Moeness G. Amin

Synthetic aperture beamformer for imaging through a dielectric wall

A coarray-based aperture synthesis scheme using subarrays and postdata acquisition beamforming is presented for through-the-wall wideband microwave imaging applications. The wall causes wave refraction and a change in the propagation speed, both effects alter the travel time between the transmitter, the target, and the receiver. Coherent combining of the pulse waveforms emitted by the different transmitters and incident at the receivers through reflections from targets and clutter requires incorporation of wall effects into the beamformer design. Simulation results verifying the proposed synthetic aperture technique for a through-the-wall imaging (TWI) system are presented. The impact of the wall ambiguities or incorrect estimates of the wall parameters, such as thickness and dielectric constant, on performance is considered.

Through-the-Wall Radar Imaging

Wall Attenuation and Dispersion, A. Hussein Muqaibel, M.A. Alsunaidi, Nuruddeen M. Iya, and A. Safaai-Jazi Antenna Elements, Arrays, and Systems for Through-the-Wall Radar Imaging, A. Hoorfar and A. Fathy Beamforming for Through-the-Wall Radar Imaging, G. Alli and D. DiFilippo Image and Localization of Behind-the-Wall Targets Using Collocated and Distributed Apertures, Y.D. Zhang and A. Hunt Conventional and Emerging Waveforms for Detection and Imaging of Targets behind Walls, F. Ahmad and R.M. Narayanan Inverse Scattering Approaches in Through-the-Wall Imaging, K. Sarabandi, M. Thiel, M. Dehmollaian, R. Solimene, and F. Soldovieri Through-the-Wall Microwave Building Tomography, P.B. Weichman, E.M. Lavely, E.H. Hill III, and P. Zemany Analytical Ray Methods for Through-the-Wall Radar Imaging, R.J. Burkholder, R.J. Marhefka, and J.L. Volakis Synthetic Aperture Radar Techniques for Through-the-Wall Imaging, T. Dogaru and C. Le Impulse SAR and Its Application for Through-the-Wall Detection and Identification of People and Weapons, J.Z. Tatoian Through-the-Wall SAR for Characterization of Building Interior Structure Using Attributed Scattering Center Features, E. Ertin and R.L. Moses Detection Approaches in Through-the-Wall Radar Imaging, C. Debes and A.M. Zoubir Detection of Concealed Targets in Through-the-Wall Imaging, L. Crocco Fast Acquisition and Compressive Sensing Techniques for Through-the-Wall Radar Imaging, M. Amin, Y-S. Yoon, and S. Kassam Radar Micro-Doppler Signatures for Characterization of Human Motion, V.C. Chen, G.E. Smith, K. Woodbridge, and C.J. Baker Index

Interference mitigation in spread spectrum communication systems using time-frequency distributions

The capability of the time-frequency distributions (TFDs) to properly represent a single as well as multiple component signals in time and frequency permits the application of a new approach for interference excision in spread spectrum communication systems. The instantaneous frequency (IF) estimate from the TFD is used to construct a finite impulse response filter that reduces the interference power with a minimum possible distortion of the desired signal. The proposed technique is therefore a case of open-loop adaptive filtering. Three- and five-coefficient zero-phase excision filters are considered. Closed-form expressions of the improvement of SNR at the receiver correlator output using the TFD-based adaptive filtering are derived for two extreme cases of time-varying interferers, namely, those of fixed frequency sinusoids and randomly changing instantaneous frequencies. Simulation results including the bit error rates are presented for both swept and frequency hopping jammers.

Spatial Filtering for Wall-Clutter Mitigation in Through-the-Wall Radar Imaging

Radio-frequency imaging of targets behind walls is of value in several civilian and defense applications. Wall reflections are often stronger than target reflections, and they tend to persist over a long duration of time. Therefore, weak and close by targets behind walls become obscured and invisible in the image. In this paper, we apply spatial filters across the antenna array to remove, or at least significantly mitigate, the spatial zero-frequency and low-frequency components which correspond to wall reflections. Unmasking the behind-the-wall targets via the application of spatial filters recognizes the fact that the wall electromagnetic (EM) responses do not significantly differ when viewed by the different antennas along the axis of a real or synthesized array aperture which is parallel to the wall. The proposed approach is tested with experimental data using solid wall, multilayered wall, and cinder block wall. It is shown that the wall reflections can be effectively reduced by spatial preprocessing prior to beamforming, producing similar imaging results to those achieved when a background scene without the target is available.

