Conference Record of the Thirty-Fourth Asilomar Conference on Signals, Systems and Computers (Cat. No.00CH37154), 2000
We consider propagation channels which are sparse, i.e., can be modeled by a finite impulse respo... more We consider propagation channels which are sparse, i.e., can be modeled by a finite impulse response filter which has a relatively large order but only a few non-zero taps. An equalizer which makes use of the sparse structure is expected to have better performance than one which does not. We compare the performance of the two types of equalizers by
2006 Fortieth Asilomar Conference on Signals, Systems and Computers, 2006
In time reversal, the received signal, including the additive noise component, is time reversed, ... more In time reversal, the received signal, including the additive noise component, is time reversed, energy normalized, and re-transmitted. The noise contained in the re-transmitted time-reversed signal has a significant impact on the time reversal detection performance. This paper carries out an asymptotic noise analysis of the time reversal generalized likelihood ratio detector, and develops an approximate closed form of the asymptotic SNR gain of the time reversal detector compared with the conventional detector. We show that a rich scattering and low noise environment is preferred for time reversal.
2013 IEEE International Conference on Acoustics, Speech and Signal Processing, 2013
ABSTRACT Real time imaging radar requires short dwell time and low hardware complexity, limiting ... more ABSTRACT Real time imaging radar requires short dwell time and low hardware complexity, limiting the application of many existing radar modulation methods. In this paper, we propose a novel Successive Multiple Frequency Continuous Wave (S-MFCW) modulation scheme to address these challenges. The S-MFCW signal model is formulated and the impact of noise is analyzed, leading to a very short dwell time and narrow signal bandwidth with excellent range and speed accuracy. From the numerical simulation of an automotive radar example, we show that S-MFCW achieves comparable range and speed performance, but orders of magnitude lower requirements on dwell time and bandwidth compared to the commonly used FMCW radar.
In this paper we consider the problem of Bayesian sequential parameter estimation of extended tar... more In this paper we consider the problem of Bayesian sequential parameter estimation of extended targets for cognitive radar with multi-antenna arrays using adaptive waveforms. The target is modeled as a complex Gaussian random process. Using iterative waveform transmission, the cognitive radar estimates the target's characteristic parameters and updates its probabilistic model based on new measurements. The adaptive waveform is designed by minimizing the conditional entropy from the posterior density of the model parameters. We analyze the performance of the developed Bayesian sequential estimation algorithm and derive expressions for the signal-to-noise ratio gain, the asymptotical posterior Cramer Rao bound, and the mutual information gain. The analysis and numerical simulations demonstrate that the adaptive sequential Bayesian estimator yields accelerated convergence of the estimate towards its true value and a smaller estimation error compared with the conventional Bayesian estimator that uses fixed waveform transmission under Gaussian or non-Gaussian noise.
In this paper, we address the problem of spectrum sharing and co-existence between radar and comm... more In this paper, we address the problem of spectrum sharing and co-existence between radar and communications systems by using a reconfigurable dual-use radar/comm system on a single RF platform. We develop a nonlinear least-squares method for range and velocity estimation of moving targets using weighted orthogonal division multiplexing (WOFDM) waveform modulation scheme. The corresponding Cramer-Rao low bounds for the target parameter estimator are derived. Weighted OFDM waveform is derived subject to a low peak to average power ratio constraint. The proposed WOFDM modulation scheme shows improved accuracy for delay estimation compared with the classic constant envelope OFDM modulation method while achieving a lower peak to average power ratio.
2013 IEEE International Conference on Acoustics, Speech and Signal Processing, 2013
In this paper we develop a new framework for Bayesian parameter estimation using adaptive wavefor... more In this paper we develop a new framework for Bayesian parameter estimation using adaptive waveforms by the minimal free energy (FE) principle in the context of cognitive radar. Unlike the conventional approach, the new method utilizes the FE principle as a unified approach for optimal estimator design and waveform design. The FE principle seeks to approximate the true density of the unknown parameters in response to sequential measurement data. In the case of a single unknown parameter we show that the estimators based on the FE principle and the conventional Bayesian estimator are identical. Moreover, the waveform design based on the FE principle results in similar water-filling solution as the traditional mutual information method.
