三维空间域多普勒功率谱及其多天线系统性能

在三维(3 dimensional,3D)空间域信道建模中,针对波达信号仰角(elevation angle,EA)在不同覆盖区散射体环境中的分布,提出了指数型EA仰角概率密度函数并对其信道特征实现建模.在假设波达信号方位谱为均匀分布时,采用近似算法在对称和非对称两种情况下导出其多普勒功率谱密度(power spectral density,PSD)闭合表达式.从分析结果可发现PSD与仰角函数式及波达信号边界角度参数βmin和βmax关系密切,并与多普勒频移密切相关.对该三维空间域中的多输入多输出(multiple input multiple output,MIMO)多天线信号衰落相关性(spatial fading correlation,SFC)进行了推导和仿真.结果表明MIMO多天线阵元间SFC与βmin和βmax关系密切,而EA仰角概率密度函数参数n对SFC影响较小.本文引入的指数型EA仰角概率密度函数可应用于多种无线通信环境下的信道参数估计,且与传统模型对比表明本模型的信道参数估计结果符合理论和经验值,拓展了对3D空间域统计信道的建模与MIMO多天线分析计算.     

Experimental determination of the azimuthal and radial mode orders of a partially coherent LG_(pl) beam(Invited Paper)

It is known that one can determine the mode orders(i.e.,the azimuthal order and radial order)of a partially coherent LG_(pl) beam(i.e.,a partially coherent vortex beam)based on the measurement of the cross-correlation function(CCF)and the double correlation function(DCF)together.The technique for measuring the CCF is known.In this Letter,we propose a method for measuring the DCF.Based on the proposed method,the determination of the mode orders of a partially coherent LG_(pl) beam is demonstrated experimentally.     

并行共焦三维检测方法的理论分析

从一般共焦系统的原理出发,重点围绕并行共焦方法的三维检测,通过引用三维点扩散函数和矩阵形式,研究了并行共焦检测方法的理论问题,确立了点阵并行共焦检测方法的检测过程。其结果表明,并行共焦检测过程总的光场分布,是各条共焦检测光路的求和,且都符合两个点扩散函数相乘的共焦形式。     

混合分集多天线系统基于相关性的分析信道建模

在考虑正交极化子信道之间相关性的前提下,指出传统分析模型无法体现极化方向对相关性的影响,并提出一种新的建模理论对传统模型进行修改补充;然后指出传统模型中使用天线在距离分集和极化分集情况下的相关系数来计算混合分集情况下的相关系数是一种不够准确的近似方法,提出直接利用天线处于混合分集情况下的统计数据进行建模,这种新的模型可以准确复现子信道间的相关系数,满足高精度的需求. 关键词： 多输入多输出系统(MIMO) 信道模型 分析模型 相关系数     

水声多输入多输出信道的分布式压缩感知估计

多输入多输出技术通过采用多个阵元进行多发多收空间复用信道可在极其有限的通信带宽下实现高速水声通信,但由于同时存在通道间干扰和多径干扰,水声MIMO信道估计变得困难。提出利用MIMO水声信道多径稀疏结构存在的相关性,在经典联合稀疏模型的基础上对MIMO观测矩阵进行重组,从而建立基于分布式压缩感知的单载波水声MIMO通信信道联合稀疏模型;同时,针对信道响应中具有相同多径位置的稀疏部分和特有稀疏部分设计区分性正交匹配追踪算法进行联合重构,进一步抑制通道间干扰的影响。最后通过仿真和海上实验进行本方法有效性的验证,实现16 kbps的MIMO水声通信。通过算法推导、仿真和实验可得到结论:利用MIMO水声信道多径相关性进行分布式压缩感知估计可提高估计性能。      

An effective PSO-based power allocation for target tracking in MIMO radar with widely separated antennas

In MIMO radar with widely separated antennas, the antennas are spaced far from each other and the target is seen from different angles. In this type of radars, each receiver collects all transmit signals and transmits them to the central processor unit. Power allocation is an important part of military operations. Therefore, it is a primary factor that requires to be taken into account in the designing of target tracking problems in MIMO radar. In fact, the power allocation finds an optimum strategy to allot power to transmit antennas with the goal of minimizing the target tracking errors under specified transmit power constraints. In this paper, the performance of power allocation for target tracking in MIMO radar with widely separated antennas is investigated. For this purpose, first, a MIMO radar with distributed antennas is configured and a target motion model using the constant velocity (CV) method is modeled. Then Joint Cramer Rao bound (CRB) for target parameters (joint target position and velocity) estimation error is computed. This is applied as a power allocation problem objective function. Because a complex Gaussian model is considered for target radar cross-section (RCS), this function becomes complicated. Due to the nonlinearity of this objective function, the proposed power allocation problem is nonconvex. Therefore, a particle swarm optimization (PSO) -based power allocation algorithm is proposed to solve it. In simulation experiments, the performance of the proposed algorithm in different conditions such as a different number of antennas and antenna geometry configurations is evaluated. Results prove the validity of the proposed algorithm.     

