Time Reversal Array Research Articles

Target scattering waves emitted from a time reversal array in shallow water

When a target exists between a sound source and a TRA in shallow water, the TRA receives 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-emitted from the TRA, it will be thought that they are converged on the position of the sound source and the target. However, since the waves converging on the sound source have a high level, the waves converging on the target position are usually masked by the high level sounds. We cannot observe the waves converged on the target. We then eliminate only the high level sounds from the sound fields. In each array element, the signals in the case of nontarget 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 wave from each element again.

Time Reversal Processing Effect on Foci Structure in a Dynamic Ocean Waveguide

AbstractTime-reversal mirror (TRM) was recently developed for underwater communication system application. This technique has been introduced as an approach to compensate for signal distortion due to multiple path effect in ocean waveguide. TRM is used in this paper for sending a message signal in ocean waveguide, received at time-reversal-array (TRA), and retransmitted to the dynamic ocean environment with an internal solitary wave. The focal structures obtained from different simulating cases are used for bidirectional transmission expediency, and the TRA is used as the relay-line or trunked radio. Ocean waveguide characteristic effects on acoustical properties are also shown in this paper, and focal structure in different ocean environments is analyzed to provide an optimal approach to underwater communication systems and for physical insight's interests.

First Experiment Result of Time-Reversal Communication in Deep Ocean

We have researched time-reversal (phase-conjugate) acoustic technology for long horizontal communication by simulations, and have proposed a method of combining time reversal and the use of an adaptive equalizer to realize communication even with a sparse time-reversal array and the communication with a moving underwater vehicle. An experiment in a real sea was executed in Suruga-bay at a frequency of 500 Hz at a distance of 10 km, and its results are described in this paper. In the trial, the projector oonlyn the array side malfunctioned. Thus only a passive time-reversal communication experiment was performed. It was confirmed that communication could be achieved using only 10 elements of the array by the proposed method at a water depth of 1,100 m.

Multiuser Communications Using Passive Time Reversal

A recent paper (Song , IEEE Journal of Oceanic Engineering, vol. 31, no. 2, pp. 170-178, 2006) demonstrated multiple-input-multiple-output (MIMO) communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in a 120-m-deep water using quaternary phase-shift keying (QPSK) modulation, achieving an aggregate data rate of 6 kb/s. Moreover, the same data rate has been achieved at 20-km range by three users using 16 quadrature amplitude modulation (16-QAM).

Experiment of time-reversal communication in deep ocean

The basic study on the long horizontal acoustic communication using time-reversal waves have been carried out by the authors, and the method of combining time reversal and adaptive filter was proposed to enhance the communication accuracy and to achieve the communication with a moving source-receiver. This time-reversal technique will be applied to the communication in the deep ocean with an autonomous underwater vehicle, ‘‘Urashima,’’ which is being developed in JAMSTEC. The experiment was executed to demonstrate the effectiveness of the proposed method in the area at the depth of 1000 m in Suruga-bay. In the experiment, the 500-Hz projectors were used, and the range between the focus and the time-reversal array (TRA) was 10 km. The measurement system of the TRA side was moored with the underwater winch to compose the virtual array by changing its depth. Unfortunately, the projector of the TRA side had a breakdown, so it was abandoned to execute active time-reversal communication. However, experiments of passive time-reversal communication could be achieved. It was verified that the proposed method makes it possible to assure the communication channel with the TRA that consists of 20 elements. [This work was supported by MEXT KAKENHI 16686050.]

Multiuser communications using passive time reversal

A recent paper [Song et al., IEEE J. Ocean. Eng. 31, 165–173 (2006)] demonstrated multipleinput/multiple-output (MIMO) multi user communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes MIMO passive time reversal communications, which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in 120-m-deep shallow water using QPSK modulation, achieving an aggregate data rate of 6 kbits/s. Moreover, the same data rate has been achieved at 20 km range by three users using 16-QAM modulation.