2007 IEEE International Conference on Acoustics, Speech and Signal Processing - ICASSP '07, 2007
We develop time reversal spotlight synthetic aperture radar (TR-SAR) for target focusing and ghos... more We develop time reversal spotlight synthetic aperture radar (TR-SAR) for target focusing and ghost images removal in SAR. Conventional SAR is not designed for imaging targets in a rich scattering environment. In this case, ghost images due to secondary re ections appear in the SAR images. We show in this paper, how, from a rough estimate of the target location obtained from a conventional SAR image and using time reversal, TR-SAR focuses on the target with improved resolution, and reduces or removes ghost images. Veri cation with experimentally measured electromagnetic data demonstrates the success of TR-SAR.
2010 International Conference on Electromagnetics in Advanced Applications, 2010
EM multipath propagation is common in radar and wireless communications. Most radar systems are d... more EM multipath propagation is common in radar and wireless communications. Most radar systems are designed assuming line-of-sight (LOS), not multipath. In this paper, we extend our prior work on Multi-Input Multi-Output (MIMO) radar in the absence of interference [1], to consider MIMO radar detection in high clutter. We develop a subspace MIMO target model and a statistical model for MIMO radar clutter that accounts for the spatial and spectral properties of radar returns. We show that, using orthogonal waveform signaling, the time reversal MIMO radar yields higher detection performance than conventional statistical MIMO radar in high clutter.
2008 5th IEEE International Symposium on Biomedical Imaging: From Nano to Macro, 2008
This paper examines a time reversal beamforming imager for detecting early stage breast cancer tu... more This paper examines a time reversal beamforming imager for detecting early stage breast cancer tumors. We use numerical simulations and electromagnetic tissue experiments to validate the imager. Microwave radiation is known to be a potential diagnostic imaging tool for breast cancer detection that could complement the standard X-ray mammography. Electromagnetic radiation waves undergo multiple scattering due to the inhomogeneities of biological tissues. In this paper, we demonstrate that our proposed time reversal imager exploits successfully the multiple path electromagnetic scattering to achieve higher resolution and robustness than the direct subtraction beamforming imager.
IEEE GLOBECOM 2007-2007 IEEE Global Telecommunications Conference, 2007
In this paper we study the multiple antenna time reversal downlink transmission in an ultra-wideb... more In this paper we study the multiple antenna time reversal downlink transmission in an ultra-wideband (UWB) communication system which consists of access points and users. The access point has multiple antennas and the user has a single antenna. We design the UWB beamformer that focuses on the intended user while minimizing its interference on unintended users and eavesdropping access points. We show that the designed UWB beamformer is equivalent to the time reversal focusing and nulling schemes and yields better performance than the conventional delay line wideband beamformer. We verify our results using experimentally measured electromagnetic data in an indoor environment.
2007 IEEE International Conference on Image Processing, 2007
Microwave radiation is well known as a diagnostic imaging method for many medical applications, f... more Microwave radiation is well known as a diagnostic imaging method for many medical applications, for example, early stage breast cancer detection. Microwave detection of breast tumors is a non-ionising, potentially low cost, in vivo modality that relies on the dielectric contrast between healthy and malignant breast tissues. The scattering environment in a breast often appears to be inhomogeneous due to changing dielectric properties of the breast tissues. Time reversal (TR) is an adaptive waveform transmission scheme that utilizes the rich scattering medium to best match to the target response. In this paper, we develop the microwave time reversal beamformer for breast tumor detection. The TR beamforming scheme is examined based upon a breast model using the twodimensional finite-difference time-domain (FDTD) method. We show that time reversal microwave beamforming is a more robust, higher resolution imaging scheme than conventional beamforming schemes.
