Time Reversal Array Research Articles

Chipless RFID identification based on time reversal algorithm

A chipless RFID identification based on time reversal algorithm is presented in this work. The key problems affecting the reliability of chipless RFID identification are analyzed by using the energy focusing feature of time reversal algorithm. Analysis carried out on the energy focusing model revealed that the time reversal array can realize the multi-antenna energy focusing at the tag, so as to improve the reading distance of chipless RFID tag. Furthermore, time reversal energy self-focusing algorithm was used to suppress the noise come from other scatters near the tag. Multi-tag identification can be realized by the application of selective focusing algorithm. MIMO time reveal antenna array and C shaped resonators chipless RFID tag simulation model is constructed to verify the correctness of the proposed method. The results show that two chipless RFID tags with a distance of 0.2 m in azimuth can be identified from 1 m away.

A Low SNR and Fast Passive Location Algorithm Based on Virtual Time Reversal

In order to achieve fast and accurate passive location of the target radiation source, this paper puts forward a Virtual Time Reversal Method Based on Uniform Linear Array (ULA-VTR) passive location algorithm. This framework employs the passive time reversal focusing theory, which does not to require the physical transmission of time reversal signals and does not require pre-estimation of the channel to realize passive direction finding. The above principle is utilized to perform the target radiation source signal through a uniform linear sensor array for detection. The article provides the signal model of linear time reversal array and the virtual retransmission process in the first place. Then utilizes ULA-VTR to establish a dual-station passive location framework and derives the coordinate transformation equation to find its observation angle, in addition, establishes the observation equation system for obtaining the site of the target radiation source. Ultimately, the paper analyses the computational complexity of the ULA-VTR algorithm and adopts Geometric Dilution of Precision(GDOP) to discuss the positioning performance. The experimental results indicate that the technique presented in this paper has higher positioning accuracy under the low SNR compared with other techniques.

A time-reversal imaging system for buried objects in layered media using complex images Green’s functions

In this paper, the three-dimensional problem of detection and localization of buried metallic objects inside a multilayered medium is studied. Decomposition of the time-reversal operator (DORT) is used as the inversion algorithm in the imaging method. In this approach, one has to analyze the time-reversal (TR) operator obtained by using the multistatic data matrix (MDM) of a TR array. To attain MDM in the forward problem, full-wave simulation results from a commercial simulation software are used. For synthetic retransmission of signals in this inverse problem, determination of the appropriate medium Green’s function is required. Here, for the first time the complex images technique is employed in calculating the required Green’s functions. The complex images technique gives a closed-form Green's function, instead of evaluating time-consuming numerical integrals resulting in a fast and accurate Green’s function calculation. Using singular value decomposition of the MDM at a frequency range of 1–3 GHz and synthesized DORT method, images are constructed. Numerical results show the accuracy of the proposed approach in the detection and localization of buried objects. Furthermore, noise effect and sensitivity analysis on layers’ permittivity values are studied. The results confirm the robustness of the method against noise and uncertain layers parameters.

Time Reversal Imaging Based on the Space-Frequency ESPRIT

A novel time reversal approach for the point target imaging is proposed in this paper. This method requires only single measurement by the time reversal array, and employs the space-frequency decomposition plus the rotation invariance of the signal subspace for the target detecting. Numerical simulations are carried out to validate the performance of the proposed method for single target located at different positions in homogenous and continuous random medium respectively. The results indicate that the proposed method performs better than the conventional signal subspace based space-frequency algorithm both in imaging resolution and efficiency.

High-Resolution Imaging Utilizing Space-Frequency DORT Combining the Extrapolated Virtual Array

Space-frequency DORT (SF-DORT) is an effective time-reversal (TR) imaging method due to its immunity to the noise and adaptability of complex environment. However, some potential drawbacks, such as low range and cross-range resolutions, make SF-DORT inferior to the Space-frequency MUSIC (SF-MUSIC). In this paper, we propose a novel high-resolution imaging method utilizing SF-DORT combining an extrapolated virtual array created by autoregressive vector extrapolation (ARVE). In this way, the aperture of the TR array is extended significantly and the drawbacks associated with SF-DORT can be improved. Simulation results demonstrate that SF-DORT combining the extrapolated virtual array achieve much higher range and cross-range resolutions than without combining.

