Combining Time Reversal Research Articles

Subwavelength Field Shaping Approach Based on Time Reversal Technique and Defective Metasurfaces

The resolution limitation is a main problem in the time-reversed field shaping system. In this paper, a subwavelength field shaping approach is proposed by combining time reversal (TR) technique and defective resonant metasurfaces. The subwavelength 2D and 3D field patterns can be effectively realized in both single-target and multi-target field-shaping systems. The resolution in both single-target and multi-target field-shaping systems aided with defective metasurfaces is approximately $0.1~\lambda $ , which is corresponding to the dimension of the applied metasurface cell. The proposed technique can be used for wireless power transfer, industrial heating, and localized wireless communications.

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.

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.

Secure transmission mechanism based on time reversal over wireless multipath channels

Broadband wireless communication is implemented primarily in a complicated environment. The complex environment with time-varying multi-path propagation characteristics will seriously affect the performance of communication. To solve the problem of insecurity in information transmission in wireless channels, in this paper a system is modeled by using the multi-input single output eavesdropping channel model and the security of information transmission through time reversal technology is ensured. Another problem is that the information focuses on the receiving point. Owing to the temporal and spatial focusing characteristics of the time reversal technology the information near the receiving point can be eavesdropped easily. To solve this problem, a secure transmission scheme based on time reversal technology with artificial noise interference on the transmitter side is proposed. One of the core technologies to solve 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 solve the problems about the physical layer wiretap channel and multi-path fading in wireless channels, a novel concept 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. It refers to the fact that information can be focused at a given moment and in space. Second, as the information is easily eavesdropped near the focus point, artificial noise is added to the sending end to disrupt the ability of the eavesdropper to eavesdrop. The artificial noise has no effect on legitimate user due to the use of null-space artificial noise in legitimate user. Based on this scheme, a closed expression, such as secure signal-to-interference and signal-to-noise ratio, an achievable secrecy rate and bit error rate are obtained, and the influences of the number of antennas, signal-to-noise ratio, and artificial noise are analyzed. The 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 the legitimate user than the the existing physical layer security schemes.

Flaw detection of wellbore systems by combining time reversal methods and nonlinear acoustic measurements

A wellbore system consisting of a steel pipe cased in cement is susceptible to cracking and delamination. Such flaws greatly reduce the integrity of the wellbore and can lead to significant leakage. To resolve the location and extent of the defects, we have combined a unique time reversal method with nonlinear acoustic evaluation. The time reversal method generates a compact high-amplitude wave only in a localized area that can be used to probe the acoustic properties spot by spot of a three dimensional region. Additionally, the nonlinear acoustic signal an object exhibits is far more sensitive to damage and defects than conventional linear analysis. The nonlinear behavior can be locally determined by observing the changes in elastic properties as the amplitude of the time-reversed focus wave is increased. The experimental setup consist of an array of piezoelectric transducers for generating the focused energy and a laser vibrometer mounted on a scanning frame for focusing and measuring nonlinearity on th...

Topological magnetic crystalline insulators and corepresentation theory

Gapless surface states of time reversal invariant topological insulators are protected by the anti-unitary nature of the time reversal operation. Very recently, this idea was generalized to magnetic structures, in which time reversal symmetry is explicitly broken, but there is still an anti-unitary symmetry operation combining time reversal symmetry and crystalline symmetry. These topological phases in magnetic structures are dubbed "topological magnetic crystalline insulators". In this work, we present a general theory of topological magnetic crystalline insulators in different types of magnetic crystals based on the co-representation theory of magnetic crystalline symmetry groups. We construct two concrete tight-binding models of topological magnetic crystalline insulators, the $\hat{C}_4\Theta$ model and the $\hat{\bf \tau}\Theta$ model, in which topological surface states and topological invariants are calculated explicitly. Moreover, we check different types of anti-unitary operators in magnetic systems and find that the systems with $\hat{C}_4\Theta$, $\hat{C}_6\Theta$ and $\hat{\bf \tau}\Theta$ symmetry are able to protect gapless surface states. Our work will pave the way to search for topological magnetic crystalline insulators in realistic magnetic materials.

