• Title/Summary/Keyword: Time Reversal(TR)

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Shifted Time Reversal Technique for Two-user Wireless Communication Using Variable Rate Back-off (두 명의 사용자를 위한 무선 통신에서 유동적인 전송률 Back-off를 이용하는 지연 Time Reversal 기술)

  • Pambudi, Sigit Aryo;Choi, Seung-Kyu;Lee, Chung-Yong
    • Journal of the Institute of Electronics Engineers of Korea TC
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    • v.48 no.5
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    • pp.33-39
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    • 2011
  • We studied the performance of a two-user time reversal multiple input single output scheme combined with the shifted transmission technique in a variable rate back-off scenario, called shifted time reversal (TR), that minimizes both intersymbol interference and multiuser interference. We compare the bit error rate performance of the shifted TR scheme to both full-rate TR and full back-off TR schemes and demonstrate its superiority to shifted zero forcing scheme when the guard interval is larger than one.

Comparison of Detection Probability for Conventional and Time-Reversal (TR) Radar Systems

  • Yoo, Hyung-Ha;Koh, Il-Suek
    • Journal of electromagnetic engineering and science
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    • v.12 no.1
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    • pp.70-76
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    • 2012
  • We compare the detection probabilities of the time-reversal(TR) detection system and the conventional radar system. The target is assumed to be hidden inside a random medium such as a forest. We propose a TR detection system based on the SAR(Synthetic Aperture Radar) algorithm. Unlike the conventional SAR images, the proposed TR-SAR system has an interesting property. Specifically, the target-related signal components due to the time-reversal refocusing characteristics, as well as some of clutter-related signal components are concentrated at the time-reversal reference point. The remaining clutter-related signal components are scattered around that reference point. In this paper, we model the random media as a collection of point scatterers to avoid unnecessary complexities. We calculate the detection probability of the TR radar system based on the proposed simple random media model.

Reconstruction of Dispersive Lamb Waves in Time Plates Using a Time Reversal Method

  • Jeong, Hyun-Jo
    • Journal of the Korean Society for Nondestructive Testing
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    • v.28 no.1
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    • pp.59-63
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    • 2008
  • Time reversal (TR) of nondispersive body waves has been used in many applications including ultrasonic NDE. However, the study of the TR method for Lamb waves on thin structures is not well established. In this paper, the full reconstruction of the input signal is investigated for dispersive Lamb waves by introducing a time reversal operator based on the Mindlin plate theory. A broadband and a narrowband input waveform are employed to reconstruct the $A_0$ mode of Lamb wave propagations. Due to the frequency dependence of the TR process of Lamb waves, different frequency components of the broadband excitation are scaled differently during the time reversal process and the original input signal cannot be fully restored. This is the primary reason for using a narrowband excitation to enhance the flaw detectability.

Structural Damage Diagnosis Method by Using the Time-Reversal Property of Guided Waves (유도초음파의 시간.역전 현상을 활용한 구조손상 진단기법)

  • Lee, U-Sik;Choi, Jung-Sik
    • Journal of the Korean Society for Precision Engineering
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    • v.27 no.6
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    • pp.64-74
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    • 2010
  • This paper proposes a new TR-based baseline-free SHM technique in which the time-reversal (TR) property of the guided Lamb waves is utilized. The new TR-based SHM technique has two distinct features when compared with the other TR-based SHM techniques: (1) The backward TR process commonly conducted by the measurement is replaced by the computation-based process; (2) In place of the comparison method, the TOF information of the damage signal extracted from the reconstructed signal is used for the damage diagnosis in conjunction with the imaging method which enables us to represent the damage as an image. The proposed TR-based SHM technique is then validated through the damage diagnosis experiment for an aluminum plate with a damage at different locations.

Time Reversa1 Reconstruction of Ultrasonic Waves in Anisotropic Media

  • Jeong, Hyun-Jo
    • Journal of the Korean Society for Nondestructive Testing
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    • v.28 no.1
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    • pp.54-58
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    • 2008
  • Time reversal (TR) of body waves in fluids and isotropic solids has been used in many applications including ultrasonic NDE. However, the study of the TR method for anisotropic materials is not well established. In this paper, the full reconstruction of the input signal is investigated for anisotropic media using an analytical formulation, called a modular Gaussian beam (MGB) model. The time reversal operation of this model in the frequency domain is done by taking the complex conjugate of the Gaussian amplitude and phase received at the TR mirror position. A narrowband reference signal having a particular frequency and number of cycles is then multiplied and the whole signal is inverse Fourier transformed. The original input signal is seen to be fully restored by the TR process of MGB model and this model can be more generalized to simulate the spatial and temporal focusing effects due to TR process in anisotropic materials.