We introduce a framework for exploring array detection problems in a reduced-dimensional space. T... more We introduce a framework for exploring array detection problems in a reduced-dimensional space. This involves calculating a structured subarray transformation matrix for the detection of a distributed signal using large aperture linear arrays. We study the performance of the adaptive subarray detector and evaluate its potential improvement in detection performance compared with the full array detector with finite data samples. One would expect that processing on subarrays may result in performance loss in that smaller number of degrees of freedom is utilized. However, it also leads to a better estimation accuracy for the interference and noise covariance matrix with finite data samples, which will yield some gain in performance. By studying the subarray detector for general linear arrays, we identify this gain under various scenarios. We show that when the number of samples is small, the subarray detectors have a significant gain over the full array detector. In addition, the subarray processing can also be successfully applied to the problem of detecting moving sources in an underwater acoustic scenario. We validate our results by computer simulations.
In this paper, we consider the problem of detecting a random spatially distributed signal source ... more In this paper, we consider the problem of detecting a random spatially distributed signal source by an array of sensors. We start with an approximate likelihood ratio (LR) detector and analyze its performance. Using the generalized likelihood ratio (GLR) approach, we then derive detectors under several assumptions on the available statistics. The performance of these detectors is evaluated, and the effect of the angular spread of the source is investigated. The detection performance behaves differently under different scenarios. We notice that the degrees of freedom (DOF) of the distributions of the detection statistics depend on both the signal angular spread and the number of data snapshots. Specifically, at a high SNR level and with small degrees of freedom, an increase of angular spread improves the detection performance. However, with large degrees of freedom, the increase of angular spread reduces detection performance. We provide a detailed discussion of the behavior of detection performance under various conditions. A comparison between the GLR detectors and conventional beamformer detectors is made by computer simulations. The results indicate that the GLR detectors perform better as the angular spread becomes large than that of the conventional beamformer detectors.
Conference Record of the Thirty-Fourth Asilomar Conference on Signals, Systems and Computers (Cat. No.00CH37154), 2000
We consider propagation channels which are sparse, i.e., can be modeled by a finite impulse respo... more We consider propagation channels which are sparse, i.e., can be modeled by a finite impulse response filter which has a relatively large order but only a few non-zero taps. An equalizer which makes use of the sparse structure is expected to have better performance than one which does not. We compare the performance of the two types of equalizers by
2006 Fortieth Asilomar Conference on Signals, Systems and Computers, 2006
In time reversal, the received signal, including the additive noise component, is time reversed, ... more In time reversal, the received signal, including the additive noise component, is time reversed, energy normalized, and re-transmitted. The noise contained in the re-transmitted time-reversed signal has a significant impact on the time reversal detection performance. This paper carries out an asymptotic noise analysis of the time reversal generalized likelihood ratio detector, and develops an approximate closed form of the asymptotic SNR gain of the time reversal detector compared with the conventional detector. We show that a rich scattering and low noise environment is preferred for time reversal.
2013 IEEE International Conference on Acoustics, Speech and Signal Processing, 2013
ABSTRACT Real time imaging radar requires short dwell time and low hardware complexity, limiting ... more ABSTRACT Real time imaging radar requires short dwell time and low hardware complexity, limiting the application of many existing radar modulation methods. In this paper, we propose a novel Successive Multiple Frequency Continuous Wave (S-MFCW) modulation scheme to address these challenges. The S-MFCW signal model is formulated and the impact of noise is analyzed, leading to a very short dwell time and narrow signal bandwidth with excellent range and speed accuracy. From the numerical simulation of an automotive radar example, we show that S-MFCW achieves comparable range and speed performance, but orders of magnitude lower requirements on dwell time and bandwidth compared to the commonly used FMCW radar.
