Time Reversal Technique Research Articles

Through the Wall Imaging Using Ultrawideband Frequency Domain Sampling Method

In this paper, a novel ultrawideband (UWB) time reversal technique for through-the-wall imaging (TWI) is introduced. The proposed algorithm is based on utilization of spatial and UWB frequency data acquired by antenna array. Scattering data are obtained by transmitting a short pulse from the central transmitter and recording the received signal at each array element. Individual multistatic scattering data matrix according to each antenna is formed by casting the fine and coarse frequency samples of the scattering information. The resulting matrix is calibrated based on transmitter signal and fed into the DORT (French acronym for decomposition of the time reversal operator) algorithm. The performance of the proposed method is investigated numerically by applying it to discrete scatterers embedded in homogeneous and continuously random medium. Numerical results are presented to show the effectiveness and high efficiency of the proposed algorithm for TWI.

Norm Criteria in the Electromagnetic Time Reversal Technique for Fault Location in Transmission Lines

This paper investigates the use of the norm criteria in the electromagnetic time reversal (EMTR) technique to locate faults in transmission lines. The energy root criterion and amplitude peak criterion, derived from 2-norm and ∞-norm, are considered and discussed contrastively. It is shown that both considered norm criteria can effectively be used in an EMTR method to locate faults. The ∞-norm originates from temporal and spatial variation of norm quantity of voltage/current wave propagation in the EMTR process. It is proven that the ∞-norm criterion makes use of the EMTR refocusing property of voltage/current waves propagating along transmission lines. In the extremely noisy circumstances, the ∞-norm criterion shows better robustness against noise compared to the 2-norm. It can locate the fault accurately even when the time-domain features of a strongly interfered signal cannot be identified. To validate the performance of the proposed criteria, two experimental tests on a reduced-scale network composed of coaxial cables and a real power grid are performed. The fault position is located accurately by the two criteria using one-terminal measurement data. It is suggested that the two criteria can be used complementarily to enhance the fault location efficiency.

Remote focusing of elastic waves in complex structures using time reversal of acoustic waves

Remotely assessing the state of damage of thin plates or beams within a complex structure is a burning issue in many industrial problems. A time-reversal technique is used to focus a short and localized elastic impulsion on a thin plate using a network of loudspeakers. The generated plate vibration is measured using a laser vibrometer. We built an experimental setup to mimic a complex structure, with several thin plates attached close to each others inside a cylinder. We show that the acoustic method allows us to put a targeted plate in vibration inside the cylinder, without exciting the other plates in the same structure. The time-reversal technique takes advantage of the strong wave reverberation caused by the presence of the complex structure around the plate, which creates virtual sound sources, and improves the focusing on the plate compared to that obtained when the plate has nothing around it. The ratio of plate vibration energy over emitted acoustic energy is about 1%. Finally, we create a small defect on a plate and assess whether the defaults can be detected from changes in the plate vibration modes. Our experimental results are compared with finite elements simulations.

Study on the ultrasonic wave convergence in the medium with bone

Low intensity pulsed ultrasound can shorten the healing time of a bone fracture; however, the present technique does not seem not to consider the complicated ultrasound propagation in the body with bone. In this study, the time reversal technique was used to effectively converge ultrasound in the bone part. A three-dimensional distal radius model was constructed by using a high resolution-peripheral quantitative CT data (spatial resolution; 61 μm). In the simulation using Finite Difference Time Domain method, one cycle of sinusoidal ultrasound wave (1 MHz) with Hanning window was transmitted from a virtual fracture point (1 mm3). The model was surrounded by water. The propagating ultrasound waves were observed by two ring shape array transducers, coaxially placed along the bone model at the distance of 30 mm from the fracture point. Then, time reversal waves were radiated from the array transducers, considering the phase shifts and amplitudes of the observed waves in the previous simulation. The time reversal waves successfully converged near the initial virtual fracture part, telling the usefulness of the technique. In the radial-tangential plane including the point, the converged area (−3dB of maximum stress value) was 42.8 mm2.