Effective permittivity of materials containing graded ellipsoidal inclusions

We prove a generic theorem stating the equivalence between a graded dielectric ellipsoid (with gradation along a family of internal confocal ellipsoids) and an anisotropic homogeneous ellipsoid. We then describe a procedure to obtain the three principal permittivities of the effective ellipsoid for any given dielectric gradation profile. Finally, we apply a multiscale approach to homogenize dispersions of ellipsoidal graded particles.     

Monte Carlo simulation code for confocal 3D micro‐beam X‐ray fluorescence analysis of stratified materials

Stratified materials are of great importance for many branches of modern industry, e.g. electronics or optics and for biomedical applications. Examination of chemical composition of individual layers and determination of their thickness helps to get information on their properties and function. A confocal 3D micro X‐ray fluorescence (3D µXRF) spectroscopy is an analytical method giving the possibility to investigate 3D distribution of chemical elements in a sample with spatial resolution in the micrometer regime in a non‐destructive way. Thin foils of Ti, Cu and Au, a bulk sample of Cu and a three‐layered sandwich sample, made of two thin Fe/Ni alloy foils, separated by polypropylene, were used as test samples. A Monte Carlo (MC) simulation code for the determination of elemental concentrations and thickness of individual layers in stratified materials with the use of confocal 3D µXRF spectroscopy was developed. The X‐ray intensity profiles versus the depth below surface, obtained from 3D µXRF experiments, MC simulation and an analytical approach were compared. Correlation coefficients between experimental versus simulated, and experimental versus analytical model X‐ray profiles were calculated. The correlation coefficients were comparable for both methods and exceeded 99%. The experimental X‐ray intensity profiles were deconvoluted with iterative MC simulation and by using analytical expression. The MC method produced slightly more accurate elemental concentrations and thickness of successive layers as compared to the results of the analytical approach. This MC code is a robust tool for simulation of scanning confocal 3D µXRF experiments on stratified materials and for quantitative interpretation of experimental results. Copyright © 2011 John Wiley & Sons, Ltd.     

荧光波长对共焦显微镜成像特性的影响

导出了共焦显微镜中不同荧光波长情况下的荧光功率传输函数、三维脉冲响应函数（3D－PSF）和三维光学传递函数（3D－OTF）。结果表明，不同的荧光波长对共焦显微镜的空间截止频率、分辨率、光学传递函数存在明显的影响。当激发波长与荧光波长的比值下降到一定程度时，可以看到明显的失锥现象。     

A method for calibrating the confocal volume of a confocal three-dimensional micro-x-ray fluorescence setup

The measurement of the confocal volume of a confocal three-dimensional micro-x-ray fluorescence(3 D-XRF) setup is a key step in the field of confocal 3 D-XRF analysis.With the development of x-ray facilities and optical devices,3 D-XRF analysis with a micro confocal volume will create a great potential for 2 D and 3 D microstructural analysis and accurate quantitative analysis.However,the classic measurement method of scanning metal foils of a certain thickness leads to inaccuracy.A method for calibrating the confocal volume is proposed in this paper.The new method is based on the basic content of the textbook,and the theoretical results and the feasibility are given in detail for the 3 D-XRF mono-chromatic x-ray condition and the poly-chromatic x-ray condition.We obtain a set of experimental confirmation using the polychromatic x-ray tube in the laboratory.It is proved that the sensitivity factor of the 3 D-XRF can be directly and accurately obtained in a real calibration process.     

NSCL从ECR离子源向CCF注入的离子束流动力学研究

The Coupled Cyclotron Facility (CCF) has been operating at the NSCL since 2001,providing up to 160MeV/u heavy ion beams for nuclear physics experiments.Recent steps,particularly the improvement of the ECR-to-K500 injection line,were taken to improve the CCF performance.For that purpose an off-line ECR source,ARTEMIS-B,was built and used to investigate the impact on beam brightness under various source operating conditions,different initial focusing systems and current analysis dipole.Beam dynamics simulations including space-charge and 3D electrostatic field effects were performed and beam diagnostics including emittance scanner were used,leading to a better understanding of the CCF beam injection process. New initial electrostatic focusing elements such as a large-bore quadrupole triplet and a quadrupole double- doublet with compensating octupole were tested,and a new beam tuning procedure was established to improve the beam brightness for the CCF.Following these efforts,a significant increase of primary beam power out of the CCF has been achieved.     

