Impulse responses of aperture antennas with arbitrary aperture distributions

On the basis of the equation of the Fresnel-Kirchhoff scalar diffraction field, it is shown that the impulse responses of parallel-fed aperture antennas of arbitrary configuration with arbitrary aperture amplitude distributions and those of end-fed rectangular aperture antennas with arbitrary aperture amplitude distributions can all be expressed by the aperture amplitude distributions, and that the impulse responses of rectangular aperture antennas with separable aperture distributions and those of circular aperture antennas with circularly symmetric distributions are special cases of the above general ones.     

Superresolution reconstruction using nonlinear gradient-based regularization

This paper discusses the problem of superresolution reconstruction. To preserve edges accurately and efficiently in the reconstruction, we propose a nonlinear gradient-based regularization that uses the gradient vector field of a preliminary high resolution image to configure a regularization matrix and compute the regularization parameters. Compared with other existing methods, it not only enhances the spatial resolution of the resulting images, but can also preserve edges and smooth noise to a greater extent. The advantages are shown in simulations and experiments with synthetic and real images.

W波段机载视频合成孔径雷达系统

合成孔径雷达(SAR)能够全天时全天候工作,在对地遥感领域具有广泛和深入的应用。视频SAR将SAR成像获得的空间维度信息拓展到时-空维度,可以获取更加丰富的遥感信息。传统SAR频段较低,导致合成孔径时间长,数据计算量大,高帧频输出难度很大;而低频段太赫兹波对目标细节感知能力强,合成孔径短,特别适合于微弱目标的视频感知。本文设计了一种工作在W波段的视频SAR系统,采用收发分置连续波固态前端体制,输出峰值功率1 W,最大发射带宽可达1 GHz;采用极坐标格式算法(PFA)结合GPU架构实现高帧率低延时成像。仿真试验表明,系统成像分辨力可达0.15 m,成像帧率约5 Hz。     

Synthetic aperture inversion for arbitrary flight paths and nonflat topography

The paper considers synthetic aperture radar (SAR) and other synthetic aperture imaging systems in which a backscattered wave is measured from positions along an arbitrary (known) flight path. The received backscattered signals are used to produce an image of the terrain. We assume a single-scattering model for the radar data, and we assume that the ground topography is known but not necessarily flat. We focus on cases in which the antenna footprint is so large that the standard narrow-beam algorithms are not useful. We show that certain artifacts can be avoided if the antenna and antenna footprint avoid particular relationships with the ground topography. We give an explicit backprojection imaging algorithm that corrects for the ground topography, flight path, antenna beam pattern, source waveform, and other geometrical factors. For the case of a non-directional antenna, the image produced by the above algorithm contains artifacts. For this case, we analyze the strength of the artifacts relative to the strength of the true image. The analysis shows that the artifacts can be somewhat suppressed by increasing the frequency, integration time, and the curvature of the flight path.     

Bistatic synthetic aperture radar imaging for arbitrary flight trajectories

In this paper, we present an analytic, filtered backprojection (FBP) type inversion method for bistatic synthetic aperture radar (BISAR). We consider a BISAR system where a scene of interest is illuminated by electromagnetic waves that are transmitted, at known times, from positions along an arbitrary, but known, flight trajectory and the scattered waves are measured from positions along a different flight trajectory which is also arbitrary, but known. We assume a single-scattering model for the radar data, and we assume that the ground topography is known but not necessarily flat. We use microlocal analysis to develop the FBP-type reconstruction method. We analyze the computational complexity of the numerical implementation of the method and present numerical simulations to demonstrate its performance.     

FDTD simulation of TE and TM plane waves at nonzero incidence in arbitrary Layered media

Plane wave scattering is an important class of electromagnetic problems that is surprisingly difficult to model with the two-dimensional finite-difference time-domain (FDTD) method if the direction of propagation is not parallel to one of the grid axes. In particular, infinite plane wave interaction with dispersive half-spaces or layers must include careful modeling of the incident field. By using the plane wave solutions of Maxwell's equations to eliminate the transverse field dependence, a modified set of curl equations is derived which can model a "slice" of an oblique plane wave along grid axes. The resulting equations may be used as edge conditions on an FDTD grid. These edge conditions represent the only known way to accurately propagate plane wave pulses into a frequency dependent medium. An examination of grid dispersion between the plane wave and the modeled slice reveals good agreement. Application to arbitrary dispersive media is straightforward for the transverse magnetic (TM) case, but requires the use of an auxiliary equation for the transverse electric case, which increases complexity. In the latter case, a simplified approach, based on formulating the dual of the TM equations, is shown to be quite effective. The strength of the developed approach is illustrated with a comparison with the conventional simulation based on an analytic incident wave specification with half-space, single frequency reflection and transmission for the edges. Finally, an example of a possible biomedical application is given and the implementation of the method in the perfectly matched layer region is discussed.     

