Phase-conjugating Array Research Articles

Retrodirective cross-eye jamming based on the phase-conjugating array

Retrodirective cross-eye jamming based on the phase-conjugating array

Source identification of a vibrating plate using phase conjugation and interior boundary element method

Noise pollution is the most serious environmental pollution, which seriously affects people’s normal life and physical and mental health, as well as normal production work. Therefore, noise control is necessary, but before noise control, the first thing to do is to identify the location of the noise source, and the noise control work is meaningless when the noise cannot be identified. Flat plate structures are a fundamental part of complex ship structures, and their vibration, noise, and their interrelationships when excited by the outside world are receiving increasing attention. The core of the research in this paper is the identification of the sound source of the vibrating plate. A method combining phase conjugation with interior boundary element method is developed for the identification of the pressure and normal velocity distribution of a vibrating plate. An interior problem is formed by enclosing the phase conjugation array plane and the plate surface. The pressures at the array elements are phase-conjugated as the specified pressure boundary condition. The impedance relationship between the surface pressure and the surface normal velocity of the plate is utilized as a specified impedance boundary condition. The interior boundary element method is applied to solve the interior problem. The identification of the surface pressure and normal velocity distribution is studied numerically. The numerical results show that with the array located in the near field the proposed method achieves subwavelength focusing to identify the surface pressure and normal velocity distribution and clearly shows the response shapes.

Tri-Mode Heterogeneous Integrated Beam-Switching/Van Atta/Phase-Conjugating Array Using Synthesized Transmission Lines

With the aid of tri-operational mode synthesized transmission lines, an innovative heterogeneous integrated phased array having the functions of beam-switching and retrodirection in three individual bands is proposed and demonstrated. The key innovation is the array feeding network, capable of automatically reconfiguring its topology into three states without applying any active switch. In the low band, the feeding circuit, and hence, the entire system function as a beam-switching array using the Butler matrix for communication and positioning. In the mid and high bands, the integrated array rearranges its feeding network into pairs of isolated delay lines and becomes a Van Atta array and a phase-conjugating array, respectively, to re-radiate the signals to the direction of arrival. The core building blocks are the three-channel selector, tri-mode coupler, and diplexer, all realized by synthesized transmission lines with rigorous design formulae. The operational principle is first introduced, followed by the design and implementation of the key components. A phase compensation scheme is discussed to complete the tri-mode array design. This unique concept is experimentally verified using the radiation characteristics of the integrated array at the end of this paper.

Two‐Ray Modeling of Recombination Effects in The Presence of a Phase Conjugation Array

ABSTRACTThis article describes by means of a simple model how signal recombination effects behave under the influence of phase conjugating retrodirective array (RDA) technology. A two‐ray ground reflection model is used to predict the operational advantages of RDA technology in multipath rich environments. The simulation results show that advantageous signal recombination occurs due to automatic self‐phasing. As the number of elements in the RDA increases, the fading effect normally observed due to out of phase multipath signal is mitigated to the extent that the system approaches that of one operating in a free space environment. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1987–1989, 2013

Identification and location of the distribution of elementary sources based on phase conjugation method

Phase conjugation method can achieve the back propagation and adaptive focusing. It can be used for acoustic source localization. Localization of the distribution of elementary sources is discussed with the discrete phase conjugation planar arrays and the discrete phase conjugation sphere array numerically. Here, it is discussed how both of the shape of array and the distance between phase conjugation array and initial source can influence the spatial resolution. Three variants of phase conjugation arrays are studied: Phase conjugation array made of monopoles, dipoles, or both of them. Corresponding to the three variants, analysis is performed in terms of evanescent and propagative waves and an acoustic sink of the three variants, which absorbs the outgoing wave of the time-reverse wave, is also discussed. The interference pattern of the wave generated by the initial source and the time-reverse wave is used to identify the combination acoustic source.

Analysis of Phase-Conjugating Arrays in Multipath Environments

The ability of a phase-conjugating array (PCA) to reduce fading in single-reflector, dual-reflector, and blocked-line-of-sight multipath environments is presented. Compared to a reference four-element antenna array, the PCA improves worst case signal fading by as much as 39 dB. Agreement between the experimental results and a ray-tracing model is observed, verifying the model as a viable method for characterizing array performance in multipath environments.

Multipath-Enabled Super-Resolution for rf and Microwave Communication using Phase-Conjugate Arrays

We demonstrate experimentally that phase-conjugate techniques can be used to achieve super-resolution focusing of electromagnetic waves in a multipath indoor environment at 2.45 GHz. The focusing phenomena was used to direct independent signals to two locations separated by approximately one-half wavelength, thereby creating two simultaneous channels at the same frequency. An increase in channel capacity is shown to be achievable by an experimental transmission of a 1 Mbps signal over two channels created using a four element phase-conjugate array.

Decision-directed passive phase conjugation: equalisationperformancein shallow water

A decision-directed extension to passive phase conjugation array demodulation is described. Initialisation by a training preamble is followed by block least-squares channel estimation via conjugate gradient and memoryless decisions, which are used by estimation in the next block. Excellent shallow-water performance is shown at ranges up to 4.6 km under windy conditions and shipping noise. The algorithm demonstrates low error rate and robust channel tracking.

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.

Phase conjugation in the ocean: Experimental demonstration of an acoustic time-reversal mirror

An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time-reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source–receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. Phase conjugation was implemented by transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal and retransmitting the time reversed signals from all the sources of the SRA. The retransmitted signal then was received at the VRA. An assortment of runs was made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable, and the experimental results were consistent with theory.