Underwater vector sensor communication in KOREX-17

The underwater acoustic communication channel is subject to multiple interactions with the ocean boundaries and refraction due to sound speed structure of water column, which produces significant time spread. This phenomenon is referred to as inter-symbol interference (ISI), which results in degradation of error performance. Recently, a time reversal technique has been used for reducing the ISI. However, this requires a large-size array with spatially separated receivers to obtain higher spatial diversity gain, and it becomes a limitation to its application in space-constrained environment. An acoustic vector sensor (combined pressure and particle velocity) can potentially yield a better communication performance relative to a system using only hydrophones. In this talk, communication data collected using a vector sensor known as IVAR (Intensity Vector Autonomous Receiver) during Korea Reverberation Experiment (KOREX-17) conducted in shallow water located at 34° 43′ N, 128° 39′ E on May 23–31, 2017. The characteristics of the channel as probed by a pressure-only versus combined pressure plus particle velocity system are discussed along with some performance results of a vector sensor communication system. [Work supported by the ADD(UD170022DD) and the National Research Foundation of Korea(NRF-2016R1D1A1B03930983).]

An augmented time reversal method for source and scatterer identification

We propose a computational method which improves the identification of sources and scatterers in a two-dimensional elastic medium using the Time Reversal (TR) technique. The identification procedure makes use of partial numerical data available at a small number of sensors. While in realistic applications these data are obtained by lab or field measurements, in the present work they are synthesized by solving the corresponding forward problem. We introduce an “augmentation” procedure that is utilized in addition to the TR technique and investigate its benefits. The augmentation procedure involves the solution of a suitable elliptic problem at selected time steps. In practice, one needs to solve, as a pre-process, a small number of “fundamental” elliptic problems which is equal to the number of measured quantities. A version of the augmentation procedure, which is easier to implement and involves almost no computational cost, is also presented. The local elastic energy in the domain is employed to decide whether the TR solution has properly refocused at the source. Cost functionals, whose minima are sought, are constructed for the source and scatterer identification problems. We conclude that augmentation improves the identification performance, in particular in the presence of noisy measurements. In some cases augmented TR succeeds in the identification whereas non-augmented TR fails.

Research Developments and Prospects on Microseismic Source Location in Mines

Abstract Microseismic source location is the essential factor in microseismic monitoring technology, and its location precision has a large impact on the performance of the technique. Here, we discuss the problem of low-precision location identification for microseismic events in a mine, as may be obtained using conventional location methods that are based on arrival time. In this paper, microseismic location characteristics in mining are analyzed according to the characteristics of the mine’s microseismic wavefield. We review research progress in mine-related microseismic source location methods in recent years, including the combination of the Geiger method with the linear method, combined microseismic event location method, optimization of relative location method, location method without pre-measured velocity, and location method without arrival time picking. The advantages and disadvantages of these methods are discussed, along with their feasible conditions. The influences of geophone distribution, first arrival time picking, and the velocity model on microseismic source location are analyzed, and measures are proposed to influence these factors. Approaches to solve the problem under study include adopting information fusion, combining and optimizing existing methods, and creating new methods to realize high-precision microseismic source location. Optimization of the velocity structure, along with applications of the time-reversal imaging technique, passive time-reversal mirror, and relative interferometric imaging, are expected to greatly improve microseismic location precision in mines. This paper also discusses the potential application of information fusion and deep learning methods in microseismic source location in mines. These new and innovative location methods for microseismic source location have extensive prospects for development.

3-D Field Intensity Shaping via Optimized Multi-Target Time Reversal

Generating given 3-D field intensity distributions in a non-homogeneous scenario is a canonical problem which is of interest in many applications. On the other side, due to its very challenging nature, very few methods have been proposed up to now in the literature. In this communication, starting from the well-known time reversal (TR) technique, a simple innovative approach, the so-called optimized multi-target TR (O-mt-TR), is presented. The strategy is based on the optimization of the phase shifts between the time-reversed fields in a number of control points within the target region. In doing so, the proposed method outperforms the simple juxtaposition of time-reversed fields pursued by the so-called mt-TR, or even succeed in cases in which the latter fails completely. This capability of the proposed method is assessed through a numerical analysis concerned with a 3-D inhomogeneous scenario, in which the results are quantitatively appraised in terms of coverage of the target areas.

Loosening Monitoring of the Threaded Pipe Connection Using Time Reversal Technique and Piezoceramic Transducers.