Serving two correlated queues with a synchronous server underexhaustive service discipline and nonzero switchover time

A system composed of two queues that accepts correlated inputs and receives services from a synchronous server is studied. An exhaustive service discipline is adopted in serving a queue. Nonzero switchover time for the server to change service from one queue to the other is assumed. A closed-form expression for mean waiting time is obtained. The validity of the analysis if verified using the results of a computer simulation     

Superresolution with second generation wavelets

Over the last 3 years or so, first-generation wavelets have been used to realize superresolution from a captured sequence of low-resolution (LR) degraded frames. Here, it is pointed out that second-generation wavelets (SGWs) are inherently more suited for image superresolution. From preliminary results that exploit subpixel displacements between LR frames to attain superresolution, it is concluded that SGWs show promise and potential to be extremely fast, efficient and versatile for superresolution.     

水下视频图像的超分辨率重建与增强

针对水下视频图像对比度低、图像模糊和噪声突出的特点,提出一种新的基于凸集投影(POCS)的超分辨率图像重建与增强算法。在POCS迭代约束的过程中,采用小波域非线性函数抑制噪声并突出细节,然后用能量非递减性约束集进行修正。实验表明,该方法具有较好的超分辨率重建与增强效果。     

Encoding and reconstruction of multiview video objects

The authors discuss how disparity-based processing can be used both for compression of multiview video data, and generation of arbitrary viewpoints from the available information of multiple cameras in the context of the MPEG-4 multimedia standard. The examples and results presented here show that viewpoint adaptation toward video objects can be accomplished with low-complexity schemes such as disparity-compensated projection, while high-quality results are presented. The ability to process multiview video is another example of the high flexibility of the MPEG-4 standard, which we expect to be applicable to various new challenging services in the multimedia market     

Maximum-likelihood expectation-maximization reconstruction of sinograms with arbitrary noise distribution using NEC-transformations

The maximum-likelihood (ML) expectation-maximization (EM) [ML-EM] algorithm is being widely used for image reconstruction in positron emission tomography. The algorithm is strictly valid if the data are Poisson distributed. However, it is also often applied to processed sinograms that do not meet this requirement. This may sometimes lead to suboptimal results: streak artifacts appear and the algorithm converges toward a lower likelihood value. As a remedy, we propose two simple pixel-by-pixel methods [noise equivalent counts (NEC)-scaling and NEC-shifting] in order to transform arbitrary sinogram noise into noise which is approximately Poisson distributed (the first and second moments of the distribution match those of the Poisson distribution). The convergence speed associated with both transformation methods is compared, and the NEC-scaling method is validated with both simulations and clinical data. These new methods extend the ML-EM algorithm to a general purpose nonnegative reconstruction algorithm.     

Utilisation of downsampling for arbitrary views in multi-view video coding

The effect of using downsampling for arbitrary views inside a multi- view sequence on the multi-view coding (MVC) efficiency is explored. A bit rate adaptive approach is proposed to consider downsampling certain views prior to encoding with relevant downscaling ratios. The inter-view references, if any, are downsampled to the same resolution and the decoded view is upsampled back to the original resolution. The results over several multi-view test sequences imply that up to 0.9 dB gain or 20% reduction in bit rate can be achieved, reducing the computational complexity in the encoder significantly at the same time.     