Time reversal communication in Rayleigh-fading broadcast channels with pinholes

The Letter presents an analysis of the time reversal in independent-multipath Rayleigh-fading channels with N inputs (transmitters) and M outputs (receivers). The main issues addressed are the condition of statistical stability, the rate of information transfer and the effect of pinholes. The stability condition is proved to be M C ≪ N eff B for broadband channels and M ≪ N eff for narrowband channels where C is the symbol rate, B is the bandwidth and N eff is the effective number (maybe less than 1) of transmitters. It is shown that when the number of screens, n − 1 , is relatively low compared to the logarithm of numbers of pinholes N eff is given by the harmonic (or inverse) sum of the number of transmitters and the numbers of pinholes at all screens. The novel idea of the effective number of time reversal array (TRA) elements is introduced to derive the stability condition and estimate the channel capacity in the presence of multi-screen pinholes. The information rate, under the constraints of the noise power ν per unit frequency and the average total power P, attains the supremum P / ν in the regime M ∧ N eff ≫ P / ( ν B ) . In particular, when N eff ≫ M ≫ P / ( B ν ) the optimal information rate can be achieved with statistically stable, sharply focused signals.

Experimental demonstration of iterative time-reversed reverberation focusing in a rough waveguide. Application to target detection

For most shallow water waveguides, the backscattered energy measured in a monostatic configuration is dominated by ocean bottom reverberation. A selected time-gated portion of the measured reverberation signal is used to provide a transfer function between a time-reversal array and a corresponding range interval on the bottom. Ultrasonic and at-sea experiments demonstrate the focusing capabilities of a time-reversal array along the rough bottom interface using these reverberation signals only. The iterative time-reversal technique facilitates robust focusing along the ocean bottom, with little signal processing effort involved and a priori information of the environment. This allows for enhanced detection and localization of proud or buried targets in complex shallow water environments. A passive implementation of the iterative time-reversal processing is used to construct reflectivity maps, similar to a sonar map, but with an enhanced contrast for the strongest reflectors (or scatterers), at the water-bottom interface. Ultrasonic and at-sea experiments show that targets on the seafloor located up to 400 wavelengths from a time-reversal array are detectable in the presence of bottom reverberation.

Basic Research on Time-Reversal Waves in Deep Ocean for Long Acoustic Communication

We have studied the focusing property of time-reversal waves and its application to acoustic communication in shallow water. In this study, this focusing property in the deep ocean and its application to long horizontal acoustic communication are discussed. The results are as follows. Even if a time-reversal array (TRA) does not expand from the sea surface to the seabed, time-reversal signals converge, and the focusing property is not significantly affected by the depths of the focus and TRA. Then, it is revealed that by using time reversal, it is possible to enssure communication channel over a long range, through the simulation. In phase modulation, time reversal can demodulate almost only by itself, while in amplitude and phase modulation, an adaptive filter to compensates further.

Phase Modulation of Wave Radiated from Time Reversal Array

In shallow water, since the propagation path of an acoustic wave exists in multiplex, the transfer of information is usually difficult. On the other hand, phase conjugate wave has the property converged on a sound source location even in shallow water and an inhomogeneous medium. A communication simulation of the information that utilized the phase conjugate wave was then performed. Phase modulation was added to the probe wave sent from the sound source, and the signal was reradiated from the array. This not only converged the emitted acoustic wave on the sound source location, but the phase of the converging wave changed in proportion to the modulation phase.

Adaptive instant record signals applied to detection with time reversal operator decomposition

Time reversal arrays are becoming common tools whether for detection or tomography. These applications require the measurement of the response from the array to one or several receivers. The most natural way to record the impulse responses for several sources is to generate pulses successively from each emitting point and record simultaneously the signals from the receivers. However, this method is very time consuming or inefficient in terms of signal-to-noise ratio. To overcome this limitation quasi-orthogonal pseudonoise signals like Kasami sequences can be used. For guided wave propagation, a very high degree of orthogonality between the signal is necessary to allow an accurate measure of the whole multipath structure of the transfer function. Hence, in this work, we propose a new family of pseudo-orthogonal signals that is adapted to the environment and more specifically, to highly dispersive media. These adaptive instant records signals are used experimentally to detect targets using the time reversal operator decomposition method. The accuracy of the 15 x 15 transfer functions acquired simultaneously, and therefore the detection capability, are demonstrated in an experimental ultrasonic waveguide as a small-scale model of shallow water propagation including bottom absorption and reverberation.

