load data streams; in-flight estimation of detector polarization parameters; and construction of maximally independent detector-set split maps. For component-separation purposes, important improvements include: maps that retain the CMB Solar dipole, allowing for high-precision relative calibration in higher-level analyses; well-defined single-detector maps, allowing for robust CO extraction; and HFI temperature maps between 217 and 857 GHz that are binned into 0′.9 pixels (Nside = 4096), ensuring that the full angular information in the data is represented in the maps even at the highest Planck resolutions. The net effect of these improvements is lower levels of noise and systematics in both frequency and component maps at essentially all angular scales, as well as notably improved internal consistency between the various frequency channels. Based on the NPIPE maps, we present the first estimate of the Solar dipole determined through component separation across all nine Planck frequencies. The amplitude is (3366.6 ± 2.7) μK, consistent with, albeit slightly higher than, earlier estimates. From the large-scale polarization data, we derive an updated estimate of the optical depth of reionization of τ = 0.051 ± 0.006, which appears robust with respect to data and sky cuts. There are 600 complete signal, noise and systematics simulations of the full-frequency and detector-set maps. As a Planck first, these simulations include full time-domain processing of the beam-convolved CMB anisotropies. The release of NPIPE maps and simulations is accompanied with a complete suite of raw and processed time-ordered data and the software, scripts, auxiliary data, and parameter files needed to improve further on the analysis and to run matching simulations. [ABSTRACT FROM AUTHOR]

Time- and depth-domain processing procedures were applied to seismic data acquired from shallow coastal areas using limited source-receiver offset ranges. A signal-to-noise ratio improvement strategy was applied in time-domain processing, and migration velocity analysis (MVA) with tomographic inversion was performed at every common image gather domain. Although the source-receiver offset was limited, the seismic image generated by the velocity from MVA represents more transparent reflections compared to the image from applied time processing only, which is likely because the shallow water depth of the survey area might yield a certain range of offsets. The processing procedure applied in this case also focused on the improvement of high-frequency content and removing as many multiples as possible in the time domain. Several small-scale faults and horizons of sedimentary units were well defined by this method. The improvement of the image with an accurate velocity by our method was demonstrated by representing the flattened events in the common image gather and velocity differences between time and depth processing. The final prestack depth migration image generated by our approaches provides refined information to interpreters that reduces uncertainties in geohazard assessments. [ABSTRACT FROM AUTHOR]

Snow water equivalent (SWE) is an important surface parameter for understanding a number of Earth system processes. Synthetic aperture radar (SAR) has considerable potential for measuring SWE of dry-snow because SAR can penetrate through the snow to the ground surface and is both amplitude- and phase-sensitive to refraction from the snow. Previous work on refraction-based SWE measurement by SAR has utilized the repeat-pass InSAR phase signal to estimate changes in SWE that occur between SAR acquisitions. These are subject to temporal decorrelation effects and consider only the refraction that occurs along the SAR beam center rather than the entire synthetic aperture. This study examines the refractive effect of dry-snow along the synthetic aperture and its impact on SAR image formation including defocusing and phase bias of the system impulse response. Snow phase compensation during time domain processing to recover the snow-free impulse response function (IRF) is described and demonstrated. The feasibility of using the mapdrift and image sharpness autofocus methods to estimate SWE is examined, and the effect of key system parameters on the estimation performance is derived. Experimental validation of the method was conducted by acquiring L-band data with the Simon Fraser University (SFU) Airborne SAR System over a pair of corner reflectors installed on the Kluane icefield in northwestern Canada. Results from both simulations and the icefield experiment are presented and compared including an analysis of errors affecting the estimation. [ABSTRACT FROM AUTHOR]

We discuss various details regarding the practical use of direct-detection acousto-optic delay lines, and develop a mathematical model for the input signal. An expression for the transient response was obtained and used to determine the acousto-optic delay line response to an boxcar input signal. This expression for the transient response was used to determine the cutoff frequency of the acousto-optic delay line and derive an equation for the impulse response to determine the acousto-optic transmission coefficient of the acoustooptic delay line as a function of frequency. The resulting equations are confirmed by numerical examples. The numerical results were in turn experimentally validated using the direct-detection acousto-optic delay line model developed in this paper. The theoretical and experimental results were compared using two criteria: the output response to a boxcar input signal and the cutoff frequency of the frequency response curve. The cutoff frequency was obtained using an oscillogram of the output pulse and the experimentally determined frequency response. A comparison of the theoretical and experimental results indicated that the time- and frequency-domain equations for the response curves have high potential for time-domain processing of broadband analog signals. [ABSTRACT FROM AUTHOR]

