Alan W. Meyer

Time-reversal processing for an acoustic communications experiment in a highly reverberant environment.

Time-reversal (T/R) communications is a new application area motivated by the recent advances in T/R theory. Although perceived by many in signal processing as simply an application of matched-filter theory, a T/R receiver offers an interesting solution to the communications problem for a reverberant channel. In this paper, the performance of various realizations of the T/R receiver for an acoustic communications experiment in air is described along with its associated processing. The experiment is developed to evaluate the performance of point-to-point T/R receivers designed to extract a transmitted information sequence propagating in a highly reverberant environment. It is demonstrated that T/R receivers are capable of extracting the transmitted coded sequence from noisy microphone sensor measurements with zero-symbol error. The processing required to validate these experimental results is discussed. These results are also compared with those produced by an equivalent linear equalizer or inverse filter, which provides the optimal solution when it incorporates all of the reverberations.

K-means reclustering: algorithmic options with quantifiable performance comparisons

This paper presents various architectural options for implementing a K-Means Re-Clustering algorithm suitable for unsupervised segmentation of hyperspectral images. Performance metrics are developed based upon quantitative comparisons of convergence rates and segmentation quality. A methodology for making these comparisons is developed and used to establish K values that produce the best segmentations with minimal processing requirements. Convergence rates depend on the initial choice of cluster centers. Consequently, this same methodology may be used to evaluate the effectiveness of different initialization techniques.

Time reversal and the spatio-temporal matched filter (L)

It is known that focusing of an acoustic field by a time-reversal mirror (TRM) is equivalent to a spatio-temporal matched filter under conditions where the Green’s function of the field satisfies reciprocity and is time invariant, i.e., the Green’s function is independent of the choice of time origin. In this letter, it is shown that both reciprocity and time invariance can be replaced by a more general constraint on the Green’s function that allows a TRM to implement the spatio-temporal matched filter even when conditions are time varying.

Iterative processing of ultrasonic measurements to characterize flaws in critical optical components

Real-time nondestructive evaluation is crucial for the safety and maintenance of critical optics in high energy, laser physics experiments. Fluence levels in short pulse, high-energy lasers can produce pits and cracks in the surfaces of the lasers optical components. These flaws in the optical glass can adversely affect the production of the laser light, or even result in a catastrophic failure of the optical component itself. Consequently, the detection, localization, and characterization of these flaws is critical. This paper describes the novel application of several signal and image-processing techniques that detect, localize, and characterize flaws in optical components. These techniques are embedded into an optic scanning system to automatically identify and report on the condition of the vacuum windows used in high fluence laser systems. These techniques exploit measurements made from two orthogonal acoustic arrays mounted on adjacent edges of the optic. After preprocessing the raw channel measurement data from two orthogonal, narrow beamwidth, transducer arrays, a two-dimensional (2-D) power image is created. A physics-based 2-D matched filter is then developed for detecting and localization. An iterative solution to sequentially search the resulting image to extract and characterize the flaws is discussed.

Bayesian Processing for the Detection of Radioactive Contraband from Uncertain Measurements

With the increase in terrorist activities throughout the world, the need to develop techniques capable of detecting radioactive contraband in a timely manner is a critical requirement. The development of Bayesian processors for the detection of contraband stems from the fact that the posterior distribution is clearly multimodal eliminating the usual Gaussian-based processors. The development of a sequential bootstrap processor for this problem is discussed and shown how it is capable of providing an enhanced signal for eventual detection.

Time reversal in a layered medium

We investigate using a time reversal array to detect voids between two layers of materials. The array is positioned on the surface of a layered material so that acoustic pulses propagate in a direction roughly normal to the layers. These pulses are reflected from both the layer interface and small voids in the interface. The reflected pulses are time‐reversed and sent back into the material. This process is iterated until it converges into an eigenstate of the time reversal operator associated with either a void or the interface. The eigenstates associated with the interface can be understood in terms of interactions between the array and its image in the interface. We look at the problem of detecting eigenstates associated with voids in the presence of the interface eigenstates. Strategies for classifying the eigenstates are derived by considering the simplified problem of a time reversal array on a single layer slab. [Work performed under the auspices of the Department of Energy by the Lawrence Livermore National Laboratory under contract No. W‐7405‐Eng‐48.]

Performance of a multichannel time‐reversal receiver design in a highly reverberative environment

The development of multichannel time‐reversal (T/R) communications in a highly reverberative environment was discussed previously [Candy et al., J. Acoust. Soc. Am. Suppl. 115, 2467 (2004)]. This paper focuses on the development and performance of a 1‐bit receiver in the multichannel case. Here, we discuss the performance of a suite of multichannel TR 1‐bit receiver realizations, similar to the multichannel case, using an acoustic 8‐element T/R array and a set of client stations. The performance of these processors on both simulated and experimental data is discussed in detail. The experiment is provided by a stairwell between two floors of a noisy building. The stairwell is populated with obstructions (pipes, rails, wall, etc.) and a 90‐deg landing—clearly a highly reverberative environment. It is shown that the 1‐bit multichannel receivers perform quite well when compared to their full‐dynamic‐range 16‐bit counterparts, and are able to reliably extract the transmitted code from the noisy measurements.

Time‐reversal communication through a highly reverberant medium

An ultrasonic time‐reversal array system is used to transmit communication signals across an aluminum slab with a large number of holes drilled through it. The hole pattern was designed to greatly attenuate the direct propagation path between each transmitter and receiver, thereby forcing the communication signal to be carried primarily by the random, scattered field. Four approaches to time‐reversal communications [see Candy et al., J. Acoust. Soc. Am. Suppl. 1 114, 2367 (2003)] were used to establish clear (negligible symbol error) communication channels through the slab. Both point‐to‐point and array‐to‐point configurations were tested. Comparisons between these approaches are shown using both simulated and experimental measurements. [Work performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W‐7405‐Eng‐48.]

Time‐reversal communications in a hostile, reverberative environment

Time‐reversal (T/R) communications is a new application area motivated by the recent advances in T/R theory. T/R receivers offer an interesting solution to the communications problem for a reverberant channel. This paper describes the performance of various realizations of the T/R receiver for an acoustic communications experiment in air. An experiment is developed to evaluate the performance of point‐to‐point T/R receivers designed to extract a transmitted information sequence propagating in a hostile, highly reverberant environment. It is demonstrated that T/R receivers are capable of extracting the transmitted coded sequence from noisy microphone sensor measurements with reasonable success. The processing required to validate these experimental results is discussed. These results are also compared with those produced by an equivalent linear equalizer or inverse filter, which provides the optimal solution when it incorporates all of the reverberations.

Experimental design and processing for time‐reversal communications in a highly reverberant environment

A suite of experiments has recently been conducted to validate the utility of using time‐reversal (T/R) theory to solve a communications problem for highly reverberant environments. This paper discusses the design and layout of those experiments as well as experimental equipment criteria and selection. Solutions to problems arising from equipment limitations encountered during the experimental design process are examined. The signal processing used to extract information from gathered data is described and it is shown that communications receivers utilizing T/R theory can be used to accurately reproduce messages broadcast through hostile, reverberant communications channels.