Time Reversal Mirror Technique Research Articles

Numerical Simulation and Application of Vortex Field Monitoring near Islands in Straits

This study addresses the issue of vortex current fields near the islands and reefs in China’s straits, which pose significant challenges to engineering construction and navigation safety in the surrounding waters. To monitor these vortex fields, the study proposes an innovative method utilizing acoustic signals. The study utilizes the numerical simulation and phase feature extraction of acoustic signals in the vortex current field, based on ray acoustic theory and time-reversal mirror technology. The study successfully monitored the central position of the vortex core and characteristic radius of the vortex current field near Barley Straw Reef using HYCOM data for the first time. Furthermore, the performance of the method was analyzed under different acoustic phase perturbations and signal-to-noise ratios. The numerical simulation results demonstrate that the acoustic method is effective in monitoring near-shore vortex fields, and the time-reversal mirror technique is useful in extracting phase difference information from acoustic signals generated by vortex currents.

Recursive Decoding Based Multi-channel M-ary Spread Spectrum Underwater Acoustic Communication

The spread spectrum communication method has strong anti-noise and anti-fading capability, but it also brings the problem of low information transmission efficiency. The paper proposes a multi-channel M-ary spread spectrum communication method, using a recursive decoding algorithm to reduce the communication bit error rate. In combination with the channel equalization technique based on the virtual time reversal mirror technique, the scheme improves the communication rate of spread spectrum communication. It is verified through computer simulation and sea trial data that the communication method can effectively reduce the mutual interference between different channel signals, which provides a new idea for the design of hydroacoustic spread spectrum communication scheme.

Multi‐user underwater acoustic communication using binary phase‐coded hyperbolic frequency‐modulated signals

Multi-user communication traditionally uses either code division multiple access (CDMA) or time-division multiple access schemes. The chirp signals resilience to multipath and the Doppler effect makes it suitable for underwater acoustic communication. This paper presents a binary phase-coded hyperbolic frequency-modulated (BPC-HFM) for multi-user underwater acoustic communication. The proposed scheme has the advantage of both linear frequency-modulated signal, which is immune to multipath and Doppler offset, while PN sequence is encoded in such a manner that a waveform possesses either a phase shift of 0 or 180 depending upon the code which results in a unique orthogonal waveform for every user. As a result, it helps to reduce multiple access interference; in addition, the time-reversal mirror technique is used to enhance the performance of the system in the high multipath environment due to the virtue of hydro-acoustic channel. Furthermore, it uses a relatively simple matched filter-based receiver. To substantiate the robustness of the proposed multi-user waveforms three types of hydro-acoustic channels using the different numbers of multiple users are chosen for simulation and the bit error rate (BER) of the system is calculated. Furthermore, the results are compared to binary phase-coded linear frequency-modulated signal and conventional Direct sequence spread spectrum CDMA.

Bionic Morse Coding Mimicking Humpback Whale Song for Covert Underwater Communication

A novel method of bionic Morse coding mimicking humpback whale vocal is presented for covert underwater acoustic communication. The complex humpback whale song is translated as bionic Morse codes based on information entropy. The communication signal is made akin to the natural singing of male humpback whales. The intruder can detect the signal but will not be able to recognize the communication signal due to unified resemblance with the natural sound. This novel technique gives an excellent low probability of recognition characteristics. A flawless stealthy underwater acoustic communication has been established which has negligible chances of deciphered with high imperceptibility. Standard mimicry Morse codes have been developed for the characters of the English language and compared with Morse coding. Covert information of one character per second can be watermarked with perfect stealth and clandestine communication. This novel concept has been verified at transmission distance of five km and less than 10−3 Bit Error Rate (BER) is achieved at Signal to Noise Ratio (SNR) down to negative seven dB. Zero BER is attained by estimating the channel by a matching pursuit algorithm and equalizing the errors by virtual time reversal mirror technique.

M-ary Cyclic Shift Keying Spread Spectrum Underwater Acoustic Communications Based on Virtual Time-Reversal Mirror.

Underwater acoustic communications are challenging because channels are complex, and acoustic waves when propagating in the ocean are subjected to a variety of interferences, such as noise, reflections, scattering and so on. Spread spectrum technique thus has been widely used in underwater acoustic communications for its strong anti-interference ability and good confidentiality. Underwater acoustic channels are typical coherent multipath channels, in which the inter-symbol interference seriously affects the performance of underwater acoustic communications. Time-reversal mirror technique utilizes this physical characteristic of underwater acoustic channels to restrain the inter-symbol interference by reconstructing multipath signals and reduce the influence of channel fading by spatial focusing. This paper presents an M-ary cyclic shift keying spread spectrum underwater acoustic communication scheme based on the virtual time-reversal mirror. Compared to the traditional spread spectrum techniques, this method is more robust, for it uses the M-ary cyclic shift keying spread spectrum to improve the communication rate and uses the virtual time-reversal mirror to ensure a low bit error rate. The performance of this method is verified by simulations and pool experiments.