Abstract Differential microphone arrays (DMAs) are characterized as compact superdirective beamformers whose beampatterns are almost frequency invariant. In this work, we present a time-domain design of N th-order DMAs, which is important in some applications where minimal delay is required, such as real-time audio communications. Moreover, design in the time domain can reduce the computational efforts, compared to the frequency-domain design, especially when short filters are sufficient. We present design examples for DMAs illustrating some of the fundamental properties of the time-domain implementation as well as the equivalence to the frequency-domain design approach. [ABSTRACT FROM AUTHOR]

Polyamide mooring lines are being proposed for station-keeping of floating wind turbines in shallow water. However, their mechanical behaviour is quite different to that of the polyester fibre ropes used for deep water offshore oil & gas platform moorings. This paper presents an empirical model which can predict the specific strain response of a polyamide 6 mooring line under stochastic loading. Particular attention is given to the non-linear load-strain behaviour, hysteresis, and the influence of loading history. The identification of the model, using 12 stochastic tests performed on wet 3-strand polyamide sub-rope samples, is described. This model reproduces the specific features of the response of fully wetted PA ropes well, with reasonable accuracy and, for the extreme (maximum and minimum) values it provides a much better prediction than the linear "dynamic stiffness" model. This new model can be integrated in the time domain processing of a suitable mooring analysis software. • Development of a model framework to predict mooring line elongation. • Identification of model parameters for polyamide. • Application to the case of representative stochastic load sequences. • Excellent correlation between model predictions and tests. [ABSTRACT FROM AUTHOR]

The standard way of suppressing movement artefacts in Doppler measurements is by means of a high-pass filter. This is because artefacts usually are of high amplitude, but have low frequencies. The immediate drawback is, then, that low-velocity blood flow is also filtered out. In this paper, a method to reduce movement artefacts in blood perfusion measurements is proposed, using simultaneous transmission and reception of multiple frequencies in a continuous-wave Doppler system. It is shown that Doppler signals originating from blood may be considered uncorrelated for a large enough frequency separation between channels, and tissue movements are more correlated. By subtracting perfusion estimates obtained by time-domain processing, correlated signals can be suppressed. The subtraction algorithm is shown to produce a linear perfusion estimate, but with twice the standard deviation compared to an estimate obtained by simply averaging channels. Movement artefacts in both in vitro and in vivo models are shown to be reduced by the algorithm. Imbalance between channels does, however, cause the artefacts to be only partly reduced. The problem can be alleviated by filtering the signals prior to subtraction, but this results in a nonlinear estimate, especially for large time constants in the filter. Some amount of filtering can still be desirable to suppress partly correlated artefacts, even if identical time-domain processing units are implemented, as could be done digitally. (E-mail: tomas.jansson@elmat.lth.se) [Copyright &y& Elsevier]

Abstract This study explores the feasibility of using laser generated ultrasonic wave to detect the interfacial damage of coating-substrate structure. Two Pearson correlation coefficient based post processing algorithms are respectively modified and used in order to evaluate the damage-induced waveform variation and thus position the damage accurately. In addition, the ultrasonic signals of frequency-domain and time-domain are separately studied to optimize the damage visualization performance of the two approaches. The proposed method helps to facilitate nondestructive examination of laser ultrasonic technique. Highlights • Pearson correlation coefficient is used to process the laser induced acoustic wave. • The proposed damage edge threshold can be used for the damage region determination. • Frequency-domain processing algorithm is suitable for the no-intact-specimen case. • Time-domain processing algorithm is suitable for the intact-specimen-provided case. [ABSTRACT FROM AUTHOR]

Ultrafast imaging based on coherent plane-wave compounding is one of the most important recent developments in medical ultrasound. It significantly improves the image quality and allows for much faster image acquisition. This technique, however, requires large computational load motivating methods for sampling and processing rate reduction. In this work, we extend the recently proposed frequency-domain beamforming (FDBF) framework to plane-wave imaging. Beamforming in frequency yields the same image quality while using fewer samples. It achieves at least fourfold sampling and processing rate reduction by avoiding oversampling required by standard processing. To further reduce the rate, we exploit the structure of the beamformed signal and use compressed sensing methods to recover the beamformed signal from its partial frequency data obtained at a sub-Nyquist rate. Our approach obtains tenfold rate reduction compared with standard time-domain processing. We verify performance in terms of spatial resolution and contrast based on the scans of a tissue mimicking the phantom obtained by a commercial Aixplorer system. In addition, in vivo carotid and thyroid scans processed using standard beamforming and FDBF are presented for qualitative evaluation and visual comparison. Finally, we demonstrate the use of FDBF for shear-wave elastography by generating velocity maps from the beamformed data processed at sub-Nyquist rates. [ABSTRACT FROM AUTHOR]

