基于改进的统计信道模型与多天线系统性能分析

为提高电磁信号的到达角度以及多普勒效应等信道参数估计的准确性,在散射体非均匀分布下引入了散射概率和有效散射体两个物理概念,提出一种合理的改进型空间信道模型,该模型能准确地描述宏小区(macrocell)和微小区(microcell)等各种移动通信环境下的重要空时信道参数,并应用于多入多出系统(multiple input multiple output,MIMO)信道性能仿真中.数值仿真结果与早期多径衰落信道模型对比,表明本模型的信道参数估计结果符合理论和经验,拓展了空间统计信道模型的研究和应用,对评估多天线MIMO系统空时处理算法和仿真无线通信系统提供有力的工具.     

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.     

基于车载通信标准街道场景的电磁散射信道模型

针对室外无线信道视距(line of sight,LOS)/非视距(non-line of sight,NLOS)传输环境下的车到车(vehicular-to-vehicular,V2V)通信系统,本文提出了一种基于标准街道散射的统计信道模型,其移动发射机(mobile transmitter,MT)与移动接收机(mobile receiver,MR)处于运动状态,街道两旁分布的散射体固定.由几何模型出发又引入了一种随机的参考信道模型,其散射体有无穷多个,均以平行于街道两侧的散射条纹形式均匀分布在三维(three dimensional,3D)空间的一个二维(two dimensional,2D)矩形内部.在室外街道通信环境下,模型推导了散射信道中发射角(angle of departure,AOD)以及到达角(angle of arrival,AOA)的概率密度函数(probability density functions,PDFs)解析式;研究了多普勒功率谱密度(power spectral density,PSD)及其时间自相关函数(autocorrelation function,ACF);分析了模型多普勒参数以及街道散射体等因素对V2V通信系统性能的影响.与城市、农村的测量信道对比分析,表明本模型仿真的统计特性符合理论与实际,拓宽了室外V2V无线通信信道建模的研究.为评估室外V2V通信系统的传输特性、仿真无线通信系统提供了有力的研究工具.     

室内直达与非直达环境无线传播综合信道建模

本文主要针对室内无线传播信道直达(line of sight, LOS)与非直达(no line of sight, NLOS)环境, 引入参考模型研究其建模设计及其相关统计特性. 文中提出了一种基于几何散射模型的综合改进室内参考信道模型, 假设将无限数量的散射体均匀分布在三维空间的一个二维(two dimensional, 2D)水平面上. 本文推导了电磁波达信号到达角(angle of arrival, AOA) 概率分布函数(probability density function, PDF)、多普勒功率谱密度(power spectral Density, PSD)、 时间自相关函数(autocorrelation function, ACF)的解析表达式, 并分析其重要参数对函数的影响. 此外, 本文还通过非现实参考模型提出了一种高效的SOC(Sum of Cisoids) 信道仿真模型, 同时提出了设计SOC信道仿真模型的两种有效参数计算方法, 并比较两者的计算性能. 仿真结果表明, 信道仿真模型的统计特性与参考模型相匹配, 即室内参考模型可以通过的SOC信道仿真模型来近似, 同时信道仿真模型可以很好的应用于评估室内无线通信系统的性能, 拓宽了室内无线信道建模的研究, 同时减少实现开支.     

多径衰落信道下混沌直扩通信的可破解性

本文在文献[1]的基础上,研究多径衰落信道条件下采用无先导卡尔曼滤波混沌拟合对混沌直扩通信的可破解性.由针对混沌直扩信号的无先导卡尔曼滤波混沌拟合的状态空间方程出发,分析了多径衰落信道对于无先导卡尔曼滤波混沌拟合过程中的跟踪误差的影响,得到了信息码状态估计的值域范围,从而提出了多径衰落信道下混沌直扩信号可被破解的充分条件定理.仿真结果表明,在满足充分条件下,混沌直扩信号无论是通过时不变信道还是时变信道,都可以被成功破解,并且具有良好的误码率性能. 关键词： 混沌通信 破解 多径衰落信道 无先导卡尔曼滤波     

改进的信道稀疏度检测正交多载波多普勒估计方法

针对正交频分复用水声移动通信中频域变采样技术联合信道稀疏度检测的多普勒估计算法在冰水混合区复杂信道环境下多普勒估计效果欠佳且计算量大的问题,提出了一种改进的多普勒估计算法。采用时域重采样技术对信号进行不同压缩因子的补偿,避免了频域变采样技术中的高阶快速傅里叶变换运算,从而降低了算法复杂度。利用梳状导频位置向量结合时域变采样技术更准确提取的导频子载波,进而获得更加精确的信道估计和信道稀疏度检测结果,提高了多普勒估计精度。仿真结果和冰下试验结果表明,与原始算法相比,改进的方法在冰水混合水域复杂多径时变信道环境下可以有效实现对信号多普勒畸变的跟踪并且计算量显著下降,能够保障正交频分复用水声移动实时通信。      

