Nicholas O'Donoughue

Time Reversal in Multiple-Input Multiple-Output Radar

Time reversal explores the rich scattering in a multipath environment to achieve high target detectability. Multiple-input multiple-output (MIMO) radar is an emerging active sensing technology that uses diverse waveforms transmitted from widely spaced antennas to achieve increased target sensitivity when compared to standard phased arrays. In this paper, we combine MIMO radar with time reversal to automatically match waveforms to a scattering channel and further improve the performance of radar detection. We establish a radar target model in multipath rich environments and develop likelihood ratio tests for the proposed time-reversal MIMO radar (TR-MIMO). Numerical simulations demonstrate improved target detectability compared with the commonly used statistical MIMO strategy.

On the Product of Independent Complex Gaussians

In this paper, we derive the joint (amplitude, phase) distribution of the product of two independent non-zero-mean Complex Gaussian random variables. We call this new distribution the complex Double Gaussian distribution. This probability distribution function (PDF) is a doubly infinite summation over modified Bessel functions of the first and second kind. We analyze the behavior of this sum and show that the number of terms needed for accuracy is dependent upon the Rician k-factors of the two input variables. We derive an upper bound on the truncation error and use this to present an adaptive computational approach that selects the minimum number of terms required for accuracy. We also present the PDF for the special case where either one or both of the input complex Gaussian random variables is zero-mean. We demonstrate the relevance of our results by deriving the optimal Neyman-Pearson detector for a time reversal detection scheme and computing the receiver operating characteristics through Monte Carlo simulations, and by computing the symbol error probability (SEP) for a single-channel M-ary phase-shift-keying (M-PSK) communication system.

Time Reversal Synthetic Aperture Radar Imaging In Multipath

Conventional spotlight synthetic aperture radar (SAR) assumes a single reflection of transmitted waveforms from targets [1]. Multiple reflections of targets due to surrounding scatterers appear as ghosting artifacts in conventional SAR images, which obscures true target image and leads to poor resolution. In this paper, we develop image formation techniques using time reversal, time reversal SAR (TR-SAR), to remove ghosting artifacts and achieve high resolution. The TR-SAR algorithm is tested using phase history data collected by a rail-mounted SAR sensor operated by Raytheon.

Detection of structural defects in pipes using time reversal of guided waves

Structural health monitoring of buried pipelines is of vital importance as infrastructures age. Ultrasonic guided waves are a popular method for inspecting buried pipes, due to their potential for long propagation. Unfortunately, the large number of wave modes present, and the effects of dispersion, in a pipeline make analysis of the received signals difficult. We plan to use Time Reversal Acoustics to compensate for these complex signals, and improve performance for the detection of faults in a pipeline. We will present theoretical performance results for conventional and Time Reversal detectors, verified with simulations conducted in PZFlex. Time Reversal shows a potential for a reduction in the power requirements of a fault detection system.

Adaptive time reversal beamforming in dense multipath communication networks

In this paper, we study the transmit time reversal beamforming in rich scattering environments in a multi-user wideband communication network. We consider the downlink time reversal transmission where a base station communicates with many users. The base station has multiple antennas and the user has a single antenna. Due to the presence of scatterers, conventional transmit beamforming suffers from crosstalk caused by nearby users. We design a wideband beamformer that focuses on the intended user while minimizing its interference to other users subject to the constraint of the sum rate capacity. We also show that the proposed beamformer is equivalent to time reversal focusing and nulling and yields higher sum data rate than the conventional delay line wideband beamformer. We verify our results using realistic scattering channels simulated by finite-difference time-domain modeling of the scattering environment.

Position location by time reversal in communication networks

Multipath effects are significant in urban or indoor communications. Current position location techniques such as TDOA suffer from multipath effects, which reduces the estimation accuracy. In this paper we propose time-reversal in a wireless communication network, where a mobile terminal wants to determine, through feedback, its own position in the request of the base station. The proposed method improves the estimation accuracy and reduces the estimation variance compared with correlation based method. We derive the closed form of the Cramer-Rao bound (CRB) on the time reversal estimation and the correlation estimation, showing that time reversal achieves a smaller CRB than the correlation method. Numerical examples are presented to illustrate the behavior of these bounds.

Ultrasonic monitoring of a pipe under operating conditions

The paper presents experimental results of applying an ultrasonic monitoring system to a real-world operating hot-water supply system. The purpose of these experiments is to investigate the feasibility of continuous ultrasonic damage detection on pipes with permanently mounted piezoelectric transducers under environmental and operational variations. Ultrasonic guided wave is shown to be an efficient damage detector in laboratory experiments. However, environmental and operational variations produce dramatic changes in those signals, and therefore a useful signal processing approach must distinguish change caused by a scatterer from change caused by ongoing variations. We study pressurized pipe segments (10-in diameter) in a working hot-water supply system that experiences ongoing variations in pressure, temperature, and flow rate; the system is located in an environment that is mechanically and electrically noisy. We conduct pitch-catch tests, with a duration of 10 ms, between transducers located roughly 12 diameters apart. We applied different signal processing techniques to the collected data in order to investigate the ongoing environmental and operational variations and the stationarity of the signal. We present our analysis of these signals and preliminary detection results.

Time reversal for damage detection in pipes

Monitoring the structural integrity of vast natural gas pipeline networks requires continuous and economical inspection technology. Current approaches for inspecting buried pipelines require periodic excavation of sections of pipe to assess only a couple of hundred meters at a time. These inspection systems for pipelines are temporary and expensive. We propose to use guided-wave ultrasonics with Time Reversal techniques to develop an active sensing and continuous monitoring system. Pipe environments are complex due to the presence of multiple modes and high dispersion. These are treated as adverse effects by most conventional ultrasonic techniques. However, Time Reversal takes advantage of the multi-modal and dispersive behaviors to improve the spatial and temporal wave focusing. In this paper, Time Reversal process is mathematically described and experimentally demonstrated through six laboratory experiments, providing comprehensive and promising results on guided wave focusing in a pipe with/without welded joint, with/without internal pressure, and detection of three defects: lateral, longitudinal and corrosion-like. The experimental results show that Time Reversal can effectively compensate for multiple modes and dispersion in pipes, resulting in an enhanced signal-to-noise ratio and effective damage detection ability. As a consequence, Time Reversal shows benefits in long-distance and lowpower pipeline monitoring, as well as potential for applications in other infrastructures.

Single antenna time reversal detection of moving target

This paper is concerned with a moving target detection using time reversal in dense multipath environments. We show that the Doppler shift in the time reversal re-transmission simplifies the detector design, yet still achieves the focusing effect. Thus, the Doppler diversity is utilized to achieve high target detectability by time reversal.

Time reversal beamforming of guided waves in pipes with a single defect

Structural health monitoring of buried pipelines is an important application for aging infrastructures. Ultrasonic guided wave inspection is an attractive tool, due to the long propagation of guided waves in the wall of a hollow cylinder. However, guided waves present a unique environment with heavily multi-modal signal propagation and complex dispersion (frequency-dependent propagation speeds). In order to alleviate these challenges, conventional techniques rely on high-voltage excitation with complex transducer arrays, but these systems are not conducive to a monitoring solution. Instead, they require periodic excavation and testing. In prior work, we have shown that Time Reversal allows for reliable detection with relatively simple antenna arrays that can be operated in low-power. This paper focuses on localization of these defects. We utilize a beamforming approach that makes use of theoretical dispersion curves to generate fault images. We show through simulations that Time Reversal Beamforming achieves high-resolution localization of a fault in the presence of strong dispersion and heavily multi-modal propagation.