Axis retrieval of a supersonic source in a reverberant space using time reversal

Localizing the axis of the Mach cone created by the supersonic displacement of a bullet in a reverberant environment is a challenging task, not only because of the high velocity of the moving source, but also because of the multiple wave reflections off of the walls. Although time reversal (TR) techniques allow static acoustic source localization in a reverberant space, they have not been explored yet on non stationary waves caused by supersonic displacements in urban canyons. The acoustic wave produced by a supersonic projectile has a conical wavefront and a N-shaped acoustic pressure signature. In this paper, this acoustic wave is reproduced using a line array of point-like sources (simulations) and loudspeakers (experiments). During the propagation of this conical wave in an urban canyon, the resulting pressure signals are measured using a time reversal array flush mounted into the ground. These acoustic signals allow to automatically retrieve with a high accuracy the location of the Mach cone axis using time reversal techniques. This inverse problem is solved using the maximization of a fourth-order statistical criterion of the backpropagated pressures. This criterion allows to estimate the intersections between the Mach cone axis and several vertical planes in the urban canyon. These estimations are then fitted to a 3D trajectory with a robust three dimensional interpolation technique based on the Random Sample Consensus (RANSAC) algorithm. This method allows to automatically retrieve the axis of the supersonic source with an angular accuracy of less than 0.5° and a misdistance of 0.5cm for both numerical simulations and experimental measurements.

Focusing Over Optical Fiber Using Time Reversal

A time-reversal array in multimode fiber is proposed for lossless remotely controlled switching using passive optical splitters. The signal to be transmitted is digitally predistorted so that it is routed through the physical layer in order to arrive at only one receiver in an array. The system performance in the presence of an additive white Gaussian noise, modal group delay, and timing error is investigated numerically for single-mode fiber and 10-mode fiber. Focusing using a two-transmitter array and 44 km of single-mode fiber is demonstrated experimentally for 3-GBd QPSK signals with a bit error rate below the forward error correction limit.

Time-Reversal Imaging Using One Transmitting Antenna Based on Independent Component Analysis

Time-reversal (TR) techniques have shown promising results for detection and localization of scatterers in multiple scattering environments. These techniques exploit the TR invariance of the wave equation. Most of TR applications require the analysis of the TR operator obtained from the MDM of a TR array. Eigenvalue decomposition of the TR operator forms the basis of techniques such as decomposition of the TR operator and TR multiple signal classification. In this letter, we have introduced a new approach that is based on the independent component analysis (ICA). The ICA takes advantage of the statistical independence of scattered signals from well-resolved scatterers and breaks the mixed received signal by each receiving antenna to independent components based on maximizing the non-Gaussianity of mixed signal components. Some preprocessing steps such as centering and whitening are also required. One advantage of the TR imaging based on ICA is that imaging and focusing can be achieved using only one transmitting antenna, which lowers the complexity of the implementation compared with other techniques.

Three-Dimensional Generalized Finite-Difference Modeling of Electromagnetic Time Reversal: Impact of the Density of Dipoles for the Localization of a Dielectric Obstacle in Free Space

This paper presents a three-dimensional (3D) full time-domain modeling of the time reversal process for the localization of a dielectric obstacle in free space using an array of transceivers made of electric dipoles. The main purpose of this study is to propose a localization method and analyze the influence of the density of dipoles in the Time Reversal Array (TRA) on the accuracy of the localization in a given electromagnetic configuration. A Generalized Finite Difference (GFD) method and a leapfrog time discretization are used to get the numerical results.

The focusing performance with a horizontal time-reversal array at different depths in shallow water

The performance of time-reversal focusing with a horizontal line array at different depths is investigated by normal mode modeling and computer simulation. It is observed that the focusing performance of a bottom-mounted horizontal time-reversal array is much better than that of a horizontal time-reversal array at other depths in shallow water. The normal mode modeling is used to explain this result. The absolute values of the modes at different depths are compared. It is shown that the number of modes whose absolute values close to zero is smaller at the bottom than that at other depths. It means that the horizontal time-reversal array deployed at the bottom can sample more modes, obtain more information of the probe source and achieve better focusing performance. The numerical simulations of time-reversal focusing performance under various conditions, such as different sound speed profiles, and different bottom parameters, lead to similar results.