The article presents an approach for calculating pass-by sound pressure radiated from railway wheels in the time domain using moving Green's functions. The Green's functions are obtained by using Finite Element (FE) and Boundary Element (BE) methods in the frequency domain, subsequent inverse Fourier transform, followed by convolution with a time series of rolling contact forces to obtain the pass-by time signals. However, traditional BE methods are computationally expensive due to the low structural damping of the wheel, necessitating a high frequency resolution. To overcome this issue, a modal approach is introduced in which the pass-by sound radiated by each wheel mode is calculated separately. This approach incorporates the dynamic response of the wheel in the time-domain processing and thus reduces the cost of the BE solution. A modal source signal is introduced to describe the excitation of each mode at each time step. The sound field radiated by unit modal amplitudes is calculated in BE and subsequently approximated by spherical harmonic (SH) equivalent sources, which allows for efficiently calculating acoustic transfer functions for varying relative positions of the wheel and a stationary receiver. Convolution of the source signal with the moving acoustic transfer function produces the pass-by pressure signal. The article investigates the directivity of the radiation from each mode and finds that most modes, including those with dominant radial deflection, radiate in mostly axial direction at high frequencies. Modes that dominate the pass-by pressure level are identified, both in frequency bands and with respect to the relative positioning of the wheel to the receiver. Finally, it is found that an SH expansion order of approximately 30 is required to satisfy the employed error measures, although lower orders may suffice for an auralisation of the signal. • Wheel pass-by noise is calculated in time-domain via moving Green's functions. • A modal approach allows separating structural and acoustic Green's functions. • Spherical harmonic sources are used to approximate the directivity of each mode. • Radial modes have a relevant axial radiation at high frequencies. • Modes that dominate the pass-by sound pressure level are identified. [ABSTRACT FROM AUTHOR]

Neural networks (NN) have spurred significant interest in automating machine condition monitoring, with many recent studies focusing on networks that take raw diagnostic signals as input. However, the complexity of these NNs has raised concerns about their physical justification and trustworthiness. Although explainable artificial intelligence (XAI) has developed promising tools to address these issues, the nature and size of raw diagnostic inputs have limited the effectiveness of XAI techniques in condition monitoring. This paper proposes a novel approach combining domain transformation and discretisation techniques with the XAI technique of Shapely value additive explanation (SHAP). This methodology allows the input domain for the network to remain in the original time-domain, while the explanation domain can be chosen as frequency or time–frequency and its discretisation adapted to the specific diagnostic case. This flexibility makes networks trained on raw signals more interpretable, while the controllable domain discretisation facilitates the implementation of XAI with manageable computational costs. The proposed methodology is tested on numerical cases and experimental signals, including the UNSW gear-wear and CWRU bearing-fault datasets. This enhancement of XAI is applicable to common black-box classifiers, providing researchers with an effective tool to give physical justification to their models and bolster trust in AI for machine condition monitoring applications. • Addressing interpretability in time-domain processing NNs for condition monitoring. • Novel methodology to explain a NN in a transformed domain to their processed data. • Novel discretisation techniques to decrease computational costs of the calculation. • Application to two numerical examples and gear and bearing fault diagnostics. [ABSTRACT FROM AUTHOR]

Modern imaging systems typically use single-carrier short pulses for transducer excitation. Coded signals together with pulse compression are successfully used in radar and communication to increase the amount of transmitted energy. Previous research verified significant improvement in signal-to-noise ratio (SNR) and imaging depth for ultrasound imaging with coded signals. Since pulse compression needs to be applied at each transducer element, the implementation of coded excitation (CE) in array imaging is computationally complex. Applying pulse compression on the beamformer output reduces the computational load but degrades both the axial and lateral point spread function, compromising image quality. In this paper, we present an approach for efficient implementation of pulse compression by integrating it into frequency domain beamforming. This method leads to significant reduction in the amount of computations without affecting axial resolution. The lateral resolution is dictated by the factor of savings in computational load. We verify the performance of our method on a Verasonics imaging system and compare the resulting images to time-domain processing. The computational savings are evaluated for a minimal sampling rate of four times the central frequency. We show that from 4- to 33-fold reduction is achieved as a function of the resulting lateral resolution, with no degradation of axial resolution. For an imaging system operating at a higher sampling rate, e.g., 10 times the central frequency, the savings can be as high as 77-fold. The efficient implementation makes CE a feasible approach in array imaging with the potential to enhance SNR as well as improve imaging depth and frame rate. [ABSTRACT FROM PUBLISHER]

