Polynomial factorization over

We describe algorithms for polynomial factorization over the binary field , and their implementation. They allow polynomials of degree up to to be factored in about one day of CPU time, distributing the work on two processors.

     

一种快速的DCT域MPEG-2到MPEG-1下采样转码技术

文章提出了一种快速的DCT域MPEG-2到MPEG-1准卷积下呆样算法。在转码过程中。头信忠保持不变，且要求将每四个相邻MPEG-2宏块变为一个下采样MPEG-1宏块：用最大最小距离方法确定下采样宏块的运动向量、用简单多数原则确定下采样宏块类型以及用加全平均方案确定下呆样宏块的量化参数。另外，对下采样视频转码失真来源进行了分析。实验结果表明我们提出的转码方案，在同样条件下与级联像素域转码器（TM5）相比，不仅其计算复杂性减少67．6％、PNSR提高0．1dB，而且具有很小的比特控制错误。     

DAB信道快速数据传输系统设计及测试分析

描述了一个利用ＥｕｒｅｋａＤＡＢ系统的快速信息数据通道进行快速数据传输的实际系统,给出了对该系统进行实际传输测试的结果。通过对测试中出现的数据丢失现象的分析,提出了进一步的改进方案。     

Image reconstruction from acoustical holograms using microprocessors

An acoustic imaging system using holographic techniques is described. Insonification of the object is achieved using a phased array scanner. Image reconstruction uses Fourier Transformations to produce a 128 × 128 pixel TV display. The system has the ability to focus in the ‘near field’ of its transducer arrays and has a frame rate of approximately 0.3 Hz.     

Heteronuclear decoupling under fast MAS by a rotor-synchronized Hahn-echo pulse train

A new heteronuclear decoupling mechanism under fast magic-angle spinning MAS is introduced. It is based on refocusing the coherences responsible for the dephase of low-gamma nuclei ((13)C, (15)N) transverse spin-polarization in the presence of strongly dipolar-coupled protons, and has the advantage that can be implemented by pulsed techniques, with all the benefits resulting from a reduced duty cycle compared with conventional decoupling by continuous rf irradiation. The decoupling efficiency of a simple rotor-synchronized Hahn-echo pulse train is analyzed both theoretically and experimentally. It was found that a substantial improvement in sensitivity and resolution can be achieved in compounds with small (1)H chemical shielding parameters even at moderate sample spinning, and some interesting applications are shown. It is also shown that much faster spinning frequencies, or alternative refocusing sequences, are needed for applications on rigid organic solids, i.e., in systems with larger (1)H chemical shifts.     

Spin-lattice relaxation and a fast T1-map acquisition method in MRI with transient-state magnetization

The magnetization under the spin-lattice relaxation and the nuclear magnetic resonance radiofrequency (RF) pulses is calculated for a signal RF pulse train and for a sequence of multiple RF pulse-trains. It is assumed that the transverse magnetization is zero when each RF pulse is applied. The result expressions can be grouped into two terms: a decay term, which is proportional to the initial magnetization M0, and a recovery term, which has no M0 dependence but strongly depends on the spin-lattice relaxation and the equilibrium magnetization Meq. In magnetic resonance pulse sequences using magnetization in transient state, the recovery term produces artifacts and can seriously degrade the function of the preparation sequence for slice selection, contrast weighting, phase encoding, etc. This work shows that the detrimental effect can be removed by signal averaging in an eliminative fashion. A novel fast data acquisition method for constructing the spin-lattice relaxation (T1) map is introduced. The method has two features: (i) By using eliminative averaging, the curve to fit the T1 value is a decay exponential function rather than a recovery one as in conventional techniques; therefore, the measurement of Meq is not required and the result is less susceptible to the accuracy of the inversion RF pulse. (ii) The decay exponential curve is sampled by using a sequence of multiple pulse-trains. An image is reconstructed from each train and represents a sample point of the curve. Hence a single imaging sequence can yield multiple sample points needed for fitting the T1 value in contrast to conventional techniques that require repeating the imaging sequence for various delay values but obtain only one sample point from each repetition.     

NMR-microscopy with TrueFISP at 11.75T

The purpose of this paper is to demonstrate that a fully balanced gradient echo technique (TrueFISP) can be used for microscopic experiments at high static magnetic field strengths. TrueFISP experiments were successfully performed on homogeneous and inhomogeneous objects at 11.75T. High-resolution TrueFISP images were obtained from phantoms, plants, formalin-fixed samples, and from an isolated beating rat heart with an in-plane resolution of 78 micro m and a slice thickness of 500 micro m. The signal-to-noise ratio (SNR) gain of TrueFISP compared to conventional gradient echo or spin echo sequences will allow faster acquisition times or an improvement in spatial resolution for microscopic experiments.     

Protected session keys context for distributed session key management

Handoffs must be fast for wireless mobile nodes (MN) without sacrificing security between the MN and the wireless access points in the access networks. We describe and analyze our new secure Session Keys Context (SKC) scheme which has all the good features, like mobility and security optimization, of the currently existing key distribution proposals, namely key-request, pre-authentication, and pre-distribution. We analyze these solutions together, and provide some conclusions on possible co-operative scenarios and on which level of the network to implement them. Finally before conclusions we provide some handoff delay simulation results with SKC and key request schemes with corresponding example handoff scenarios with a next generation radio link layer.     

An efficient algorithm for superresolution in medium field imaging

In this paper, we study the problem of reconstruction of a high-resolution (HR) image from several blurred low-resolution (LR) image frames in medium field. The image frames consist of blurred, decimated, and noisy versions of a HR image. The HR image is modeled as a Markov random field (MRF), and a maximum a posteriori (MAP) estimation technique is used for the restoration. We show that with the periodic boundary condition, a HR image can be restored efficiently by using fast Fourier transforms. We also apply the preconditioned conjugate gradient method to restore HR images in the aperiodic boundary condition. Computer simulations are given to illustrate the effectiveness of the proposed approach. This research was conducted with support from the Army Research Office Grant DAAD 19-03-1-0261 and the National Science Foundation Grant CCF-0429481. Research supported in part by RGC Grant Nos. 7130/02P, 7046/03P, 7035/04P and 7035/04P and FRG/04-05/II-51.     

Nanoscale intrinsic heterogeneities in Ag-Ge-Se glasses and their correlation with physical properties

Ag-Ge-Se glass forming system is dominated by liquid immiscibility. Glasses in the Se-GeSe₂-Ag₂Se triangle may have different morphologies according to their composition. In this work inhomogeneous morphology is observed in Ag_x(Ge_ySe_1−y)_100−x bulk glasses with y = 0.20, 0.25 at.% fraction. Electron microscopy reveals that the scale of heterogeneity strongly depends on the sample composition. The physical properties of these glasses are also dependent on their composition and can be easily correlated to their morphology. Accordingly, the electric conductivity evolves from semiconducting to ionic conductivity whereas a percolation transition occurs in a narrow Ag concentration range.Mössbauer spectrometry let us to analyse the local order of these intrinsically heterogeneous glasses employing ⁵⁷Fe as a probe.