Time-domain channel estimator based on cyclic correlation for OFDM systems with guard interval

Channel impulse response （CIR） can be estimated on the basis of cyclic correlation in time-domain for orthogonal frequency division multiplexing （OFDM） systems, This article proposes a generalized channel estimation method to reduce the estimation error by taking the average of different CIRs. Channel impulse responses are derived according to the different starting points of cyclic correlation. In addition, an effective CIR length estimation algorithm is also presented. The whole proposed methods are more effective to OFDM systems, especialiy to those with longer cyclic prefix. The analysis and the simulation results verify that the mean square error performance is 4-5 dB better than the conventional schemes under the same conditions.     

Time-domain inverse scattering method for cross-borehole radar imaging

In this paper, we consider a solution method of the inverse problem of imaging two-dimensional (2D) objects buried underground by cross-hole radar data in the time domain. In addition to less information on the targets due to restriction on the arrangement of transmitters and receivers than for full-view cases such as imaging of objects in free space, the large search region between boreholes makes solving the inverse problem difficult. Although iterative optimization approaches take long computing time, these approaches give much better image qualities for high-contrast objects than linear inversions such as a diffraction tomography. However, the reconstruction in a large search region with limited-view measurements often fails trapped in a local minimum. To overcome this difficulty, we propose a two-step iterative approach: the first step is to reduce the search region to a smaller one and the second step is the accurate reconstruction of the targets in the small region. Both steps are based on an iterative optimization approach, i.e., the forward-backward time-stepping method previously proposed. This two-step approach is tested for detection of tunnel-like objects surrounded by a heterogeneous background medium to evaluate its performance. Numerical results indicate the efficiency of the approach and its ability of circumventing local minima.     

Experimental results for cascadable micropower time delay estimator

Experimental results of a micropower integrated circuit (IC) for the measurement of the relative time delay between two signals are presented. A complete experimental characterisation shows that the IC achieves an accuracy of 3 mus in the whole measurement range with a power consumption of 12 muW. This improves on all low-power designs previously reported in the literature     

Spherical wave near-field imaging and radar cross-sectionmeasurement

The paper presents a new inverse synthetic aperture radar (ISAR) algorithm intended for radar cross-section (RCS) imaging and measurement from scattered fields. The method, based on a spherical-wave near-field illumination of the target, overcomes the requirement for an expensive compact range facility to produce a plane wave illumination. The formulation and the implementation of the algorithm are described. Some experimental results obtained in an anechoic chamber are presented to show RCS results similar to the conventional plane wave methods     

Accelerating near-field 3D imaging approach for joint high-resolution imaging and phase error correction

The computational complexity and memory requirements of large-scale data seriously affect the application of compressed sensing (CS) in near-field three-dimensional (3-D) imaging system. In addition, as influenced by the measurement environment, the error in echo phase results in imaging defocusing. This paper proposes a CS near-field 3-D imaging approach based on nonuniform fast Fourier transform and phase error correction. It applies the fast Gaussian gridding nonuniform fast Fourier transform technique and Separable Surrogate Functionals with only matrix and vector multiplied to accelerate imaging speed and reduce memory requirements; it adopts the phase error correction technique to realize highly-focused imaging; in addition, a sparse observation approach based on Logistic sequence is proposed in this paper for easy availability of engineering realization for CS imaging. As indicated by numerical analysis and actual measurement in anechoic chamber, the approach proposed in this paper, compared with traditional imaging approaches, has the following advantages: accurate high resolution 3-D image of target can be obtained by applying small amount of observation data (10%); the computational complexity falls from O(LN) to O(3N) and memory occupation quantity drops from O(LN) to O(N); it can effectively perform highly-focused imaging for echo signal with phase error; the measurement matrix designed has better non-coherence and easy availability for engineering realization.