Frequency Dispersion Compensation in Time Reversal Techniques for UWB Electromagnetic Waves

The invariance of the wave equation under time reversal enables optimal refocusing of ultrawideband waves by time-reversal (TR) arrays. This forms the basis of recently developed TR techniques for selective (re-)focusing and inverse scattering applications. However, in electromagnetic sensing applications involving lossy or dispersive media, such as earth media, time invariance is lost and TR techniques can be significantly degraded. To alleviate this problem, a method to compensate dispersion in TR applications is proposed here. The method is based on short-time Fourier transforms and is shown to improve refocusing.

A high frequency time reversal array deployment in a very shallow water environment

A new rigid 24-element acoustic source receiver array in the 10 to 15 kHz frequency band has recently been developed [Folegot et al., submitted to OCEANS2005-Europe]. Despite of a height of 10 m, it has been specified to be easily deployed from a pier and to allow low cost at-sea experiments with water depth up to 12 m. The array has been deployed in a pool of 50 m length and 10 m depth, where calibrated surface plane waves have been generated. The objective of the trial was to undertake a first deployment of the system, to calibrate the instrument and provide some scientific results based on time reversal in a very controlled experimental setup. The Time Reversal Operator has been measured in several configurations. Subsequently, its eigenvectors associated with target reflections have been emitted by the mirror in order to focus separately on each target. The sensitivity of the method against target depth, target type, emission basis, and surface roughness has been investigated. The collected data will help to improve the understanding of environmental factors affecting acoustic propagation in the 10–15 kHz range and will make an important contribution to the development of advanced acoustic communications methods and tomographic capabilities.

A numerical study of time-reversed UWB electromagnetic waves in continuous random media

We investigate superresolution effects on ultrawideband (UWB) electromagnetic waves (EM) produced by time-reversal (TR) arrays in continuous random media. The investigation is done via numerical simulations employing the finite-difference time-domain (FDTD) method. A Gaussian random background medium is considered. We compare the effect of statistical parameters, viz. variance, and correlation length, on the focusing resolution of transmitted waves by the TR array (TRA). The effect of depolarization is also briefly considered.

Ａｃｔｉｖｅ Ｔｉｍｅ Ｒｅｖｅｒｓａｌによる音響通信の基礎研究

We have researched on the application of time reversal waves to the underwater acoustic technology. Time reversal waves enable a desired signal to converge to the focus, even in unknown and inhomogeneous media. As time reversal waves converge to a “point” at any range, their focusing property is quite different from that of the conventional beam forming method. In this study, the basic research on time reversal communication is carried out. By using time reversal, the reflected and refracted waves are converged to the focus, the signal level received at the focus is increased and the symbol interferences are reduced. However, in order to ensure the communication channel only with time reversal, it is necessary to install many transducers of time reversal array. So, in this study, it is proposed to combine the focusing effect of time reversal waves and conventional adaptive filter. After most of the multi-path waves are utilized by using time reversal, unfocused waves, which appear like “noise”, are removed by adaptive filter. The simulations in shallow water are performed and it is concluded that this proposed method makes more accurate communication to the long distance possible than the conventional method.

Single-channel time reversal in elastic solids

Reverberant volume time reversal in 3D elastic solids (doped glass and Berea sandstone) using a single channel are presented. In spite of large numbers of mode conversions (compressional to shear wave conversions at the walls), time reversal works extremely well, providing very good spatial and time focusing of elastic waves. Ceramics were bonded to the surface as sources (100–700 kHz); a broadband laser vibrometer (dc—1.5 MHz) was used as detector. Temporal and spatial time-reversal focusing are frequency dependent and depend on the dissipation characteristics of the medium. Doped glass (inverse dissipation Q between 2000 to 3000) shows time-reversed spatial focal resolution at about half of the shear wavelength. The Berea sandstone (Q=50) yields a wider focusing width (a bit more than the shear wavelength) due to its lower Q. Focusing in the doped glass is better because the time-reversal (virtual) array created by wave reflections is larger than in the highly attenuating sandstone. These are the first results reported in granular media, and are a first step toward geophysical and field applications.