Combining Time Reversal and Fast Marching Method in Wireless Indoor Positioning

Most of current wireless indoor positioning methods could not accurately obtain channel model, the mapping between spatial position and received signal features. The main factor for a precise channel model in an indoor environment is multipath effect. Time reversed (TR) wireless indoor positioning method has been validated to effectively reduce signals fading or time delay affected by multipath effect. However, these advantages are depended on a prior known channel model, without this condition, the accuracy of TR method will be seriously deteriorated. To solve the shortcoming of a general TR method in an unknown channel model application, we present a combining Time Reversal and Fast Marching Method (TR-FMM) positioning method. This method locates a target with two stages. In the stage one, the precise channel model of an indoor environment is estimated by FMM and simultaneous algebraic reconstruction technique (SART). In this stage, Time of Flight (TOF) information generated by some fixed spatial position anchors are used to fulfill the indoor channel model estimation, then the needed channel impulse response (CIR) for TR method will be obtained based on the estimated channel model. In the stage two, with the obtained CIR, any new joint mobile target will be accurately located by a general TR wireless indoor positioning method. Some numerical simulations have been presented to validate the proposed method. Simulative results depict the positioning deviation is less than 3cm for a newly joined mobile target with 1cm scale in a moderate complex indoor configure, and the accuracy of the positioning is improved 30 times comparing to a general TR method. The positioning time in the stage 2 is less than 3 minutes in a PC with 1.6 GHz dual CPUs and 2G Bytes memory. Obviously, the proposed method has great advantage in high accuracy and low complexity for wireless indoor positioning system.

Recent experiment results of long-range time-reversal communication in deep ocean

In the Japan Agency for Marine-Earth Sicence and Technololgy (JAMSTEC), a project to develop new autonomous underwater vehicle (AUV) is being planned, which will have the capability to cruise long distances over several hundred kilometers. Achieving acoustic communication with such a long-range AUV, even at a low data transmission rate, will be important. Time reversal is an attractive solution for such a long-range communication, by converging multipath signals and decreasing intersymbol interference (ISI). Thus, we have researched on time-reversal communication in the deep ocean, have proposed a method of combining time reversal and adaptive equalization, and have executed various at-sea experiments in the deep ocean. In the first experiments, both active and passive time-reversal communication were performed at the range of 10 km and it was shown that time reversal could enable communication under many multipath interferences. The subsequent experiments were carried out in various ranges for passive time-reversal communication. In our latest trial, communication at the range up to 1,000 km was demonstrated at the data rate of 100 bps at the frequency of 500 Hz. In this paper, the results of these experiments are described.

Performance of UWB Linked Relay Network with Time-Reversed Transmission in the Presence of Channel Estimation Error

In this paper, we present the bit error probability (BEP) of a relay network employing UWB links between its nodes. Since UWB systems can resolve many paths and are thus rich in multipath diversity, the use of the Rake diversity combining is very effective. However, because of the complexity, it is impractical in the scenarios with portable terminals or in the networks with multiple relays. Time-reversal technique, in which the transmitting nodes can do the pre-diversity filtering of UWB signal before transmission, is known as an approach to achieve the performance equivalent to the Rake combining without increasing the receiver complexity. The performance of such a network with simple correlators for each link at the receiver side is analyzed considering some realistic channel estimation errors. The results show that there is a great potential in combining time reversal and UWB techniques with respect to both reducing the receiver complexity and improving the system performance. The system robustness to imperfect channel estimation is also shown in the numerical analysis and compared to traditional UWB system with All-rake receiver. The analytical expressions are also validated by computer simulations which confirm the accuracy of the approximations used in obtaining these results.