     

Time-domain near-field analysis of short-pulse antennas .II.Reactive energy and the antenna Q

For pt. I see ibid., vol.47, no.2, p.271-79 (1999). The time-domain (TD) multipole expansion, developed in the first part of this two-part sequence, is extended here to analyze the power-flow and energy balance in the vicinity of a pulsed antenna. Using the spherical transmission line formulation, we derive expressions for the pulsed power-flow and energy and identify the radiative and the reactive constituents. For time-harmonic fields, the reactive concepts are well understood in terms of the stored energy, but this interpretation is not applicable for short-pulse fields where there is no stored energy. By considering the TD energy balance, we clarify the transition of the near-zone pulsed reactive energy to the radiation power and show that the pulsed reactive energy discharges back to the source once the pulse has been radiated. We thus introduce a TD Q factor that quantifies the radiation efficiency. In particular, we show that super resolution using short-pulse fields involves large TD reactive energies and Q and is, therefore, not feasible. The general TD concepts discussed are demonstrated through a numerical example of radiation from a circular disk carrying a pulsed current distribution     

The bi-polar planar near-field measurement technique, part II:near-field to far-field transformation and holographic imaging methods

A novel customized bi-polar planar near-field measurement technique is presented in a two-part paper. This bipolar technique offers a large scan plane size with minimal “real-estate” requirements and a simple mechanical implementation, requiring only rotational motions, resulting in a highly accurate and cost-effective antenna measurement and diagnostic system. Part I of this two-part paper introduced the bi-polar planar near-field measurement concept, discussed the implementation of this technique at the University of California, Los Angeles (UCLA), and provided a comparative survey of measured results. This paper examines the data processing algorithms that have been developed and customized to exploit the unique features of the bi-polar planar near-field measurement technique. Near-field to far-field transformation algorithms investigated include both interpolatory and non-interpolatory algorithms due to the a typical arrangement of the bi-polar near-field samples. The algorithms which have been tailored for the bi-polar configuration include the optimal sampling interpolation (OSI)/fast Fourier transform (FFT), Jacobi-Bessel transform, and Fourier-Bessel transform. Additionally, holographic imaging for determination of antenna aperture fields has been incorporated to facilitate antenna diagnostics. Results for a simulated measurement of an array of infinitesimal dipoles and a measured waveguide-fed slot array antenna are included. Appropriate guidelines with respect to the advantages and disadvantages of the various processing algorithms are provided     

CMOS chip for invasive ultrasound imaging

A very small transmit/receiver chip has been developed for use in an arterial ultrasonic imaging system. In this technique, a solid-state ultrasonic imaging head placed within a small medical catheter is used to provide high quality 360° images of arteries as small as 2 mm in diameter. Novel design and packaging techniques have been used to allow four easily testable 0.86 mm×1.65 mm mixed-signal CMOS die to be placed on a multichip carrier within this 1.83 mm diameter imaging probe. Each chip contains interface circuitry for sixteen transducers including 20 MHz transmit pulsers and receive current amplifiers with approximately 1.3 pA/rt-Hz equivalent input noise performance. The techniques described here are generally applicable to any probe or device with extreme size and performance requirements     

Programmable ultrasound imaging using multimedia technologies: anext-generation ultrasound machine

High computational and throughput requirements in modern ultrasound machines have restricted their internal design to algorithm-specific hardware with limited programmability. The authors have architected a programmable ultrasound processing system, Programmable Ultrasound Image Processor (PUIP), to facilitate engineering and clinical ultrasound innovations. Multiple high-performance multimedia processors were used to provide a computing power of 4 billion operations per second. Flexibility was achieved by making the system programmable and multimodal, e.g., B-mode, color flow, cine and Doppler data can be processed. They have successfully designed and implemented the PUIP to fit within an ultrasound machine. It provides a platform for rapid testing of new concepts in ultrasound processing and enables software upgrades for future technologies. Current and future clinical applications include extended fields of view, quantitative measurements, three-dimensional ultrasound reconstruction and visualization, adaptive persistence, speckle reduction, edge enhancement, image segmentation, and motion analysis. The PUIP is a significant step in the evolution of ultrasound machines toward more flexible and generalized systems bridging the gap between many innovative ideas and their clinical use in ultrasound machines     

Progress in multimodality imaging: truly simultaneous ultrasound and magnetic resonance imaging