Threaded pipe connections are commonly used in the oil and gas industry in particular to connect casting strings, drill pipe strings, production and transportation risers, and pipelines. As the most critical components in the entire chain, maintaining a sealed and secure connection while being subjected to environmental loads and pollution is very important and necessary to reduce potential leakage risk and guarantee the safety of the entire chain. In this paper, an effective approach using time reversal technique and lead zirconate titanate (PZT) transducer was developed to monitor the looseness of the threaded pipe connection. Two threaded pipeline segments connected with a metal coupling were assembled to simulate the threaded connection in the pipeline system. Two PZT patches were mounted on the surface of one pipeline segment and the pipe coupling, respectively. By loosening the threaded connection with different rotation angles, several looseness scenarios were experimentally investigated. For each looseness condition, the developed time reversal-based approach was performed and the corresponding response signal was acquired and analyzed. The experimental results demonstrate that the peak value of the focused signal detected by the PZT sensor decreases with the increase of the looseness degree. The entire test conducted from tightened connection to loosened connection was repeated eight times to validate the repeatability of the developed method and the consistency of the detection results. In addition, the reliability of the developed method was studied by involving high disturbances when the signal was measured. All the test results show that the developed method has a great potential to be employed in practical applications for monitoring the looseness condition of the threaded pipe connection, especially in an environment with severe noises and disturbances.

Array processing for the localisation of noise sources in hot flows

Abstract This paper investigates the problem of localizing a sound source in a heated flow using a microphone array. Applications are found in studies dealing with the identification of sound sources in hot turbulent jets, or with the sound radiation from installed turbofans. Two configurations have been investigated: a shear layer flow (wind-tunnel type) and a jet flow. In the present study acoustic data are generated using a simulation based on the Linearized Euler Equations. For heated flows, refraction by temperature gradients is superimposed with refraction by velocity gradients, and the objective of this study is to assess whether this effect is important and how it can be accounted for in different source localisation methods. For this purpose, a time-reversal-based imaging method has been compared with a beamforming-based method in which the time-delays are computed based on ray tracing. For the shear flow, the results show that for high subsonic Mach numbers and steep thermal gradients, the thermal stratification must be taken into account to ensure a satisfactory precision of localisation for both methods. However, including the gradients of velocity and temperature is less crucial for imaging sound sources in the jet flow. The results indicate also that the localisation error is lower with the beamforming and ray-tracing technique than with the time-reversal technique, the latter being more sensitive to the limited array aperture.

Convective Wave Equation and Time Reversal Process for Source Refocusing

The convective wave equation deals with wave propagation in a moving media. We focus on the underwater acoustic wave equation where the convective element is the flow of water inside a river, along its length. The main thrust of this paper is the ill-posed “refocusing” problem. The initial condition simulates an explosion in a small compact region and the response is recorded over time at several microphones. Having only partial and noisy information we expect that small perturbations will destroy the ability to recover the complete initial data. We use the time reversal (TR) technique to determine the location of the original explosion, given limited spatial observations. We test the effectiveness of this scheme under conditions including dissipation, dispersion, etc. We use finite differences and implement absorbing boundary conditions to simulate an unbounded region.

Device‐to‐device communications using EMTR technique

Device-to-device (D2D) communication is a promising 5G technology, which helps to increase spectrum efficiency and sum-rate, as well as to decrease experienced latency. The main challenges of D2D communication are power consumption of paired devices, channel estimation between device pairs, and interference management between devices that use the same time and frequency resources. The electromagnetic time reversal (EMTR) technique is used in D2D communications to focus the signal power in both time and space domains for power efficiency of end users, to simplify the structure of transceivers, to help channel estimation in a low complexity way, and to nullify the effect of interference. First, in the time domain, the effect of using EMTR technique is investigated and its performance is compared with a similar non-EMTR system. Then, EMTR technique is used in an OFDM-based system and its advantages in the context of signal to interference plus noise ratio (SINR) and sum-rate are presented analytically and through computer simulations. Simulation results show a significant gain in SINR, sum-rate and received signal power for the proposed EMTR-based D2D system.