We measured precursory acoustic emissions (AE) preceding the release of landslides in an inclined chute (3.0 m × 0.5 m × 0.5 m) filled with well-graded sandy soil. To detect elastic waves, we used an intelligent distributed acoustic sensing (DAS) technology. This method is based on fiber-optic technology that provides AE measurements at a high spatial and temporal resolution across large distances. The objective was to investigate acoustic emissions as precursors of slope failure and to assess the potential for landslide early warning based on this technology. We used a micro-sprinkler irrigation system to mimic triggering of landslides by rainfall. Fiber-optical cables were embedded within the soil material at three different depths. Additionally, we monitored occurrence of AE using independent piezoelectric transducers. Hydraulic monitoring of the wetted slope provided information on its hydrologic evolution and an audiovisual inspection system recorded deformations of the surface. Failure of the material was typically preceded by collapse of the slope toe followed by slowly accelerating retrogressive failure. This progressive failure resulted in an unambiguous acoustic signature captured by the DAS. Time-domain processing of waveforms provided insights into the prevailing failure mechanisms. The imminence of global failure was indicated by elevated AE activity. The fiber-optic AE detection using the iDAS™ (intelligent Distribute Acoustic Sensor) allows acquisition of failure-related elastic waves and may provide a valuable warning tool for large scale (km-scale) monitoring of shallow landslides and other rapid mass movements. [ABSTRACT FROM AUTHOR]

Channel estimation is one of the most significant technologies in the orthogonal frequency division multiplexing OFDM system and its accuracy will influence the performance of the whole system directly. Among all kinds of channel estimation algorithms, the least square LS estimation and the linear minimum mean-squared error LMMSE estimation are the most classic ones. However, both the two estimator are not able to reach a compromise between accuracy and computational complexity. The discrete Fourier transform DFT -based channel estimation can get a better performance by a time domain processing. [ABSTRACT FROM AUTHOR]

In this paper a new interference technique for material permittivity determination in the extremely high frequency range using a vector network analyzer is described. The novelty of this technique is the absence of a reference interference channel and there is no need to rotate the experimental sample that simplifies the measurement process and increases its accuracy. In order to approve this technique, the respective experimental setup was designed. The setup structure and theoretical background of interference fringe minimum formation were described. Based on the quasi-optical principle of the setup operation, the formula for the calculation of the material permittivity was derived. Two different dielectric materials were subjected to test measurements using the created experimental setup. Obtained average values of permittivity in the $50\div 70$ GHz range are $\varepsilon =$ 10.67 ± 0.09 and $\varepsilon =$ 2.35 ± 0.03 for sapphire and optical glass, respectively. For the purposes of result verification, the time-domain processing technique was also applied to calculate the permittivity of these materials. Besides, the obtained data are compared with those reported in the literature. [ABSTRACT FROM PUBLISHER]

In this work two methods for gear-rattling detection in mechanical systems are studied. For this purpose, an experimental quick-return mechanism was constructed with gears driven by an electric motor. Rattling is detected from the mechanical vibrations acceleration and the electric motor current. Vibrations signal is measured by a biaxial accelerometer and electrical current is obtained from the motor electronic driver. Signals are processed in time-domain and time-frequency domain. Rattling level is measured through a proposed performance evaluation index that is obtained from the application of band-pass filters and the RMS value of the signal for the time-domain processing. For the time-frequency analysis, the wavelet transform is used to isolate the rattling and the signal is post-processed to compute the RMS value. It is shown that the use of the electric motor current for the detection of rattling produce better results than the mechanical vibrations acceleration. With respect to the time-domain method versus the time-frequency method, it was found that the application of the wavelet transform significantly improves the indexes that are used for gear-rattle detection. [ABSTRACT FROM AUTHOR]