Multimodality medical imaging takes advantage of the strengths of different imaging modalities to provide a more complete picture of the anatomy under investigation. Many complementary modalities have been combined to form such systems and some are gaining use clinically. One combination that has not been developed, in large part due to technical difficulties, is a combined magnetic resonance (MR) and ultrasound (US) imaging system. Such a system offers the potential to combine the strengths of these modalities in a wide range of diagnostic and therapeutic applications. The goal of this study was to evaluate the feasibility of performing simultaneous multimodality US and MR imaging. An US imaging system capable of operation in a clinical MR imager was developed, and methods to perform simultaneous imaging were investigated. Simultaneous imaging was feasible without any mutual interference by either filtering the transmitted and received US signal, or by synchronizing data acquisition between the two imaging systems. Spatial registration between the two modalities was achieved by using a reference phantom with implanted glass beads in orthogonal planes. Excellent agreement was observed between spatial measurements of an object made with both modalities, and the feasibility of using this system in vivo was demonstrated in a rabbit model. Simultaneous US and MR imaging is achievable, and can provide complementary information about an object under investigation. This demonstration of technical feasibility and the development of a prototype system open up the potential to investigate the promising clinical applications of this combined technology.     

Detection performance theory for ultrasound imaging systems

A rigorous statistical theory for characterizing the performance of medical ultrasound systems for lesion detection tasks is developed. A design strategy for optimizing ultrasound systems should be to adjust parameters for maximum information content, which is obtained by maximizing the ideal observer performance. Then, given the radio-frequency data, image and signal processing algorithms are designed to extract as much diagnostically relevant information as possible. In this paper, closed-form and low-contrast approximations of ideal observer performance are derived for signal known statistically detection tasks. The accuracy of the approximations are tested by comparing with Monte Carlo techniques. A metric borrowed and modified from photon imaging, Generalized Noise Equivalent Quanta, is shown to be a useful and measurable target-independent figure of merit when adapted for ultrasound systems. This theory provides the potential to optimize design tradeoffs for detection tasks. For example it may help us understand how to push the limits of specific features, such as spatial resolution, without significantly compromising overall detection performance.     

Time-delay- and time-reversal-based robust Capon beamformers for ultrasound imaging

Currently, the nonadaptive delay-and-sum (DAS) beamformer is extensively used for ultrasound imaging, despite the fact that it has lower resolution and worse interference suppression capability than the adaptive standard Capon beamformer (SCB) if the steering vector corresponding to the signal of interest (SOI) is accurately known. The main problem which restricts the use of SCB, however, is that SCB lacks robustness against steering vector errors that are inevitable in practice. Whenever this happens, the performance of SCB may hecome worse than that of DAS. Therefore, a robust adaptive beamformer is desirable to maintain the robustness of DAS and adaptivity of SCB. In this paper we consider a recent promising robust Capon beamformer (RCB) for ultrasound imaging. We propose two ways of implementing RCB, one based on time delay and the other based on time reversal. RCB extends SCB by allowing the array steering vector to be within an uncertainty set. Hence, it restores the appeal of SCB including its high resolution and superb interference suppression capabilities, and also retains the attractiveness of DAS including its robustness against steering vector errors. The time-delay-based RCB can tolerate the misalignment of data samples and the time-reversal-based RCB can withstand the uncertainty of the Green's function. Both time-delay-based RCB and time-reversal-based RCB can be efficiently computed at a comparable cost to SCB. The excellent performances of the proposed robust adaptive beamforming approaches are demonstrated via a number of simulated and experimental examples.     

Three-dimensional ultrasound imaging systems for prostate cancerdiagnosis and treatment

3D ultrasound imaging system for imaging the prostate can be interfaced to any conventional ultrasound machine, and can accommodate side-firing transrectal ultrasound transducers. After acquiring a series of 2D ultrasound images, a 3D image is reconstructed. The 3D image is available to the physician, allowing the prostate to be viewed interactively in multiple simultaneous planes, allowing better visualization of its internal architecture. This approach allows the physician to record and view the whole prostate in successive examinations, making 3D TRUS well-suited to performing prospective or follow-up studies. The results indicate that 3D ultrasound imaging of the prostate has great potential as a tool for the diagnosis and follow-up of prostate disease     

Real-time vessel segmentation and tracking for ultrasound imaging applications

A method for vessel segmentation and tracking in ultrasound images using Kalman filters is presented. A modified Star-Kalman algorithm is used to determine vessel contours and ellipse parameters using an extended Kalman filter with an elliptical model. The parameters can be used to easily calculate the transverse vessel area which is of clinical use. A temporal Kalman filter is used for tracking the vessel center over several frames, using location measurements from a handheld sensorized ultrasound probe. The segmentation and tracking have been implemented in real-time and validated using simulated ultrasound data with known features and real data, for which expert segmentation was performed. Results indicate that mean errors between segmented contours and expert tracings are on the order of 1%-2% of the maximum feature dimension, and that the transverse cross-sectional vessel area as computed from estimated ellipse parameters a, b as determined by our algorithm is within 10% of that determined by experts. The location of the vessel center was tracked accurately for a range of speeds from 1.4 to 11.2 mm/s.     