Biologically inspired covert underwater acoustic communication by mimicking dolphin whistles

Focusing on the need of covert underwater acoustic communication, a camouflage underwater acoustic communication approach is proposed in this paper. The idea is to deceive the interceptor to actively exclude the communication signal as it resembles the original whistle. Based on the time and frequency modulation feature of dolphin whistles, a series of segmented chirp rate modulated signals are designed to mimic the dolphin whistles by sound characteristics. A specialized communication frame structure is designed to facilitate the information demodulation, where the original whistle is used as the synchronization signal and the mimicry signal is followed to convey information. Besides, the original synchronization whistle is also used as a transmit reference to extract the whistle contour and furthermore to get the center frequency of each chirp. Virtual time reversal mirror technique is employed to mitigate channel effect. Fractional Fourier Transform and correlation approaches are adopted to realize demodulation respectively. The feasibility and efficiency of the proposed method are verified by both simulation and sea experiment. Bit error rate less than 10-3 with chirp duration of 20ms at a distance of 5.5km is achieved in the sea trial.

Reducing the effects of acoustic heterogeneity with an iterative reconstruction method from experimental data in microwave induced thermoacoustic tomography.

An iterative reconstruction method has been previously reported by the authors of this paper. However, the iterative reconstruction method was demonstrated by solely using the numerical simulations. It is essential to apply the iterative reconstruction method to practice conditions. The objective of this work is to validate the capability of the iterative reconstruction method for reducing the effects of acoustic heterogeneity with the experimental data in microwave induced thermoacoustic tomography. Most existing reconstruction methods need to combine the ultrasonic measurement technology to quantitatively measure the velocity distribution of heterogeneity, which increases the system complexity. Different to existing reconstruction methods, the iterative reconstruction method combines time reversal mirror technique, fast marching method, and simultaneous algebraic reconstruction technique to iteratively estimate the velocity distribution of heterogeneous tissue by solely using the measured data. Then, the estimated velocity distribution is used subsequently to reconstruct the highly accurate image of microwave absorption distribution. Experiments that a target placed in an acoustic heterogeneous environment are performed to validate the iterative reconstruction method. By using the estimated velocity distribution, the target in an acoustic heterogeneous environment can be reconstructed with better shape and higher image contrast than targets that are reconstructed with a homogeneous velocity distribution. The distortions caused by the acoustic heterogeneity can be efficiently corrected by utilizing the velocity distribution estimated by the iterative reconstruction method. The advantage of the iterative reconstruction method over the existing correction methods is that it is successful in improving the quality of the image of microwave absorption distribution without increasing the system complexity.

The study of adaptive time reversal mirror as a crosstalk mechanism for underwater communication

The time reversal mirror technique has been widely applied to mitigate the inter-symbol interference in underwater channels. Meanwhile, an adaptive time reversal mirror is introduced to improve the crosstalk quality between receivers in underwater communication. To explore the effectiveness of this method, this study extended the analysis of adaptive time reversal mirror as a crosstalk mechanism and explored the mechanism in an experiment using a towing tank as a testing platform. The advantage of this process is its simplicity in examining the effects of the array configuration of this crosstalk mechanism. Results of parametric experiments are discussed i.e. the effects of number of receivers, spacing between sources and noise energy threshold. Experimental data at 10 and 16 kHz with a 5-kHz bandwidth demonstrate as much as an 8-dB signal to noise ratio improvement for four receivers dual sources over a 30 m communication range in a 3.3 m depth testing platform. The results indicate array configuration affects this mechanism drastically.

Detecting closing delaminations in laminated composite plates using nonlinear structural intensity and time reversal mirrors

Closing delaminations in composite laminated structures exhibit a nonlinear dynamic response when excited by high frequency elastic waves. The contact acoustic nonlinear effects taking place at the damage interface act as a mechanism of energy redistribution from the driving frequency to the nonlinear harmonic frequencies. In this paper, we extend the concept of nonlinear structural intensity (NSI) to the analysis of closing delaminations in composite laminated plates. NSI is calculated using a method based on a combination of finite element and finite difference techniques, which is suitable for processing both numerical and experimental data. NSI is proven to be an effective metric to identify the presence and location of closing delaminations. The highly directional nature of orthotropic composites results in vibrational energy propagating in a different direction from that of the initial elastic wave. This aspect reduces the ability to effectively interrogate the damage and, therefore, the sensitivity to the damage. The time reversal mirror technique is explored as a possible approach to overcome the effect of the material directionality and increase the ability to interrogate the damage. Numerical simulations show that this technique is able to overcome the material directionality and to drastically enhance the ability to interrogate the damage.