Improvement of confocal microscope performance by shaped annular beam and heterodyne confocal techniques

In order to further improve the performance of a confocal microscope (CM) used for measurement of surface profiles and 3D microstructures, a shaped annular beam heterodyne confocal measurement method based on annular pupil filter technique and reflection confocal microscopy, is proposed to expand the measurement range and to improve the defocused property of CM. The approach proposed uses a confocal dual-receiving light path arrangement and a heterodyne subtraction of two signals received from detectors with axial offset to enable CM to be used for bipolar absolute measurement and to improve the defocused property of CM, and it uses the annular pupil filter technique to produce a binary optical shaped annular beam, which expands the measurement range by expanding the full-width at half-maximum of intensity curve received from two detectors in a heterodyne confocal microscopy system. Theoretical analyses and experimental results indicate that a shaped annular beam heterodyne microscope has a measurement range expanded from 4 to 14 μm, achieved an axial resolution of 2 nm and improved the defocused property, when ε=0.5 and NA=0.65. It can be therefore concluded that the shaped annular beam heterodyne confocal measuring method proposed is a new approach to ultraprecision measurement of surface profiles and 3D microstructures.     

Direction-of-arrival estimation for co-located multiple-input multiple-output radar using structural sparsity Bayesian learning

This paper addresses the direction of arrival(DOA) estimation problem for the co-located multiple-input multipleoutput(MIMO) radar with random arrays. The spatially distributed sparsity of the targets in the background makes compressive sensing(CS) desirable for DOA estimation. A spatial CS framework is presented, which links the DOA estimation problem to support recovery from a known over-complete dictionary. A modified statistical model is developed to accurately represent the intra-block correlation of the received signal. A structural sparsity Bayesian learning algorithm is proposed for the sparse recovery problem. The proposed algorithm, which exploits intra-signal correlation, is capable being applied to limited data support and low signal-to-noise ratio(SNR) scene. Furthermore, the proposed algorithm has less computation load compared to the classical Bayesian algorithm. Simulation results show that the proposed algorithm has a more accurate DOA estimation than the traditional multiple signal classification(MUSIC) algorithm and other CS recovery algorithms.     

Some basic questions on helicity inferred from vector magnetograms

This paper summarizes current helicity measurements in the solar active regions (ARs). There is a basic agreement with the “hemispheric sign rule (HSR)” of the current helicity among different vector magnetographs through two solar cycles, but there is a large dispersion of the fraction of ARs following the HSR. In our sample, there are 50%–78% ARs in solar cycle 22 and 44%–79% ARs in cycle 23 following the HSR. A variation is also found in the fraction of the ARs following the HSR between different instruments even when the same ARs are selected. The difference also exists for the same instrument when the selected ARs are different. There are some differences in the variation of HSR with solar cycle for the individual helicity parameter inferred from different instruments. Factors which influence the correlation of different data sets are analyzed.     

“Orthogonalized” sparsity enhanced mismatch models for wireless heterogeneous networks

Performance of MIMO precoder for heterogeneous networks can be hindered by a lack of accurate channel state information. The sparsity enhanced mismatch model (SEMM) has been proposed recently to account for the channel estimate mismatch problem by exploiting the inherent sparse characteristics of MIMO interference channels. When (single user-MIMO) SU-MIMO precoder design takes into account the SEMM, it was shown to have better transmission performance compared to the conventional norm ball mismatch model (NBMM) in a single-user multi-victims scenario. However, when communicating and interference channels are highly correlated, which can happen frequently in ultra-dense heterogeneous networks, performance of the SEMM precoder degrades and in some cases, underperforms the NBMM precoder. An “orthogonalized” SEMM (OSEMM) is proposed herein to modify the SEMM such that it is better suited for correlated channels. The concept of orthogonalization of channels is not new but this work uses it to enhance the SEMM, which creates synergy between transmission performance and robustness toward channel mismatch error. Two variants of the OSEMM are proposed, namely the OSEMM-LQ and OSEMM-SVD, to modify the basis expansion model that is an integral part of the SEMM. The resulting mismatch model influences the design of the SU-MIMO precoder that aims to maximize certain transmission criterion. Even though SU-MIMO precoding is considered, a new channel correlation definition, which acts as a metric for the OSEMM, is given that allows for user selection in a multiuser scenario such that optimal performance can be attained for the targeted user. Analytical and simulation results are given that highlight the difference in performance between the two variants of the OSEMM.     