The terahertz region presents promising opportunities for quantitative gas sensing in practical environments thanks to its access to molecular fingerprints and immunity to particulate scattering. With Terahertz time-domain spectroscopy (THz-TDS), molecular absorption and dispersion spectra can be rendered using femtosecond-laser-based interferometric system. However, procurement of accurate spectroscopic parameters in this uncharted spectral region and quantitative reduction of the broadband data remain challenging. We propose and validate two end-to-end machine learning models based on Gaussian process regression (GPR) for efficient THz-TDS sensing in both time- and frequency-domain (TD and FD). Quantitative CO sensing is accomplished for proof-of-concept demonstration of the method. Both TD- and FD-GPR models demonstrate accurate predictions in the presence of measurement noise and ambient interference, and the root-mean-square errors are below 0.6 % with both simulated and experimental data. It is concluded that while the FD model benefits from immunity to dispersion distortions, the TD model noses out for its baseline-free nature and better elimination of temporally introduced noises. This work presents the first experimental demonstration of THz-TDS for quantitative gas sensing exploiting both TD and FD signals, showing promising prospects for real-time, multi-species, multi-parameter sensing under challenging scenarios such as chemically reactive or open-path atmospheric environments. • Terahertz spectroscopy has prospects for gas sensing in challenging environments. • Absorption and dispersion spectra can be measured with interferometric THz system. • Gaussian process regression efficiently interprets THz time-/frequency-domain data. • Direct inference of gas properties from temporal profile is successful and accurate. • Time-domain processing benefits from better immunity to power fluctuation and noise. [ABSTRACT FROM AUTHOR]

To obtain sufficient resolution, the operating band of synthetic aperture radar (SAR) must extend over several tens of megahertz. Consequently, SAR reception occurs across frequency bands allocated for other broadcasting stations and SAR often suffers radio frequency interference (RFI). This study proposes a novel approach to suppress RFI in SAR. The authors combine the classical spectral analysis method singular spectrum analysis, with an efficient subspace tracking method fast approximated power iteration (FAPI) which has the most economical computational cost among state-of-the-art algorithms. Furthermore, they supply a gap of FAPI so that it can achieve rank tracking based on eigenvalues distribution of the covariance matrix of the observed SAR record and in that way can detect the number of RFIs along the SAR azimuth time. The proposed algorithm is a time domain processing method and performs on SAR range records one-by-one. Finally, well-focused SAR imagery is obtained from the 'decontaminated' SAR data using classical SAR imaging algorithms. The proposed algorithm could be applied in complicated interference circumstances no matter the styles of RFI. Imaging results of artificial and real data have proved the validity and robustness of this algorithm. [ABSTRACT FROM AUTHOR]

ABSTRACT We discuss the problem of source control in controlled-source electromagnetic (CSEM) surveying and compare and contrast equal energy transient square-wave and transient pseudo-random binary sequence source signatures for the same towed-streamer electromagnetic survey line over the Peon gasfield in the Norwegian sector of the North Sea. The received response of the transient square-wave data was 11 dB greater than that of the pseudo-random binary sequence data, due to diffusive attenuation of higher frequencies present in the more broadband pseudo-random binary sequence signature. Deconvolution of the pseudo-random binary sequence data recovers the total impulse response function, increases the signal-to-noise ratio by 32.6 dB and separates most of the air wave from the earth impulse response by the causality principle. The recovered impulse responses have more detailed information in the frequency domain than the transient square-wave data. The pseudo-random binary sequence data were acquired with a 10 Hz source bit rate but contain no information about the Peon gasfield at frequencies above 2 Hz. The bit rate could have been reduced to 4 Hz, increasing the signal energy below 2 Hz by 150% and thus, potentially, increasing the signal-to-noise ratio by a further 4 dB. Because the total earth impulse response can be recovered from the broad-bandwidth pseudo-random binary sequence data, further time-domain processing may be applied, including correlated noise removal, which can increase the signal-to-noise ratio by as much as 20 dB, and air wave removal using the causality principle. The information in the arrival time of the peak of the earth response provides the potential for traveltime to resistivity mapping to provide a starting model for inversion. [ABSTRACT FROM AUTHOR]