Variable range focusing in a time-reversal mirror

Recently, a time-reversal mirror (or phase conjugate array) was demonstrated experimentally in the ocean, which spatially and temporally re-focused an incident acoustic field back to the original position of the probe source. [Kuperman et al., accepted for publication in J. Acoust. Soc. Am.]. 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 [G. A. Grachev, Acoust. Phys. 39(1), 33–35 (1993)]. This technique has been verified in a second phase conjugate experiment conducted in the Mediterranean Sea in May 1997. The theory behind variable range focusing will be presented along with experimental results demonstrating the ability to shift the focal range ±1 km for a probe source in the vicinity of 15 km from the source–receive array.

Retrofocusing of a time reversing acoustic array in shallow-water random media

A time reversing or phase conjugate array operating in a random medium has been shown to focus acoustic radiation better than an equivalent array operating in a nonrefractive medium, making such arrays attractive for a large class of underwater acoustic problems. The present work extends this result to include the lifetime of the array’s focus, and shallow-water random refraction. Analytic predictions of focal properties, based on the parabolic approximation to the second moment of the Green’s function connecting the source with the array, are made for acoustic propagation through an unbounded dynamic random medium. The necessary projected autocorrelation function for the spatial and temporal evolution of the acoustic medium has been constructed from available oceanographic shallow-water fluctuation statistics. Focus size and focus lifetime are predicted to depend on acoustic frequency, source–array range, and array size. Focus lifetime predictions suggest that there is substantial opportunity for retrofocusing application of this technology. Influence of the ocean surface and deterministic water column variations will be addressed in future research.

Acoustical beamforming and targeting using phase conjugation of ultrasonic fields

Phased arrays were used to locate waterborne targets, and to focus ultrasonic fields onto the targets using phase conjugation in computer simulations. Array performance was evaluated in the presence of small uncertainties added to the amplitudes and phases of the array elements. For the cw case, the free-space Green’s function was used to calculate the conjugated field in space. The propagation of a wideband pulse utilizing time reversal concepts is simulated using a finite-difference code that allows the introduction of inhomogeneous media. Again, the performance of the array with uncertainties in the phase, amplitude, and pulse-shape information is studied. The focusing capability of a high-intensity phase conjugation array operating in the nonlinear regime will suffer from the nonlinear response of the propagating medium and transducer devices. This study examines the performance penalties of uncertainties in the linear regime as a first step to implementing a digital time-reversal system with high-intensity, nonlinear arrays in the laboratory. [Work supported by the ONR.]

Phase-conjugate array focusing in a moving medium

Theoretical formulas are developed that demonstrate the effect of a constant convection velocity on the focusing characteristics of a phase-conjugate (or time-reversing) acoustic array. The convection velocity influences the conjugate-array focus field by displacing it relative to its location in a nonmoving medium. For a planar array and a convection velocity that is less than the acoustic speed, the focal displacement is parallel to the plane of the array, and is proportional the propagation distance and the convection-velocity component parallel to the plane of the array. The convection-velocity component perpendicular to a planar phase-conjugate array only changes the phase of the array’s response. It does not influence the size or location of the phase-conjugate array’s focus.

Narrow-band performance of phase-conjugate arrays in dynamic random media

The theoretical narrow-band performance of acoustic phase-conjugate arrays in the presence of static and dynamic random media is presented. For a static random medium, analytical formulas are derived for the mean focus field of a Gaussian-shaded volumetric phase-conjugate array. The results suggest that random refraction allows phase-conjugate arrays to ‘‘super focus,’’ that is, produce a focal region smaller than the free-space diffraction limit. More specifically, vertical phase-conjugate arrays are predicted to have horizontal directivity. In a dynamic random medium, phase-conjugate array performance is degraded by changes in the medium that occur between the time that the acoustic signal is launched from its source and the time that the array’s transmission is received back at the source location (the round-trip time delay). Formal results are obtained for an intrinsic signal-to-noise ratio for the signal sent from the phase-conjugate array based on a combination of multiple scattering from static random refraction and single scattering from dynamic random refraction in the acoustic medium. These formal results are reduced to analytical formulas for a random medium characterized by the statistics of oceanic internal waves. The intrinsic signal-to-noise ratio is found to be proportional to: the inverse square of the round-trip time delay, the inverse square of the acoustic frequency, the inverse first power of the array-source range, and a simple function that combines the size of the array and the parameters of the random medium. For a typical deep-water oceanic medium at acoustic frequencies near 10 kHz, phase-conjugate array performance may be unaffected for round-trip time delays as long as a minute.

Narrow-band performance of phase-conjugate arrays in dynamic random media.

Phase-conjugate arrays in underwater acoustics are of interest because of their ability to produce retrodirective fields. The theoretical narrow-band performance of acoustic phase-conjugate arrays in the presence of static and dynamic random media will be presented. For a static random medium, our results suggest that random refraction allows phase-conjugate arrays to ‘‘super-focus,’’ that is, produce a focal region smaller than the free-space diffraction limit. For a dynamic random medium, the round-trip time delay between the source and the array degrades phase-conjugate array performance. When the dynamics of the medium are characterized by the statistics of oceanic internal waves, analytical results can be obtained for the intrinsic or ‘‘maximum-possible’’ signal-to-noise ratio produced by the array at the source location. For a typical deep-water oceanic medium, internal wave dynamics do not preclude interesting applications of phase-conjugate arrays. [Work supported by ONR.]