Statistically optimized minislot allocation for initial andcollision resolution in hybrid fiber coaxial networks

In a two-way hybrid fiber coaxial (HFC) network, the headend broadcasts in downstream channels, whereas all stations share the upstream channels. Hence, collision occurs when multiple stations send their bandwidth requests in a minislot. The headend determines how many minislots to allocate to manage collisions. This paper proposes a minislot allocation (SOMA) algorithm to optimize minislot throughput based on statistical estimation. A time proportional scheme is adopted to estimate the number of new requests in the initial resolution process. In addition, the number of retry requests in the collision resolution process is estimated by looking up a predetermined table of the most likely number of requests (MLR). In addition, SOMA is modified to reduce the request access delay by relaxing its allocation policy in a specific situation. We use a self-similar traffic model for simulation and analysis to compare SOMA with the optimal bound and the 3-ary tree algorithm     

Time-domain tests for Gaussianity and time-reversibility

Statistical signal processing algorithms often rely upon Gaussianity and time-reversibility, two important notions related to the probability structure of stationary random signals and their symmetry. Parametric models obtained via second-order statistics (SOS) are appropriate when the available data is Gaussian and time-reversible. On the other hand, evidence of nonlinearity, non-Gaussianity, or time-irreversibility favors the use of higher-order statistics (HOS). In order to validate Gaussianity and time-reversibility, and quantify the tradeoffs between SOS and HOS, consistent, time-domain chi-squared statistical tests are developed. Exact asymptotic distributions are derived to estimate the power of the tests, including a covariance expression for fourth-order sample cumulants. A modification of existing linearity tests in the presence of additive Gaussian noise is discussed briefly. The novel Gaussianity statistic is computationally attractive, leads to a constant-false-alarm-rate test and is well suited for parametric modeling because it employs the minimal HOS lags which uniquely characterize ARMA processes. Simulations include comparisons with an existing frequency-domain approach and an application to real seismic data. Time-reversibility tests are also derived and their performance is analyzed both theoretically and experimentally     

Charge-coupled imaging devices: Experimental results

The design and fabrication of a 96-element 3-phase linear charge-coupled device are described. A transfer efficiency of ∼95 percent over 288 transfers at a 1-MHz clock rate was measured. The use of the device as an analog delay line is demonstrated and its imaging properties are illustrated with reproductions of black and white text and a picture with gray scale. The results demonstrate the feasibility of using self-scanned imaging devices in practical applications. Configurations are presented for both an improved linear and an area imaging device. In both cases the problem of image smear, which occurs if stored charge is transferred along the light-sensitive region and if significant light integration takes place during this transfer, can be avoided.     

A novel portable bipolar near-field measurement system for millimeter-wave antennas: construction, development, and verification

As new antenna designs require higher frequencies and smaller sizes, traditional large-scale antenna measurement systems become ill-suited for such measurements. External mixing, room-sized chambers, and expensive test equipment add large costs and burdens to antenna measurement systems. A smaller and more cost-effective system is presented in this paper. Using the bipolar planar scanning technique developed at UCLA, a portable millimeter-wave antenna measurement system has recently been constructed. The system was designed to fit on the end of a standard optical table, and enjoys the spacesaving and accuracy inherent to the bipolar planar configuration. Simple construction of the chamber allows for relatively easy assembly and disassembly, and allows movement of the system from one table to another, if needed. Antennas of diameters up to 24 in can be accommodated, and scan planes of up to a diameter of 60 in can be measured. Millimeter-wave frequencies from around 30 GHz to 67 GHz can be measured, with potential extension to higher frequencies. Planar nearfield- to-far-field techniques are used to construct the antenna's far-field patterns from the measured near field. In particular, the post processing follows the OSI/EFT method for pattern reconstruction and diagnostics. The design of the scanner configuration allows the incorporation of the phase-retrieval techniques developed for the bipolar configuration. These phaseless measurements allow the use of scalar millimeter-wave test equipment, with much lower cost than comparable vector test equipment.