This paper presents a microphone array technique aimed at enhancing speech quality in a reverberant environment. This technique is based on the central idea of single-input-multiple-output equivalent source inverse filtering (SIMO-ESIF). The inverse filters required by the time-domain processing in the technique serve two purposes: de-reverberation and noise reduction. The proposed approach could be useful in telecommunication applications such as automotive hands-free systems, where noise-corrupted speech signal generally needs to be enhanced. SIMO-ESIF can be further enhanced against uncertainties and perturbations by including an adaptive generalized side-lobe canceller. The system is implemented and validated experimentally in a car. As indicated by numerous performance measures, the proposed system proved effective in reducing noise in human speech without significantly compromising the speech quality. In addition, listening tests were conducted to assess the subjective performance of the proposed system, with results processed by using the analysis of variance and a post hoc Fisher’s least significant difference (LSD) test to assess the pairwise difference between the noise reduction (NR) algorithms. [ABSTRACT FROM AUTHOR]

In this paper, we propose two simple orthogonal frequency-division multiplexing (OFDM) mean-delay estimation schemes, i.e., a channel impulse response (CIR)-based scheme and a differential scheme, which are applicable for adaptive algorithms including improved channel estimation and fast handover. Instead of time-domain processing which requires additional inverse fast Fourier transform (IFFT), the mean delay can be obtained in the proposed CIR-based scheme using the estimated frequency-domain CIR. When cell-specific preamble sequences are employed, the mean delay can also be estimated without CIR estimation in the proposed differential scheme using cross correlation of differentially correlated received signals, enabling further complexity reduction and performance improvement when the CIR estimation is not sufficiently accurate. The performance of the proposed schemes is verified by computer simulation. [ABSTRACT FROM AUTHOR]

When laser Doppler vibrometers are used in the presence of ambient vibration, it is essential to compensate for the additional vibration signal content. In practice, compensation is realised by independently determining the instrument vibration and subtracting it from the erroneous measurement. When these vibrations are transient in nature, time domain-based processing should be used to carry out the correction. However, recent implementation of such an approach on stationary signals showed a factor of eight increase in performance over the previously established frequency domain-based alternative. Therefore, the work described in this paper initially focuses on determining the cause of the inconsistency and proposes a revised frequency domain approach. This revised approach offers near-equivalent performance to its time domain-based equivalent, with the latter approach offering only a factor of 0.26 increase in performance. However, despite the advantages of selecting the time domain-based technique, it typically requires high oversampling factors to allow for the accurate synchronisation of the various transducer type signals. Up until now, the only method available to determine the relationship between the sampling frequency and the performance would be experimentally, which is laborious and time consuming. Therefore, the significance of this paper is the development and experimental validation of an analytical model which predicts the sampling frequency dependence of the time domain correction technique performance. Using this, a framework was developed which allows for the optimal implementation of either correction technique and specifies the required acquisition parameters. • Revised frequency domain based LDV instrument vibration correction technique presented. • Enhanced signal processing shown to yield factor of seven performance improvement. • Recently proposed time domain processing method performance rigorously examined. • Model relating signal synchronisation and performance developed and validated • Framework defined for selection of correction technique and hardware specification. [ABSTRACT FROM AUTHOR]

Block transmission techniques, with appropriate cyclic prefix and frequency-domain processing schemes, have been shown to be excellent candidates for digital transmission over severely time-dispersive channels, allowing good performance with implementation complexity that is much lower than traditional time-domain processing schemes. Orthogonal frequency- division multiplexing (OFDM) modulation is the most popular block transmission technique. Single-carrier (SC) modulation using frequency-domain equalization (FDE) is an attractive alter- native approach based on this principle. In this paper, we propose two new receiver structures for multiple-input-multiple-output (MIMO) channels employing SC (MIMO-SC) modulation and FDE schemes. These receivers have a hybrid structure with frequency-domain feedforward and time-domain feedback filters for intersymbol interference (ISI) and interference cancellation. The proposed schemes are compared with different MIMO systems employing OFDM modulation (MIMO-OFDM) receivers in terms of performance [bit error rate (BER) and throughput] and complexity. Our performance results show the superiority of MIMO-SC approaches relative to MIMO-OFDM in terms of the BER performance for the simulated scenarios. Also, the simulation results show that the proposed hybrid MIMO-SC receivers yield a higher throughput than a MIMO-OFDM system. [ABSTRACT FROM AUTHOR]

High peak-to-average-power ratio (PAPR) is a particular disadvantage of orthogonal frequency-division multiplexing (OFDM) systems and can significantly degrade the power efficiency at the transmitter. The aim of this paper is to propose a post-inverse fast Fourier transform (IFFT) PAPR reduction method based on intelligent linear symbol combining with the objective to minimize the PAPR at the transmitter. Unlike the methods so far reported in the literature, the proposed method has a relatively low complexity and does not require external randomization sequences. This paper will also investigate the impact of the proposed method on the system's bit error rate (BER) performance and provide a closed-form analytical expression that takes into account the BER dependency on the side information bits and multiblock combination. It will be shown that the proposed technique can provide significant PAPR reductions at manageable levels of signal processing requirements while the BER degradation is relatively small. [ABSTRACT FROM AUTHOR]