In this paper we consider the problem of Bayesian sequential parameter estimation of extended tar... more In this paper we consider the problem of Bayesian sequential parameter estimation of extended targets for cognitive radar with multi-antenna arrays using adaptive waveforms. The target is modeled as a complex Gaussian random process. Using iterative waveform transmission, the cognitive radar estimates the target's characteristic parameters and updates its probabilistic model based on new measurements. The adaptive waveform is designed by minimizing the conditional entropy from the posterior density of the model parameters. We analyze the performance of the developed Bayesian sequential estimation algorithm and derive expressions for the signal-to-noise ratio gain, the asymptotical posterior Cramer Rao bound, and the mutual information gain. The analysis and numerical simulations demonstrate that the adaptive sequential Bayesian estimator yields accelerated convergence of the estimate towards its true value and a smaller estimation error compared with the conventional Bayesian estimator that uses fixed waveform transmission under Gaussian or non-Gaussian noise.
In this paper, we address the problem of spectrum sharing and co-existence between radar and comm... more In this paper, we address the problem of spectrum sharing and co-existence between radar and communications systems by using a reconfigurable dual-use radar/comm system on a single RF platform. We develop a nonlinear least-squares method for range and velocity estimation of moving targets using weighted orthogonal division multiplexing (WOFDM) waveform modulation scheme. The corresponding Cramer-Rao low bounds for the target parameter estimator are derived. Weighted OFDM waveform is derived subject to a low peak to average power ratio constraint. The proposed WOFDM modulation scheme shows improved accuracy for delay estimation compared with the classic constant envelope OFDM modulation method while achieving a lower peak to average power ratio.
2013 IEEE International Conference on Acoustics, Speech and Signal Processing, 2013
In this paper we develop a new framework for Bayesian parameter estimation using adaptive wavefor... more In this paper we develop a new framework for Bayesian parameter estimation using adaptive waveforms by the minimal free energy (FE) principle in the context of cognitive radar. Unlike the conventional approach, the new method utilizes the FE principle as a unified approach for optimal estimator design and waveform design. The FE principle seeks to approximate the true density of the unknown parameters in response to sequential measurement data. In the case of a single unknown parameter we show that the estimators based on the FE principle and the conventional Bayesian estimator are identical. Moreover, the waveform design based on the FE principle results in similar water-filling solution as the traditional mutual information method.
2007 IEEE International Conference on Acoustics, Speech and Signal Processing - ICASSP '07, 2007
We develop time reversal spotlight synthetic aperture radar (TR-SAR) for target focusing and ghos... more We develop time reversal spotlight synthetic aperture radar (TR-SAR) for target focusing and ghost images removal in SAR. Conventional SAR is not designed for imaging targets in a rich scattering environment. In this case, ghost images due to secondary re ections appear in the SAR images. We show in this paper, how, from a rough estimate of the target location obtained from a conventional SAR image and using time reversal, TR-SAR focuses on the target with improved resolution, and reduces or removes ghost images. Veri cation with experimentally measured electromagnetic data demonstrates the success of TR-SAR.
2010 International Conference on Electromagnetics in Advanced Applications, 2010
EM multipath propagation is common in radar and wireless communications. Most radar systems are d... more EM multipath propagation is common in radar and wireless communications. Most radar systems are designed assuming line-of-sight (LOS), not multipath. In this paper, we extend our prior work on Multi-Input Multi-Output (MIMO) radar in the absence of interference [1], to consider MIMO radar detection in high clutter. We develop a subspace MIMO target model and a statistical model for MIMO radar clutter that accounts for the spatial and spectral properties of radar returns. We show that, using orthogonal waveform signaling, the time reversal MIMO radar yields higher detection performance than conventional statistical MIMO radar in high clutter.