Bionic underwater acoustic communication using dolphin sounds

An underwater acoustic communication scheme using intrinsic dolphin sounds is proposed to overcome the sound exposure derived from traditional fixed carrier modulation for covert communication in this paper. The communication signal can be detected but it could easily be excluded in the process of recognition/classification by an adversary. This is because the signal resembles dolphin sounds. Properties of dolphin sounds are investigated in this paper. Synchronization in this scheme is achieved by using dolphin whistles while dolphin clicks are used as information carrier. The time interval between dolphin clicks conveys information. Channel equalization is performed by passive time reversal mirror technique, whereas channel estimation is done through matching pursuit method under the theory of compressive sensing. The efficiency and feasibility of the proposed method are verified by lake trial. The received signal and emission mimic bio-signal are very similar. Bit error rate less than 10-4 is achieved above 29 bps to a distance of about 2 km.

The Computational Study of Microwave-Induced Thermo-Acoustic Tomography for Biologic Tissue Imaging Based on Pseudo-Spectrum Time Domain and Time Reversal Mirror Technique

Microwave-Induced Thermo-Acoustic Tomography (MITAT) own much concerns in recent years in biomedical imaging field. High contrast and resolution compared with conventional microwave or ultrasound imaging system especially for malignant tumors are outstanding characters of it. In this paper, the induced thermo-acoustic wave propagating in a mimic biologic tissue is simulated by numeric method Pseudo-Spectrum Time Domain (PSTD). Due to the excellent performance in noise-depress and the stability for the fluctuation of the model parameters, Time Reversal Mirror (TRM) imaging technique is studied computationally for the simulative received thermo-acoustic signals. Some thermo-acoustic objects with different initial pressure distribution are designed and imaged by TRM technique to represent the complex biologic tissue case in a random media. The quality of images generated by TRM technique based on PSTD method hints the potential of the MITAT technique.

Short baseline positioning with an improved time reversal technique in a multi-path channel

The existence of a multi-path channel under the water greatly decreases the accuracy of the short baseline positioning system. In this paper, the application of a time reversal mirror to the short baseline positioning system was investigated. The time reversal mirror technique allowed the acoustic signal to better focus in an unknown environment, which effectively reduced the expansion of multi-path acoustic signals as well as improved the signal focusing. The signal-to-noise ratio (SNR) of the time reversal operator greatly increased and could be obtained by ensonifying the water. The technique was less affected by the environment and therefore more applicable to a complex shallow water environment. Numerical simulations and pool experiments were used to demonstrate the efficiency of this technique.

Near-field source localization method and application using the time reversal mirror technique

In order to develop the acoustic keyboard for Personal Computer (PC), it is necessary to seek high-precision near-field source localization algorithm for identifying the keyboard characters. First of all, the focusing property of Time Reversal Mirror (TRM) is introduced, and then a mathematical model of microphone array receiving typing sound is established according to the realization of acoustic keyboard from which the TRM localization algorithm is carried out. The results through computer simulation show that the localization Root Mean Square Error (RMSE) performance of the algorithm can reach 10−3, which demonstrates that the algorithm possesses a high accuracy for the actual near-field acoustic source localization, with potential of developing the computer acoustic keyboard. Furthermore, for the purpose of testing its effect on actual near-field source localization, we organize three experiments for acoustic keyboard characters localization. The experiment results show that the positioning error of TRM algorithm is less than 1 cm within a provided acoustic keyboard region. This will provide theoretical guidance for the further research of computer acoustic keyboard.

Pseudo-Spectrum Time Domain and Time Reversal Mirror technique using in Microwave-induced Thermo-Acoustic Tomography System

Microwave-Induced Thermo-Acoustic Tomograpphy (MITAT) has attracted more concerns in recent years in biomedical imaging field. It has both the high contrast of the microwave imaging and the high resolution of ultrasound imaging. As compared to optoacoustics, which uses instead a pulsed light for evoking optoacoustic response, thermo-acoustic imaging has the advantage of deeper tissue penetration, attaining the potential for wider clinical dissemination, especially for malignant tumors. In this paper, the induced thermo-acoustic wave propagating in a mimic biologic tissue is simulated by numeric method Pseudo-Spectrum Time Domain (PSTD). Due to the excellent performance in noisedepress and the stability for the fluctuation of the model parameters, Time Reversal Mirror (TRM) imaging technique is studied computationally for the simulative received thermo-acoustic signals. Some thermo-acoustic objects with different initial pressure distribution are designed and imaged by TRM technique to represent the complex biologic tissue case in a random media. The quality of images generated by TRM technique based on PSTD method hints the potential of the MITAT technique.