A comparison of time-reversal and cross-spectral beamforming for localizing experimental rod-airfoil interaction noise sources

Abstract This paper compares the results of different implementation methods of Time-Reversal (TR) and Conventional Beamforming (CB) array processing techniques for localizing experimental flow-induced rod-airfoil interaction noise sources. Experiments were conducted in an anechoic wind tunnel for low Mach number cross-flow whereby the far-field acoustic pressure was recorded using two line arrays (LAs) of microphones located above and below the rod-airfoil test-model for a range of flow speeds. TR simulations were carried out for the highest flow speed considered, without and with the rigid-wall modeling of the scattering surfaces which include the experimental facility and the airfoil. The predicted location, resolution and strength of the flow-induced dipole source was noted across different frequency bands wherein it was observed that modeling the airfoil during TR simulation helps to identify location of the scattered field source and simultaneously improves resolution. A Dipole phase-correction method for TR is presented (wherein the scattering surfaces are not modeled) which yields a single focal spot, thereby unambiguously localizing the dipole source. However, the predicted source location and focal-resolution in the Dipole phase-correction TR maps were found to be nearly the same as that obtained by TR without phase-correction and without scatterer modeling. It was shown that the CB maps based on monopole/dipole steering vector formulations were highly comparable to their counterpart TR maps in terms of source characteristics, predicted location, strength and focal-resolution. This demonstrates the equivalence of the TR and CB methods for aeroacoustic source localization when the experimental facility and airfoil were not modeled during TR simulations. Additionally, the Point-Time-Reversal-Sponge-Layer (PTRSL) damping and the deconvolution CLEAN-SC techniques used for enhancing the resolution of TR and dipole CB source maps, respectively, were compared. It was shown that while both methods were equally effective in suppressing side-lobes, the former produced a commensurate reduction in focal spot size whilst the latter yields a nearly constant focal spot size across the frequency bands. Moreover, the simultaneous use of the PTRSL damping and scatterer (in particular, the airfoil) modeling during TR not only yields a further enhanced resolution but also improves the accuracy of the scattered field source location in the low-frequency range.

Evanescent-Wave Reconstruction in Time Reversal System

AbstractThe evanescent wave re-construction performance in the time reversal system with periodic sub-wavelength grating is studied in this paper. Both the analysis and calculation results show that the evanescent components of the initial wave can’t be re-constructed directly by far field time reversal processing. The results indicate the uncertainty of the super-resolution performance of the time reversal technique with randomly arranged scatters. A new method to achieve super-resolution imaging by using evanescent-to-propagation wave conversion also been proposed at the end of this paper.

Time-Reversal Based Secure Transmission Scheme for 5G Networks over Correlated Wireless Multi-Path Channels

Broadband wireless communication users for 5G networks are primarily implemented in a complicated environment; the complex environment of time-varying multi-path propagation characteristics will seriously affect the performance of communication. One of the core technologies to overcome this problem is to introduce the environment adaptive technique--time reversal in the wireless link. Further, the problem of a Wiretap Channel in physical layer security research has become a popular research topic in recent years. To resolve the physical layer wiretap channel and multi-path fading problems in wireless channels, a novel concept of combining time reversal technology with physical layer security technology is proposed. In this paper, a physical layer secure transmission scheme based on the joint time reversal technique and artificial noise at the sending end is proposed for the wireless multi-path channel. First, in a typical wiretap channel model, the time reversal technique is used to improve the security of the information transmission process by using the properties of spatial and temporal focusing. Second, as the information is easily eavesdropped near the focus point, artificial noise is added to the sending end to disrupt the eavesdropping capability of the eavesdropper. Finally, due to the complexity of the multi-path channels, the influence of the antenna correlation on the system security performance is considered. Compared with the existing physical layer security schemes, theoretical analysis and simulation results show that the proposed scheme has a higher secrecy signal-to-noise ratio, a higher rate of secrecy, and a lower bit error rate of legitimate user.

Primary Sensorimotor Cortex Drives the Common Cortical Network for Gamma Synchronization in Voluntary Hand Movements.