Time Delay Focusing of Ultrasonic Array Transducers on a Defect Using the Concept of a Time Reversal Process

  • Jeong, Hyun-Jo;Lee, Jeong-Sik;Lee, Chung-Hoon;Jun, Ghi-Chan
    • Journal of the Korean Society for Nondestructive Testing
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    • v.29 no.6
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    • pp.550-556
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    • 2009
  • In an application of a time reversal(TR) focusing of array transducer on a defect inside the test material, we employ a new time delay focusing technique based the TR process. In order to realize this idea, a multi-channel ultrasonic system is constructed capable of applying necessary time delays to each channel. The TR-based focusing procedure first measures the backscattered signals after firing one of the array elements. A phase slope method is then used to determine the time-of-flights of the backscattered signals received by all elements of the array. These time delays are used to adjust the time of excitation of the elements for transmission focusing on the defect. In addition to the TR focusing, the classical phased array focusing is also considered for comparison. Experimental results show that the TR-based time delay focusing produces much stronger backscattered signals than the phased array focusing, demonstrating the enhanced capability of the TR focusing.

Nonlinear Time Reversal Focusing and Detection of Fatigue Crack

  • Jeong, Hyun-Jo;Barnard, Dan
    • Journal of the Korean Society for Nondestructive Testing
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    • v.32 no.4
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    • pp.355-361
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    • 2012
  • This paper presents an experimental study on the detection and location of nonlinear scattering source due to the presence of fatigue crack in a laboratory specimen. The proposed technique is based on a combination of nonlinear elastic wave spectroscopy(NEWS) and time reversal(TR) focusing approach. In order to focus on the nonlinear scattering position due to the fatigue crack, we employed only one transmitting transducer and one receiving transducer, taking advantage of long duration of reception signal that includes multiple linear scattering such as mode conversion and boundary reflections. NEWS technique was then used as a pre-treatment of TR for spatial focusing of reemitted second harmonic signal. The robustness of this approach was demonstrated on a cracked specimen and the nonlinear TR focusing behavior is observed on the crack interface from which the second harmonic signal was originated.

Capacity and Secrecy Rate Analysis of a Frequency-Domain Equal-Gain-Combining TR Scheme for Distributed Antenna Systems in Multi-User Multi-Path Fading Channels (다중 사용자 다중 경로 페이딩 채널에서 분산 안테나 시스템을 위한 주파수 영역 Equal-Gain-Combining TR 기법의 Capacity와 Secrecy Rate 분석)

  • Kim, Myoung-Seok;Lee, Chungyong
    • Journal of the Institute of Electronics and Information Engineers
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    • v.49 no.10
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    • pp.47-53
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    • 2012
  • Time-reversal (TR) precoding focuses the energy of the effective channel in time and improves receive performance of a single tap receiver. Frequency-domain equal-gain-combining (FD-EGC) TR scheme, which works in linear block precoding fashion, has better temporal focusing performance than the traditional TR. Also, the FD-EGC improves receive performance of minimum mean square error receiver with distributed antenna systems (DAS). The detailed receive performance of the FD-EGC was analyzed in our previous work. In this paper, we focused on capacity analysis of the FD-EGC in DAS. We derived a scaling law which shows how the use of multiple antenna can increase the capacity of the FD-EGC precoding compared with that of no precoding. In addition, we analyze the secrecy rate of the FD-EGC which shows how high-rate messages can be transmitted towards an intended user without being decoded by the other users from the view point of information theoretic security.

Hybrid Time-Reversal Method for Structural Health Monitoring (구조물 건전성 모니터링을 위한 하이브리드 시간-반전기법)

  • Lee, U-Sik;Kim, Dae-Hwan;Jun, Yong-Ju
    • Proceedings of the KSR Conference
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    • 2010.06a
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    • pp.546-548
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    • 2010
  • This paper proposes a new baseline-free TR-based SHM method in which the time-reversal (TR) property of the guided Lamb waves is utilized. The new TR-based SHM method has two distinct features when compared with the other existing SHM techniques: (1) The measurement- based backward TR process is replaced by the computation-based process (2) In place of the comparison method most commonly used for SHM, the TOF information of the damage signal extracted from the reconstructed signal is utilized for the damage diagnosis. For the damage diagnosis, the imaging method is adopted to efficiently detect damage by representing the damage as an image. The proposed TR-based SHM technique is then validated through the damage diagnosis experiment for an aluminum plate with a damage at different locations.

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Time-reversal microwave focusing using multistatic data

  • Won-Young Song;Soon-Ik Jeon;Seong-Ho Son;Kwang-Jae Lee
    • ETRI Journal
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    • v.46 no.2
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    • pp.333-346
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    • 2024
  • Various techniques for noninvasively focus microwave energy on lesions have been proposed for thermotherapy. To focus the microwave energy on the lesion, a focusing parameter, which is referred to as the magnitude and phase of microwaves radiated from an external array antenna, is very important. Although the finite-difference time-domain (FDTD)-based time-reversal (TR) focusing algorithm is widely used, it has a long processing time if the focusing target position changes or if optimization is needed. We propose a technique to obtain multistatic data (MSD) based on Green's function and use it to derive the focusing parameters. Computer simulations were used to evaluate the electric fields inside the object using the FDTD method and Green's function as well as to compare the focusing parameters using FDTD- and MSD-based TR focusing algorithms. Regardless of the use of Green's function, the processing time of MSD-based TR focusing algorithms reduces to approximately 1/2 or 1/590 compared with the FDTD-based algorithm. In addition, we optimize the focusing parameters to eliminate hotspots, which are unnecessary focusing positions, by adding phase-reversed electric fields and confirm hotspot suppression through simulations.