Slow light with a remarkably low group velocity is a promising solution for buffering and time-domain processing of optical signals. It also offers the possibility for spatial compression of optical energy and the enhancement of linear and nonlinear optical effects. Photonic-crystal devices are especially attractive for generating slow light, as they are compatible with on-chip integration and room-temperature operation, and can offer wide-bandwidth and dispersion-free propagation. Here the background theory, recent experimental demonstrations and progress towards tunable slow-light structures based on photonic-band engineering are reviewed. Practical issues related to real devices and their applications are also discussed. [ABSTRACT FROM AUTHOR]

In this paper, a fundamental frequency (F0) tracking algorithm is presented that is extremely robust for both high quality and telephone speech, at signal to noise ratios ranging from clean speech to very noisy speech. The algorithm is named “YAAPT,” for “yet another algorithm for pitch tracking.” The algorithm is based on a combination of time domain processing, using the normalized cross correlation, and frequency domain processing. Major steps include processing of the original acoustic signal and a nonlinearly processed version of the signal, the use of a new method for computing a modified autocorrelation function that incorporates information from multiple spectral harmonic peaks, peak picking to select multiple F0 candidates and associated figures of merit, and extensive use of dynamic programming to find the “best” track among the multiple F0 candidates. The algorithm was evaluated by using three databases and compared to three other published F0 tracking algorithms by using both high quality and telephone speech for various noise conditions. For clean speech, the error rates obtained are comparable to those obtained with the best results reported for any other algorithm; for noisy telephone speech, the error rates obtained are lower than those obtained with other methods. [ABSTRACT FROM AUTHOR]

Synthetic aperture radar signal processing issues related to the exploitation of a pulse-to-pulse encoding transponder using pseudorandom codes discussed analytically. Namely the focusing algorithm, the code synchronization procedure and the properties of the code induced gain against non-encoding point scatterers and distributed ones. A time-domain processing algorithm and a code synchronization procedure are proposed and validated on simulated data and on a European Remote Sensing Satellite-2 data set containing prototypes of such a device. The interaction of the transponder signal with terrain backscattering is analyzed by deriving parameters that are useful for performance assessment. These are related to the relevant parameters in radiometric calibration, interferometric applications, and tagging. [ABSTRACT FROM AUTHOR]

Wave-type weakly electric fish are specialists in time-domain processing: behaviors in these animals are often tightly correlated with the temporal structure of electrosensory signals. Behavioral responses in these fish can be dependent on differences in the temporal structure of electrosensory signals alone. This feature has facilitated the study of temporal codes and processing in central nervous system circuits of these animals. The temporal encoding and mechanisms used to transform temporal codes in the brain have been identified and characterized in several species, including South American gymnotid species and in the African mormyrid genus Gymnarchus. These distantly related groups use similar strategies for neural computations of information on the order of microseconds, milliseconds, and seconds. Here, we describe a suite of mechanisms for behaviorally relevant computations of temporal information that have been elucidated in these systems. These results show the critical role that behavioral experiments continue to have in the study of the neural control of behavior and its evolution. [ABSTRACT FROM AUTHOR]

Objective To evaluate blood-volume flow-rate measurement in the fetal descending thoracic aorta using a non-invasive, non-Doppler, ultrasound technique. Methods This was a cross-sectional, observational study. Volume flow measurements were obtained from the descending thoracic aorta in 59 human fetuses between 20 and 40 weeks' gestation. These were uncomplicated pregnancies that resulted in the live births of appropriately grown infants. The measurements were obtained using a time domain processing technique: color velocity imaging quantification (CVI-Q). Results The blood-volume flow rate increased consistently from the second trimester until term. The mean values ranged between 100 mL/min at 20 weeks' gestation and approximately 350 mL/min at term. The normalized (weight-adjusted) volume flow rates decreased with increasing gestation, from a maximum of 626 mL/min/kg at 23 weeks to a minimum of 45.6 mL/min/kg at 37 weeks. Conclusions The regular measurement of blood-volume flow in the descending fetal thoracic aorta is feasible using CVI-Q. Although there is a considerable learning curve, with adequate training there are potential clinical applications for this non-Doppler technique. However, limitations exist with the currently available technology for clinical use in fetal vascular studies. [ABSTRACT FROM AUTHOR]