Imaging Through Unknown Walls Using Different Standoff Distances

In through-the-wall imaging, errors in wall parameters cause targets to be imaged away from their true positions. The displacement in target locations depend on the accuracy of the estimates of the wall parameters as well as the target position relative to the antenna array. A technique using two or more standoff distances of the imaging system from the wall is proposed for application under wall parameter ambiguities. Two different imaging schemes can then be applied to correct for errors in wall characteristics. The first scheme relies on forming target displacement trajectories, each corresponding to a different standoff distance, and assuming different values of wall thickness and dielectric constant. The target position is then determined as the trajectories crossover point. In the second scheme, an image sequence is generated. Each specific image in this sequence is obtained by summing those corresponding to different standoff distances, but with the same assumed wall parameters. An imaging-focusing metric can then be adopted to determine the target position. The paper analyzes the above two schemes and provides extensive simulation examples demonstrating their effectiveness

Generalized Coprime Array Configurations for Direction-of-Arrival Estimation

A coprime array uses two uniform linear subarrays to construct an effective difference coarray with certain desirable characteristics, such as a high number of degrees-of-freedom for direction-of-arrival (DOA) estimation. In this paper, we generalize the coprime array concept with two operations. The first operation is through the compression of the inter-element spacing of one subarray and the resulting structure treats the existing variations of coprime array configurations as well as the nested array structure as its special cases. The second operation exploits two displaced subarrays, and the resulting coprime array structure allows the minimum inter-element spacing to be much larger than the typical half-wavelength requirement, making them useful in applications where a small interelement spacing is infeasible. The performance of the generalized coarray structures is evaluated using their difference coarray equivalence. In particular, we derive the analytical expressions for the coarray aperture, the achievable number of unique lags, and the maximum number of consecutive lags for quantitative evaluation, comparison, and design of coprime arrays. The usefulness of these results is demonstrated using examples applied for DOA estimations utilizing both subspace-based and sparse signal reconstruction techniques.

Compressed sensing technique for high-resolution radar imaging

Compressed sensing (CS) has recently attracted much interest because of its important offerings and versatility. High-resolution radar imaging applications such as through-the-wall radar (TWR) imaging or inverse synthetic aperture radar (ISAR) are two key application areas that can greatly benefit from CS. Both applications require probing targets using radar signals with large bandwidth for collecting, and then processing, a large number of data samples for achieving high resolution imaging. These applications are also characterized by sparse imaging where targets of interest are few and have larger cross-section than clutter objects. Reducing the number of samples without compromising the imaging quality reduces the acquisition time and saves signal bandwidth. This reduction is important when surveillance is performed within small time window and when targets are required to remain stationary without translation or rotation motions, to avoid blurring and smearing of images. In this paper, we discuss applicability of compressed sensing to indoor radar imaging, using synthesized TWR data.

Three-Dimensional Wideband Beamforming for Imaging Through a Single Wall

Through-the-wall imaging and urban sensing is an emerging area of research and development. The incorporation of the effects of signal propagation through wall material in producing an indoor image is important for reliable through-the-wall mission operations. We have previously analyzed wall effects, such as refraction and change in propagation speed, and designed a wideband beamformer for 2D imaging using line arrays. In this letter, we extend the analysis to 3D imaging via delay-and-sum beamforming in the presence of a single uniform wall. The third dimension provides valuable information on target heights that can be used for enhancing target discrimination/identification. Supporting simulation results are provided.

Time-frequency MUSIC

A new method for the estimation of the signal subspace and noise subspace based on time-frequency signal representations is introduced. The proposed approach consists of the joint block-diagonalization (JBD) of a set of spatial time-frequency distribution matrices. Once the signal and noise subspaces are estimated, any subspace based approach, including the multiple signal classification (MUSIC) algorithm, can be applied for direction of arrival (DOA) estimation. Performance of the proposed time-frequency MUSIC (TF-MUSIC) for an impinging chirp signal using three different kernels is numerically evaluated.

Autofocusing of Through-the-Wall Radar Imagery Under Unknown Wall Characteristics

The quality and reliability of through-the-wall radar imagery is governed, among other things, by the knowledge of the wall characteristics. Ambiguities in wall characteristics smear and blur the image, and also shift the imaged target positions. An autofocusing technique, based on higher order statistics, is presented which corrects for errors under unknown walls. Simulation results show that the proposed technique provides high-quality focused images with target locations in close proximity to true target positions.