任意稀疏结构的复稀疏信号快速重构算法及其逆合成孔径雷达成像

为同时提高压缩感知(CS)中任意稀疏结构多量测向 量(MMV)模型的重构性能和重构速度,提出基于改进线性Bergman 迭代(LBI)的任意稀疏 结构MMV模型重构算法。首先给出了任意稀疏结构MMV模型,并对模型进行了分析,利用LBI 实现MMV模型的二维重构;然后,通过 设置预条件子的方 法对感知矩阵的条件数进行优化,以通过提高收敛速度而提高重构速度,并从理论和仿真两 个方面对算法 的收敛性和运算量进行了分析;最后通过仿真结果表明,本文算法能够高质量地重构任意稀 疏结构MMV模型, 同时在重构速度方面具有明显的优势。基于实测数据不同信噪比(SNR)条件下的逆合成孔径雷达(ISAR)成像结果,验证了算法的有效性。     

The sampling and reconstruction of time-varying imagery with application in video systems

Sampling is a fundamental operation in all image communication systems. A time-varying image, which is a function of three independent variables, must be sampled in at least two dimensions for transmission over a one-dimensional analog communication channel, and in three dimensions for digital processing and transmission. At the receiver, the sampled image must be interpolated to reconstruct a continuous function of space and time. In imagery destined for human viewing, the visual system forms an integral part of the reconstruction process. This paper presents an overview of the theory of sampling and reconstruction of multidimensional signals. The concept of sampling structures based on lattices is introduced. The important problem of conversion between different sampling structures is also treated. This theory is then applied to the sampling of time-varying imagery, including the role of the camera and display apertures, and the human visual system. Finally, a class of nonlinear interpolation algorithms which adapt to the motion in the scene is presented.     

Electromagnetic coupling to a conducting wire behind an aperture of arbitrary size and shape

The electromagnetic coupling to a conducting wire behind an aperture in a plane conducting screen is analyzed. The aperture can be of arbitrary size and shape. The wire can be of finite length, with or without terminating loads, or of infinite length. The electric current on the wire and the equivalent magnetic current over the aperture region are calculated by the method of moments. An equivalent circuit for the effect of the aperture on the transmission line mode of the wire is derived.     

Fast MR image reconstruction for partially parallel imaging with arbitrary k-space trajectories

Both acquisition and reconstruction speed are crucial for magnetic resonance (MR) imaging in clinical applications. In this paper, we present a fast reconstruction algorithm for SENSE in partially parallel MR imaging with arbitrary k-space trajectories. The proposed method is a combination of variable splitting, the classical penalty technique and the optimal gradient method. Variable splitting and the penalty technique reformulate the SENSE model with sparsity regularization as an unconstrained minimization problem, which can be solved by alternating two simple minimizations: One is the total variation and wavelet based denoising that can be quickly solved by several recent numerical methods, whereas the other one involves a linear inversion which is solved by the optimal first order gradient method in our algorithm to significantly improve the performance. Comparisons with several recent parallel imaging algorithms indicate that the proposed method significantly improves the computation efficiency and achieves state-of-the-art reconstruction quality.     

Very simple method to find the minimum polynomial of an arbitrary nonzero element of a finite field

A method is described of finding the minimum polynomial of an arbitrary, nonzero element of GF(2m). This method requires finite arithmetic on only a single element of the field, and not on each of the k+1 coefficients over GF(2m), where k is the degree of the minimum polynomial. Applications include the design of error-correcting codes and of apparatus to process them, as well as cryptographic systems.     

Modified circuit model for the step-recovery diode to include nonzero transition time

A diode model is proposed which enables the effect of the intrinsic transition time to be accounted for in any frequency-domain analysis. The validity of this model is established by mathematically analysing the model as used in a transition-time test set. Practical measurements show that the evaluated model is independent of the method of measurement.     

Convolution backprojection image reconstruction for spotlight modesynthetic aperture radar

Convolution backprojection (CBP) image reconstruction has been proposed as a means of producing high-resolution synthetic-aperture radar (SAR) images by processing data directly in the polar recording format which is the conventional recording format for spotlight mode SAR. The CBP algorithm filters each projection as it is recorded and then backprojects the ensemble of filtered projections to create the final image in a pixel-by-pixel format. CBP reconstruction produces high-quality images by handling the recorded data directly in polar format. The CBP algorithm requires only 1-D interpolation along the filtered projections to determine the precise values that must be contributed to the backprojection summation from each projection. The algorithm is thus able to produce higher quality images by eliminating the inaccuracies of 2-D interpolation, as well as using all the data recorded in the spectral domain annular sector more effectively. The computational complexity of the CBP algorithm is O(N (3)).