2008 5th IEEE International Symposium on Biomedical Imaging: From Nano to Macro, 2008
This paper examines a time reversal beamforming imager for detecting early stage breast cancer tu... more This paper examines a time reversal beamforming imager for detecting early stage breast cancer tumors. We use numerical simulations and electromagnetic tissue experiments to validate the imager. Microwave radiation is known to be a potential diagnostic imaging tool for breast cancer detection that could complement the standard X-ray mammography. Electromagnetic radiation waves undergo multiple scattering due to the inhomogeneities of biological tissues. In this paper, we demonstrate that our proposed time reversal imager exploits successfully the multiple path electromagnetic scattering to achieve higher resolution and robustness than the direct subtraction beamforming imager.
IEEE GLOBECOM 2007-2007 IEEE Global Telecommunications Conference, 2007
In this paper we study the multiple antenna time reversal downlink transmission in an ultra-wideb... more In this paper we study the multiple antenna time reversal downlink transmission in an ultra-wideband (UWB) communication system which consists of access points and users. The access point has multiple antennas and the user has a single antenna. We design the UWB beamformer that focuses on the intended user while minimizing its interference on unintended users and eavesdropping access points. We show that the designed UWB beamformer is equivalent to the time reversal focusing and nulling schemes and yields better performance than the conventional delay line wideband beamformer. We verify our results using experimentally measured electromagnetic data in an indoor environment.
2007 IEEE International Conference on Image Processing, 2007
Microwave radiation is well known as a diagnostic imaging method for many medical applications, f... more Microwave radiation is well known as a diagnostic imaging method for many medical applications, for example, early stage breast cancer detection. Microwave detection of breast tumors is a non-ionising, potentially low cost, in vivo modality that relies on the dielectric contrast between healthy and malignant breast tissues. The scattering environment in a breast often appears to be inhomogeneous due to changing dielectric properties of the breast tissues. Time reversal (TR) is an adaptive waveform transmission scheme that utilizes the rich scattering medium to best match to the target response. In this paper, we develop the microwave time reversal beamformer for breast tumor detection. The TR beamforming scheme is examined based upon a breast model using the twodimensional finite-difference time-domain (FDTD) method. We show that time reversal microwave beamforming is a more robust, higher resolution imaging scheme than conventional beamforming schemes.
We introduce a framework for exploring array detection problems in a reduced-dimensional space. T... more We introduce a framework for exploring array detection problems in a reduced-dimensional space. This involves calculating a structured subarray transformation matrix for the detection of a distributed signal using large aperture linear arrays. We study the performance of the adaptive subarray detector and evaluate its potential improvement in detection performance compared with the full array detector with finite data samples. One would expect that processing on subarrays may result in performance loss in that smaller number of degrees of freedom is utilized. However, it also leads to a better estimation accuracy for the interference and noise covariance matrix with finite data samples, which will yield some gain in performance. By studying the subarray detector for general linear arrays, we identify this gain under various scenarios. We show that when the number of samples is small, the subarray detectors have a significant gain over the full array detector. In addition, the subarray processing can also be successfully applied to the problem of detecting moving sources in an underwater acoustic scenario. We validate our results by computer simulations.
In this paper, we consider the problem of detecting a random spatially distributed signal source ... more In this paper, we consider the problem of detecting a random spatially distributed signal source by an array of sensors. We start with an approximate likelihood ratio (LR) detector and analyze its performance. Using the generalized likelihood ratio (GLR) approach, we then derive detectors under several assumptions on the available statistics. The performance of these detectors is evaluated, and the effect of the angular spread of the source is investigated. The detection performance behaves differently under different scenarios. We notice that the degrees of freedom (DOF) of the distributions of the detection statistics depend on both the signal angular spread and the number of data snapshots. Specifically, at a high SNR level and with small degrees of freedom, an increase of angular spread improves the detection performance. However, with large degrees of freedom, the increase of angular spread reduces detection performance. We provide a detailed discussion of the behavior of detection performance under various conditions. A comparison between the GLR detectors and conventional beamformer detectors is made by computer simulations. The results indicate that the GLR detectors perform better as the angular spread becomes large than that of the conventional beamformer detectors.
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