Application of time reversal mirror technique in microwave-induced thermo-acoustic tomography system

Microwave-induced thermo-acoustic tomography (MITAT) is a promising technique with great potential in biomedical imaging. It has both the high contrast of the microwave imaging and the high resolution of the ultrasound imaging. In this paper, the proportional relationship between the absorbed microwave energy distribution and the induced ultrasound source distribution is derived. Further, the time reversal mirror (TRM) technique based on the pseudo-spectral time domain (PSTD) method is applied to MITAT system. The simulation results show that high contrast and resolution can be achieved by the TRM technique based on PSTD method even for the received signals with very low signal-to-noise ratio (SNR) and the model parameter with random fluctuation.

Computational Study of Time Reversal Mirror Technique for Microwave-Induced Thermo-Acoustic Tomography

Microwave-Induced Thermo-Acoustic Tomography (MITAT) is a promising technique in biomedical imaging. It has predominant advantages in both contrast and resolution compared with conventional microwave or ultrasound imaging system for malignant tumors. In this paper, Time Reversal Mirror (TRM) imaging technique based on numeric method Pseudo-Spectrum Time Domain (PSTD) for MITAT imaging is studied computationally. The proportion relationship between the deposited microwave energy and the induced thermo-acoustic initial source is deduced by analytic method. Due to the globe higher order character and the efficiency, the PSTD method is employed to implement the forward core of the TRM and to model the complex biologic tissue. In order to illustrate the performance of the TRM technique applied in MITAT, some thermo-acoustic objects with different initial pressure distributions are simulated, then designed and imaged by this technique to represent the complex biologic tissue case in a random media. Under the consideration of noise and the velocity parameter change of the model, the contrast and resolution of the images generated by using TRM technique based on PSTD method are also studied.

A Numerical Investigation of the Time Reversal Mirror Technique for Trans-skull Brain Cancer Ultrasound Surgery

Introduction: The medical applications of ultrasound on human brain are highly limited by the phase and amplitude aberrations induced by the heterogeneities of the skull. However, it has been shown that time reversing coupled with amplitude compensation can overcome these aberrations. In this work, a model for 2D simulation of the time reversal mirror technique is proposed to study the possibility of targeting any point within the brain without the need for craniotomy and to calculate the acoustic pressure field and the resulting temperature distribution within the skull and brain during a High Intensity Focused Ultrasound (HIFU) transcranial therapy. Materials and Methods: To overcome the sensitivity of the wave pattern to the heterogeneous geometry of the skull, a real MRI derived 2D model is constructed. The model should include the real geometry of brain and skull. The model should also include the couplant medium which has the responsibility of coupling the transducer to the skull for the penetration of ultrasound. The clinical substance used as the couplant is water. The acoustic and thermal parameters are derived from the references. Next, the wave propagation through the skull is computed based on the Helmholtz equation, with a 2D finite element analysis. The acoustic simulation is combined with a 2D thermal diffusion analysis based on Pennes Bioheat equation and the temperature elevation inside the skull and brain is computed. The numerical simulations were performed using the FEMLAB 3.2 software on a PC having 8 GB RAM and a 2.4 MHz dual CPU. Results: It is seen that the ultrasonic waves are exactly focalized at the location where the hydrophone has been previously implanted. There is no penetration into the sinuses and the waves are reflected from their surface because of the high discrepancy between the speed of sound in bone and air. Under the focal pressure of 2.5 MPa and after 4 seconds of sonication the temperature at the focus reached 51 °C and the temperature of the pre-target bone increased to 56.31 °C. In the post-target region the temperature of the sphenoid bone increased to 47.1 °C while the temperature of the occipital bones reached up to 46 °C. It is also shown that by using a cold water cooling system and cooling down the pre-target bones to 20 °C before sonication, the maximum pre-target bone temperature will not exceed 40 °C and hence the pre-target bone cells will remain intact. Discussion and Conclusion: In this study, it is well demonstrated that by using the time reversal mirror technique it is possible to target any point within the skull without the need for craniotomy. Although at higher acoustic frequencies compared to the lower ones such as 300 kHz the ultrasound undergoes more severe aberrations while passing through media having geometrical heterogeneity and discrepant sound velocities, the simulations performed in this work show that even at such frequencies it is still possible to correct these aberrations using the time reversal mirror technique. The thermal simulations show that by using this method the temperature of the deep seated tumors can be increased to cytotoxic temperature in a few seconds.

A time-reversal mirror with variable range focusing

Recently a time-reversal mirror (or phase-conjugate array) was demonstrated experimentally in the ocean that spatially and temporally refocused an incident acoustic field back to the original position of the probe source [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)]. Here this waveguide time-reversal mirror technique is extended to refocus at ranges other than that of the probe source. This procedure is based on the acoustic-field invariant property in the coordinates of frequency and range in an oceanic waveguide [Brekhovskikh and Lysanov, Fundamentals of Ocean Acoustics, 2nd ed. (Springer-Verlag, Berlin, 1991), pp. 139–145]. Simulations are combined with experimental data to verify this technique.