荧光共焦扫描系统成像特性的优化

对荧光共焦扫描系统用光强点扩散函数进行傅里变换得到系统三维传递函数的数学模型，并由此求得环形透镜和各种有限大小探测器系统的光学传递函数。用计算机模拟和光学传递九数值计算，分析了采用不同环形透镜及探测器对系统成像特性的影响     

Application of Three Dimensional Confocal Micro X-Ray Fluorescence Technology Based on Polycapillary X-Ray Lens in Analysis of Rock and Mineral SamplesSCIEI北大核心CSCD

Confocal three dimensional (3D) micro X-ray fluorescence (XRF) spectrometer based on a polycapillary focusing X-ray lens (PFXRL) in the excitation channel and a polycapillary parallel X-ray lens (PPXRL) in the detection channel was developed. The PFXRL and PPXRL were placed in a confocal configuration. This was helpful in improving the signal-to-noise ratio of the XRF spectra, and accordingly lowered the detection limitation of the XRF technology. The confocal configuration ensured that only the XRF signal from the confocal micro-volume overlapped by the output focal spot of the PFXRL and the input focal spot of the PPXRL could be detected by the detector. Therefore, the point-to-point information of XRF for samples could be obtained non-destructively by moving the sample located at the confocal position. The magnitude of the gain in power density of the PFXRL was 10(3). This let the low power conventional X-ray source be used in this confocal XRF, and, accordingly, decreased the requirement of high power X-ray source for the confocal XRF based on polycapillary X-ray optics. In this paper, we used the confocal 3D micro X-ray fluorescence spectrometer to non-destructively analyzed mineral samples and to carry out a 3D point-to-point elemental mapping scanning, which demonstrated the capabilities of confocal 3D micro XRF technology for non-destructive analysis elements composition and distribution for mineral samples. For one mineral sample, the experimental results showed that the area with high density of element of iron had high density of copper. To some extent, this reflected the growth mechanisms of the mineral sample. The confocal 3D micro XRF technology has potential applications in such fields like the analysis identification of ore, jade, lithoid utensils, "gamble stone" and lithoid flooring.     

Confocal three-dimensional micro-X-ray fluorescence spectroscopy used to retrieve the information covered in printed paper

Confocal three-dimensional micro-X-ray fluorescence (3D-μXRF) spectroscopy based on laboratory X-ray tube was employed to unravel the three-dimensional (3D) distribution of metal element (Fe) inner the paper. According the distribution of the Fe in the paper, the information covered in the printed paper could be retrieved. Before the experiment, the performance of the confocal 3D-μXRF setup was described. The two-dimensional (2D) mapping of printed character covered by the opaque tape and contaminated by the black ink was scanned by confocal 3D-μXRF setup. Furthermore, 2D and 3D mapping of a printed number covered by the paper with printed character was performed by micro-X-ray fluorescence and confocal 3D-μXRF setup, respectively. The scanning results indicated that confocal 3D-μXRF was more suitable for retrieving the information covered in the paper. Furthermore, the confocal 3D-μXRF setup has potential application on the identification of unseen letter or painting that was covered by others, such as ink.     

毛细管X光透镜三维共聚焦微束X射线荧光技术在岩矿样品分析中的应用

利用毛细管X光透镜搭建了三维共聚焦微束X射线荧光谱仪,处在激发道的多毛细管X射线会聚透镜和处在探测道的多毛细管X射线平行束透镜处于共聚焦状态,该共聚焦结构降低了X射线荧光光谱的背底,从而有利于降低的X射线荧光分析技术的探测极限。在上述共聚焦结构中,多毛细管X射线会聚透镜和多毛细管X射线平行束透镜的焦斑重合形成共聚焦微元,探测器只能探测到来自该共聚焦微元内的X射线荧光信号,当该共聚焦微元在样品移动时,就可利用该共聚焦技术原位无损得到样品内部的三维X射线荧光信息。该共聚焦技术使用的多毛细管X射线会聚透镜具有103量级的功率放大倍数,从而降低了该共聚焦技术对高功率X射线源的依赖程度,即利用低功率普通X射线源就可以设计毛细管X光透镜共聚焦X射线荧光技术。利用上述共聚焦微束X射线荧光谱仪对两种岩矿样品进行三维无损分析,在其中一种岩石中发现：Fe浓度大的区域Cu的浓度也大,这在一定程度上反映了岩矿自然生长的机理。实验结果证明：该共聚焦X射线荧光技术可以在不破坏样品情况下分析岩矿样品中元素成分组成和元素三维分布情况。该共聚焦三维微束X射线荧光技术在矿石勘察、玉器选材和鉴别、石质食用器皿、“赌石”和家用石质地板检测等领域具有潜在的应用。     

任意分集方式多输入多输出无线通信系统的统计信道建模

针对极化分集多输入多输出系统的传统统计模型存 在着对不同极化方向上的功率变化设置不合理的问题, 提出一种功率矩阵对其进行纠正; 对采用不同分集方式的多输入多输出系统信道建模的原理进行分析, 提出对功率矩阵和相关矩阵进行复合计算的方法, 建立了一种适用于任意分集方式多输入多输出系统的统计信道模型, 该模型能够确切反映出同时受到距离分集和极化分集影响的多输入多输出系统之统计特征.