Experiment Results of Time-Reversal Communication at the Range of 300 km

We have researched on long horizontal time-reversal communication in the deep ocean, to be used for communication with autonomous underwater vehicle (AUV) in future, and proposed the method of combining time reversal with adaptive equalization. The pervious at-experiments were carried out at the ranges from 10 to 100 km with a virtual time-reversal array. In our latest sea trial, the experiment for passive time-reversal communication was executed with a real receiver array at the range of 300 km. This was the first implementation of time-reversal communication at such a range as an experiment intended for communication including the source movement. In this experiment, the source was suspended from the research vessel, thus drifted due to the ship motion. The proposed method could achieve error-free demodulation in spite of such drifting motion. The M-sequence signal and chirp pulse were used as the probe signal for passive time reversal. The results in case of M-sequence were better than those in case of chirp pulse.

레일레이 및 라이시안 페이딩 채널 환경 내의 시역전 펄스의 공간 특성

We perform an analysis of the characteristics of the time-reversal pulse in Rayleigh or Ricean fading channel environments. We verify it by using Monte Carlo simulation. In a time-reversal system, each antenna in the time-reversal array receives signals from the transmitter and reverse the received signal in the time axis and then resend it to the original transmitter. We assume that the channel characteristics varies very slowly and the spatial separation between the antennas is not large. Hence the signals received by each antenna are correlated. In this paper, the effect of the correlation on the time-reversed pulse is examined, which includes the spatial properties of the time-reversal pulse such as the focus size, and spatial power distribution.

A realization of sound focused personal audio system using acoustic contrast control

A personal audio system that does not use earphone or any wire would have great interest and potential impact on the audio industries. In this study, a line array speaker system is used to localize sound in the listening zone. The contrast control [Choi, J.-W. and Kim, Y.-H. (2002). J. Acoust. Soc. Am. 111, 1695-1700] is applied, which is a method to make acoustically bright zone around the user and acoustically dark zone in other regions by maximizing the ratio of acoustic potential energy density between the bright and the dark zone. This ratio is regarded as acoustic contrast, analogous with what is used for optical devices. For the evaluation of the performance of acoustic contrast control, experiments are performed and the results are compared with those of uncontrolled case and time reversal array.

Zero Padding Multistatic Data Matrix and Parameter Optimisation of DORT Method in Selective Focusing

With the introduction of DORT (Decomposition de l’Operateur de Retournement Temporel) method, selective focusing on the target has been made possible. The DORT method measures the multistatic data matrix (MDM) based on inter-element impulse responses of the propagating medium for all pairs of elements in time reversal array (TRA). From these responses, the time reversal operator (TRO) is deduced and diagonalised. Each eigenvector and eigenvalue of the TRO is associated to a specific scatterer in the medium. Back-propagation of these eigenvectors performs selectively focusing signal on different scatterers. However, the issue of how varying certain DORT parameters to achieve optimisation has not been explored. Therefore, the DORT simulations are developed to study the effect of tuning various parameters and optimise them in performance versus cost. Besides, zero padding MDM method is proposed to improve focusing performance compared to conventional method. Moreover, zero padding MDM can achieve time saving...

Time reversal method for guided wave inspection in pipes

The application of the time reversal method in pipe-like structures based on finite element method (FEM) is investigated. A steel pipe model measuring 70 mm x 3.5 mm is used to analyze the reflection coefficient of the L(0,2) mode with the time reversal process. Simulation results show that the time reversal array method is beneficial to the improvement of the signalto-noise ratio of a guided wave inspection system. As the intercepting window is widened, more energy is included in re-emitted signals, which leads to a large reflection coefficient of the L(0,2) mode. In parallel, a circumferential locating method based on the time reversal method is described. The time reversal process used for guided wave inspection leads to the temporal and spatial focusing. When the time reversal signals are re-emitted, the angular profile obtained at the axial location of the defect can be used to determine the circumferential location of the defect. Except for a pipe with one defect, the circumferential locating method has been verified on another pipe model with two defects. Meanwhile, the elements number of the time reversal array has been discussed for enhancing the discrimination of the defect circumferential location.