Inconel 600 Primary Water Stress Corrosion Cracking (PWSCC) in Nuclear Pressurized Water Reactors (PWRs) has necessitated the repair/replacement of various small bore nozzles. These repairs/replacements must be designed to avoid unwanted vibrations. So, to this end, new RTD-Thermowell-Nozzle replacement designs were developed and subjected to flow testing over a velocity range from 9.14 to 33.53 m/s (30–110 ft/s), and temperatures ranging from 121°C to 316°C (250–600°F). The replacement nozzles are welded on the pipe OD, rather than on the pipe ID. A split, tapered ferrule is used to support the nozzle tip inside the pipe bore. This maintains high thermowell tip-resonance frequencies with the objective of avoiding self-excitation from vortex shedding that is believed to have caused failures in an earlier design during initial, precritical plant startup testing. The flow testing was complicated by the small size of the thermowell tips (5.08 mm or 0.2 in ID), which necessitated use of a complement of low temperature and high temperature instrumentation. Since the high temperature device had an internal resonance (750 Hz) within the frequency range of interest (0–2500 Hz), adequate sensor correlations had to be derived from low temperature tests. The current nozzle/thermowell design was tested concurrently with two slight variations of the replacement design. The acceleration signals were acquired during incremental and continuous flow sweeps, nominally at 5 kHz sampling rates and for time domain processing as high as 25 kHz. Whereas vortex-shedding frequencies were predicted to prevail between 400 and 1500 Hz, no such response was observed at these frequencies. Rather, the thermowell tips responded due to turbulent buffeting with a peak response that was related directly to flow velocity. Lift direction response was always larger than drag direction response. The thermowell tips also responded at their natural tip frequencies in a narrow band random fashion. At the higher flow rates, one replacement design experienced an instability mode leading to high tip stresses. Although this instability did not repeat, this particular design was eliminated from consideration. The second replacement design performed almost identically to the current in-plant design. The experimental data were used to extract forcing functions and thermowell responses that were used as input into the design calculations. [Copyright &y& Elsevier]

Provides information on a method of real-time arterial color flow imaging using an intravascular ultrasound imaging system to estimate relative blood speed imaging. Details on real-time resonant frequency A-scans processed with an finite-impulse response filter bank; Relationship between the aperture diffraction pattern and scatterer speed; Use of time domain processing for non-real-time quantitative blood speed imaging.

With the increasing requirement for resolution, the negligence of topography variations causes serious phase errors, which leads to the degradation of the focusing quality of the synthetic aperture (SAR) imagery, and geometric distortion. Hence, a precise and fast algorithm is necessary for high-resolution airborne SAR. In this paper, an extended back-projection (EBP) algorithm is proposed to compensate the phase errors caused by topography variations. Three-dimensional (3D) variation will be processed in the time-domain for high-resolution airborne SAR. Firstly, the quadratic phase error (QPE) brought by topography variations is analyzed in detail for high-resolution airborne SAR. Then, the key operation, a time-frequency rotation operation, is applied to decrease the samplings in the azimuth time-domain. Just like the time-frequency rotation of the conventional two-step approach, this key operation can compress data in an azimuth time-domain and it reduces the computational burden of the conventional back-projection algorithm, which is applied lastly in the time-domain processing. The results of the simulations validate that the proposed algorithm, including frequency-domain processing and time-domain processing can obtain good focusing performance. At the same time, it has strong practicability with a small amount of computation, compared with the conventional algorithm. [ABSTRACT FROM AUTHOR]

Adaptive filtering algorithms can be used on the time-domain processing of navigation receivers to suppress interference and maintain the navigation and positioning function. The filter length can affect the interference suppression performance and hardware utilization simultaneously. In practical engineering, the filter length is usually set to a large number to guarantee anti-jamming performance, which means a high-performance receiver requires a high-complexity anti-jamming filter. The study aims at solving the problem by presenting a design method for the optimal filter order in the time-domain anti-jamming receiver, with no need for detailed interference information. According to interference bandwidth and jam-to-signal ratio (JSR), the approach designed a band-stop filter by Kaiser window for calculating the optimal filter order to meet interference suppression requirements. The experimental results show that the time-domain filtering processing has achieved good interference suppression performance for engineering requirements with optimal filter order in satellite navigation receivers. [ABSTRACT FROM AUTHOR]