Experimental demonstration of time reversed reverberation focusing in an oceanic waveguide

The robust focusing and pulse compression provided by time reversal techniques may be exploited for active sonar. Backscattering from the rough water–bottom interface can be used as a surrogate probe source [Lingevitch et al., J. Acoust. Soc. Am. 111, 2609–2614]. This method uses a selected time-gated portion of the reverberation signals (or backscattered signals) to provide a transfer function between a time-reversing array (TRA) and a corresponding range interval on the bottom. The potential of a TRA for studying the application of shallow water bottom scattering is investigated experimentally using both sea-data collected in July 2004 in the Mediterranean sea off the Italian coast, and ultrasonic water tank data.

Analysis of the Time-Reversal Operator for a Small Spherical Scatterer in an Electromagnetic Field

The time-reversal operator (TRO) for a planar array of crossed dipole elements illuminating a small conducting and/or dielectric sphere is investigated in order to determine the general properties of an electromagnetic time-reversing array system. The behavior of such a system for a given frequency is analyzed by studying the eigenvalues and eigenvectors of the TRO. Each eigenvector specifies a set of complex driving currents for the array elements that produce received voltages that are proportional to the conjugates of the drive currents. The proportionality constant is equal to the square root of the associated eigenvalue and is the same for all elements. The eigenvalues and eigenvectors can be determined by performing a singular value decomposition (SVD) on the multistatic data matrix of the array. The eigenvalues of the TRO are the squares of the singular values, and the eigenvectors are identical to the singular vectors. We have shown that the maximum number of singular vectors associated with the sphere is equal to the number of orthogonal orientations of the dipole moments induced in the sphere when irradiated by the array, so there is a maximum of six for a conducting sphere but only three are significant when the conductivity is small and the sphere may be considered being just a dielectric. Numerical results are presented for linear and circular arrays to show the general behavior of the system.

Broadband performance of a time reversing array with a moving source

The automatic spatial and temporal focusing properties of a time-reversing array (TRA) make it an attractive technology for active and passive sonar systems that may be deployed in unknown multipath environments. However, in these and other potential underwater applications of TRAs, either the source, the array, or both are likely to be moving. In this paper we present broadband-signal TRA performance predictions that include the influence of the Doppler effect on the time-reversal process for broadband signals transmitted from an arbitrarily moving source to a stationary vertical TRA through a shallow ocean environment. Here, the impact of source motion on TRA performance is predicted from analysis and numerical simulations using a formulation of the Doppler shifted field based on Fourier superposition of stationary but spatially distributed time-harmonic sources. Quantitative results for the size and location of the TRA’s retrofocus are presented as well as the correlation of the TRA retrofocus signal with the time-reversed original signal for various source motions in range-independent and range-dependent shallow water sound channels. Overall, source motion is predicted to have little effect on TRA operations with source speeds less than 20 m/s for signals having a center frequency of 500 Hz at source–array ranges of a few kilometers.

Effects of time-reversing array deformation in an ocean wave guide

Active acoustic time reversal is a technique for focusing sounds recorded in complex unknown environments back to their remote point(s) of origin. It can be accomplished with a transducer array—a time-reversing array (TRA)—that sends and receives sound. Nearly all prior work on TRA performance has involved stationary arrays. This letter describes how random array deformation influences TRA retrofocusing in shallow ocean environments. For harmonic signals, randomly drifting array elements degrade TRA performance by ∼20% when the average horizontal wavenumber times the root-mean-square horizontal element displacement approaches 0.5. TRA focusing should be less sensitive to vertical element drift.