Time reversal processing to forward scattering waves of underwater targets

This paper presents the detection of underwater targets using a time reversal. When a target exists between a sound source and a time reversal array in shallow water, the time reversal array receive the sound waves from the sound source and the waves scattering by the target. If the time reversal processing to them is carried out and they are re-transmitted from the time reversal array, it will be thought that they are converged at the position of the sound source and the target. However, since the waves converging at the sound source have a high level, the waves converging at the target position are usually masked by the high level sounds. Then, we cannot observe the waves converged at the target. We eliminate only the high level sounds from the sound fields. In each array element, the signals in case of non target are subtracted from the signals including the target. As a result of subtraction, the components of the scattering wave by the target are left on the array elements. The time reversal fields of the scattering wave are constructed by radiating the components of the scattered waves from each element again.

Time-Reversal Communication in Deep Ocean: Result of Second At-Sea Experiment

We have researched long horizontal time-reversal (phase-conjugate) communication through simulations and have proposed a method of combining the effect of time reversal and the processing of an adaptive equalizer for realizing communication with a sparse time-reversal array and communication with a moving target. In the first at-sea experiment, only passive time-reversal experiments were performed. In this second experiment, both trials of active and passive time-reversal communications were executed. As results, it is confirmed that it is possible to achieve bi-directional communication by active and passive time reversals, and that time reversal is superior to conventional methods. It is also proved that at least ten source-receivers for the time-reversal array can achieve time-reversal communication and that the performance of time-reversal communication is hardly affected by the depth of the time-reversal array.

Basic at‐sea experiment for long horizontal time‐reversal communication in deep ocean

In our research program, a long cruising autonomous underwater vehicle (AUV) has been developed and a project on a second AUV is planned, for wide‐areal observation in the deep ocean. In such AUV operation, it is preferred to ensure acoustic communication with AUV even at low data‐transmission rate. Time‐reversal techniques have a possibility to achieve such long horizontal communication. We have proposed a method of combining time reversal and an adaptive equalizer to realize communication even with a sparse time‐reversal array (TRA) and communication with a moving target. To verify the performance of such time‐reversal communication, at‐sea experiments were executed at the various distances. In the experiment of 10 km at a water depth of 1000 m, bidirectional time‐reversal communication between the focus point and TRA was achieved successfully and the effects of the TRA depth and its channel number are investigated. In the experiments of 20, 30, 40 km at the slight sloping area, the performance on the distances are discussed. In the experiment of 100 km at a water depth of 4000 m, the convergence by passive time reversal was verified.

On the Sensitivity of Time-Reversal Imaging Techniques to Model Perturbations

Time-reversal (TR) techniques involve physical or synthetic retransmission of signals acquired by a set of transceivers in a time-reversed fashion, and can be used for a host of applications, including ultrawideband imaging of obscured targets. This work investigates the performance of some TR-based techniques under non-ideal conditions, and is divided into three main parts. In the first part, we investigate the robustness of TR-based imaging techniques via the decomposition of TR operator (DORT) algorithm and the multiple signal classification (MUSIC) algorithm under clutter and (additive) noise. In the second part, we examine the effect of TR-invariance breaking (due to losses in the intervening media) on the performance of both DORT and (TR-based) MUSIC. In the third part, effects of translational perturbations of the TR array are examined, as a class of model perturbations. These ensuing effects are then exploited, in a controlled manner, as a means to extract second-order spatial statistics of the probed media.

Sound beam scanning in sediment layer using phase conjugated pseudo sound source

When an object that is buried underwater in a sediment layer is detected from the diagonal, detection is often hampered by reflection from the bottom surface. Stated in different terms, the sound-wave energy that reaches the object is small because of reflection and refraction that occurs at the bottom surface when a sound wave is applied to the object in the sediment for diagonal detection. To alleviate that problem, a pseudo sound source can be set up in the sediment layer. The sound wave radiated from the pseudo sound source is then received by a time-reversal array. The sound wave can be injected into the sediment layer by reversing the received signal with time, and radiating it again from the time-reversal array. The beam can hit the target through movement of this pseudo sound source along the bottom surface. The sound wave that hits the target is reflected and returns in water. The sound wave is subsequently reflected many times at the surface and bottom, and is diffused over time. The passive-phase conjugate processing is then given to the received diffusing signal to reduce this diffusion. The pulse is compressed by this processing, thereby obtaining the target position.