A time reversal damage imaging method for structure health monitoring using Lamb waves

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metallic plate structure. The temporal focusing effect of the time reversal Lamb waves is investigated theoretically. It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation. Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations. The results show that the reconstructed time reversed wave exhibits close similarity to the reversed narrowband tone burst signal validating the theoretical model. To enhance the similarity, the cycle number of the excited signal should be increased. Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure. In this work, the time reversal technique is used for the recompression of Lamb wave signals. Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal. It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely, but the cycle number of the excited signal should be chosen reasonably.

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.

Current Flow-Induced Effect on Time-Reversal Communication

We previously studied long horizontal time-reversal (phase-conjugate) communication based on simulations and at-sea experiments. In these previous experiments, time-reversal communication at ranges of 10 to 100 km was achieved. In the implementation of time-reversal communication, it is prerequisite that the reciprocity of the medium is retained. Therefore, it is necessary to investigate the effects of nonreciprocal disturbances, such as movements of source-receiver or current flow. The effect of the movement of source receiver was studied in previous works. In this paper, the effect of current flow is studied on the basis of simulations. The results show that the effects of current flow appear mainly as stationary phase deviation. The method of combining time reversal and adaptive equalization, which we have proposed in our previous works, can compensate the current flow-induced effect and enable communication if the current flow speed is not so high.

Basic research on time-reversal communication with a moving source-receiver.

A basic research on time reversal, to be used for communication with a long cruising AUV, has been researched by our research group. In such application, it is preferred to maintain its cruising speed. However, time reversal does not compensate the effect of moving source-receiver in principle. So in this study, simulations with a normal mode method considering the effect of source-receiver movement are executed in which the carrier frequency is 500 Hz and symbol rate is 100 bps. The method of combining time reversal and adaptive decision feedback equalizer is applied. In both cases of shallow water and the deep ocean, demodulation cannot be realized with only time reversal as expected, while the proposed method achieves communication at the moving speed up to 1.0 m/s.

Experiment results of time-reversal communication in the deep ocean at the range of 300 km.

The research is being conducted to realize communication with a long cruising AUV in the deep ocean using time reversal. We have proposed the method of combining time reversal and adaptive equalization. In our previous at‐sea experiments, the performance of time‐reversal communication was confirmed at the range of 10–100 km with a virtual array. In our latest sea‐trial, experiments of passive time‐reversal communication were executed at the range of 300 km with a real receiver array. As a probe signal, chirp pulses and M‐sequence pulses were used. The data transmission speed was 50 and 100 bps. The source was suspended from the research vessel and the 20 channel receiver array was moored. Thus, Doppler effect was induced due to the ship drifting at the speed of approximately 0.25 m/s at maximum. Nevertheless, such effect is compensated and demodulation with no error is achieved with the proposed method. The results of using chirp pulses are inferior to the results of using M‐sequences because of sidelobes of the chirp correlation. The results of using longer M‐sequence are degraded due to Doppler effect.

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.

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.

A Time Reversal Transmission Approach for Multiuser UWB Communications

We propose and evaluate the performance of the time reversal technique in impulse radio ultrawideband (UWB) communications. The evaluation was based on measured channel impulse responses of a 4times1 multiple input single output (MISO) system in the UWB frequency bandwidth of 3 to 5 GHz with both vertical and horizontal polarization at the receiver. The results show that there is a great potential in combining time reversal and UWB technique with respect to both reducing the receiver complexity and improving the system performance. Simultaneous communication is illustrated with 5 users, each with a bit error rate (BER) of less than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> at an average signal to noise ratio (SNR) of 15 dB

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.]

Time-Reversal Communication with Moving Source-Receiver in Shallow Water

The applications of time-reversal waves to underwater acoustic technologies have been researched by the authors, particularly for long horizontal communication with autonomous underwater vehicle (AUV). In this study, the influence of a moving source-receiver to time-reversal communication is investigated because AUV is desired to maintain its speed. The simulations based on the normal mode method including such effect are executed. As for the results, it is clarified that the convergency of time reversal is not so affected and that the proposed method of combining time reversal and an adaptive filter can compensate the influences of the moving source-receiver adequately whose speed is not so high.