Background: Gamma synchronization (GS) may promote the processing between functionally related cortico-subcortical neural populations. Our aim was to identify the sources of GS and to analyze the direction of information flow in cerebral networks at the beginning of phasic movements, and during medium-strength isometric contraction of the hand.Methods: We measured 64-channel electroencephalography in 11 healthy volunteers (age: 25 ± 8 years; four females); surface electromyography detected the movements of the dominant hand. In Task 1, subjects kept a constant medium-strength contraction of the first dorsal interosseus muscle, and performed a superimposed repetitive voluntary self-paced brisk squeeze of an object. In Task 2, brisk, and in Task 3, constant contractions were performed. Time-frequency analysis of the EEG signal was performed with the multitaper method. GS sources were identified in five frequency bands (30–49, 51–75, 76–99, 101–125, and 126–149 Hz) with beamformer inverse solution dynamic imaging of coherent sources. The direction of information flow was estimated by renormalized partial directed coherence for each frequency band. The data-driven surrogate test, and the time reversal technique were performed to identify significant connections.Results: In all tasks, we depicted the first three common sources for the studied frequency bands that were as follows: contralateral primary sensorimotor cortex (S1M1), dorsolateral prefrontal cortex (dPFC) and supplementary motor cortex (SMA). GS was detected in narrower low- (∼30–60 Hz) and high-frequency bands (>51–60 Hz) in the contralateral thalamus and ipsilateral cerebellum in all three tasks. The contralateral posterior parietal cortex was activated only in Task 1. In every task, S1M1 had efferent information flow to the SMA and the dPFC while dPFC had no detected afferent connections to the network in the gamma range. Cortical-subcortical information flow captured by the GS was dynamically variable in the narrower frequency bands for the studied movements.Conclusion: A distinct cortical network was identified for GS in voluntary hand movement tasks. Our study revealed that S1M1 modulated the activity of interconnected cortical areas through GS, while subcortical structures modulated the motor network dynamically, and specifically for the studied movement program.

Statistical Learning Over Time-Reversal Space for Indoor Monitoring System

As embedded in wireless signals, information of an indoor environment is captured during radio propagation, motivating the development of emerging wireless sensing technologies. In this paper, we propose a smart radio system that leverages the informative wireless radios to enable intelligent environment and extend human senses to perceive the world. In particular, owing to the time-reversal (TR) technique that captures changes in multipath profiles, the proposed TR indoor monitoring system (TRIMS) is capable of monitoring indoor events and detecting motion through walls in real time. A statistic model of intraclass TR resonance strength is developed and treated as the feature for TRIMS. Moreover, a prototype of TRIMS is implemented using commercial WiFi devices with three antennas. We investigate the performance of TRIMS in different single family houses with normal resident activities. In general, TRIMS can have a perfect detection rate with almost zero false alarm rates for seven target events, whereas during a two-week experiment TRIMS achieves a detection rate of 95.45% in the indoor multievent monitoring. The proposed TRIMS illustrates the potential of smart radio applications in smart homes, thanks to the ubiquitous WiFi.

Optimization Algorithm for DOA Estimation of a Shallow Sea Target Based on Active Time Reversal

Time reversal technique is applied to the DOA estimation of a shallow sea target, and a method based on active time reversal (ATR) is proposed to achieve correct estimation under multipath and low signal-to-noise (SNR) conditions. Combining the classical ray theory with array signal processing theory, the conventional multipath DOA estimation model based on uniform line array and the ATR-based DOA estimation model are set up respectively. The Capon algorithm is employed to simulate the models and compare it with conventional one. The simulation results show that the ATR-based estimation model can better estimate the azimuth angle of the target than the conventional counterpart, provide higher resolution and better suppress side lobes with the same signal-to-noise ratio (SNR), especially the low SNR.

A computational flow-induced noise and time-reversal technique for analysing aeroacoustic sources.

A simulation technique to analyse flow-induced noise problems that combines computational fluid dynamics (CFD), the boundary element method (BEM) and an aeroacoustic time-reversal (TR) source localisation method is presented. Hydrodynamic data are obtained from a high-fidelity CFD simulation of flow past a body and aeroacoustic sources are extracted based on Lighthill's acoustic analogy. The incident pressure field on the body due to the aeroacoustic sources is combined with a BEM representation of the body to obtain the spectrum of the direct, scattered and total acoustic pressure fields at far-field microphone locations. The microphone data are then used as input for the time-reversal simulations which are implemented by numerically solving two-dimensional linearized Euler equations. Decomposing the far-field pressure enables the TR simulation of the direct, scattered and total acoustic fields to be performed separately which yields the location and nature of the corresponding aeroacoustic sources. To demonstrate the hybrid CFD-BEM-TR technique, the sound generated by a cylinder in low Mach number cross-flow is considered. The nature of the aeroacoustic sources at the vortex shedding frequency and its second harmonic for the direct, scattered and total fields are identified.

Axial localization using time reversal multiple signal classification in optical scanning holography.

This paper presents a method to identify the axial location of targets in an optical scanning holography (OSH) system. By combining time reversal (TR) technique with the multiple signal classification (MUSIC) method in OSH, axial location can be detected with high resolution. Both simulation and experimental work have been carried out to verify the feasibility of the proposed work.