非正交多址光通信系统信道均衡的虚拟时反方法

室内非正交多址(non-orthogonal multiple access, NOMA)可见光通信系统(visible light communications, VLC)可实现高速率多用户通信,但多径效应会造成通信可靠性及用户公平性明显下降,为此,提出一种虚拟时间反转技术(virtual time reversal mirror, VTRM)对NOMA-VLC进行信道均衡,消除多径效应对通信过程的影响。分析了多用户场景下室内NOMA-VLC系统模型及其通信光链路的特性,依据光链路增益的稀疏特性采用稀疏度自适应匹配追踪(sparsity adaptive matching pursuit,SAMP)算法,估计NOMA-VLC系统的信道冲激响应(channel impulse response, CIR)。在此基础上,采用VTRM方法对NOMA-VLC进行信道均衡,通过VTRM的时空聚焦特性减少信道衰落影响,并重构接收信号,抑制多径效应。理论分析和实验结果表明：经论文算法均衡后的NOMAVLC系统,在满足前向纠错(forward error correction,FEC)误码率阈值的同...     

一种2D—RAKE接收机

在DS—CDMA无线移动环境中，存在严重的多址干扰和多径衰落。本文利用自适应天线阵列分离出多径信号，利用RAKE消除多径衰落。导频位符号辅助(PSA)信道估计用来对BAKE的输入端进行加权，天线阵列的加权系数的自适应更新是采用RLS算法而得。结果表明，该方案具有很强的抗多径干扰和多址干扰的能力。     

多天线信道空间衰落相关性近似算法及其复杂性研究

信道空间衰落相关性(SFC)主要取决于波达信号的功率方位谱(PAS)和多天线阵列收发模式.深入研究了移动通信系统中多天线阵列SFC近似计算法及其复杂性.首先导出在典型PAS为均匀分布、高斯分布以及拉普拉斯分布下的SFC函数的闭合表达式.再研究在波达信号PAS小角度扩展时的近似计算法,建立多输入多输出(MIMO)多天线接收信道模型,深入分析所选择的天线阵列和电波传播参数对MIMO系统信道容量的影响.通过理论计算和仿真实验得出近似计算法在特定条件下具有很好的拟合度,定量分析了近似计算法在对MIMO多天线系统分析时的适用性和计算效率.该算法能极大地减低理论计算复杂性,提高分析和仿真MIMO多天线系统的效率.     

室内可见光通信多径信道建模及MIMO-ACO-OFDM系统性能分析

当通信速率较高时,室内光信号的多径效应不可忽视,当发送端存在时间弥散性时,提出一种室内可见光通信-多输入多输出(VLC-MIMO)多径衰落信道建模方法.由于光正交频分复用(OFDM)技术能有效抵抗码间干扰,故将非对称限幅光OFDM(ACO-OFDM)和MIMO相结合,建立MIMO-ACO-OFDM系统.用迭代法计算每对LED和光电探测器(PD)之间的冲激响应,连接LED和PD阵列的几何中心作为等效视线传输(LOS)信道,建立了多径信道模型.分析表明,当PD在房间中间时多径信道路径增益大,多径干扰弱;当PD在墙角时信号衰减大,多径干扰强.采用迫零(ZF)和最小均方误差(MMSE)检测,理论推导了系统误码率性能,建立了蒙特卡罗仿真模型.仿真表明,低阶调制时系统有更好的性能.当调制阶数小于64时,MMSE比ZF更有优势,随着信噪比增大两中检测方法得到的系统性能趋于相同;当LED半功率角变小时光束会聚强,PD在房间中心时性能最好,越往墙边性能越差;当PD视场角变小时,虽然在房间中心时接收功率不变,但多径干扰变弱性能变好.     

A large‐solid‐angle X‐ray Raman scattering spectrometer at ID20 of the European Synchrotron Radiation Facility

An end‐station for X‐ray Raman scattering spectroscopy at beamline ID20 of the European Synchrotron Radiation Facility is described. This end‐station is dedicated to the study of shallow core electronic excitations using non‐resonant inelastic X‐ray scattering. The spectrometer has 72 spherically bent analyzer crystals arranged in six modular groups of 12 analyzer crystals each for a combined maximum flexibility and large solid angle of detection. Each of the six analyzer modules houses one pixelated area detector allowing for X‐ray Raman scattering based imaging and efficient separation of the desired signal from the sample and spurious scattering from the often used complicated sample environments. This new end‐station provides an unprecedented instrument for X‐ray Raman scattering, which is a spectroscopic tool of great interest for the study of low‐energy X‐ray absorption spectra in materials under in situ conditions, such as in operando batteries and fuel cells, in situ catalytic reactions, and extreme pressure and temperature conditions.     

Algorithms for converting 3D surface roughness spectra when calculating frequency-angle reverberation characteristics

On the basis of an earlier developed model of sound scattering by surface disturbances characterized by 3D roughness spectra, we propose and test a sufficiently fast algorithm for calculating the spectral power density of surface reverberation that takes into account the propagation conditions, distance, and beam pattern of the receiver system. For a distance of up to 1000 m, a water area depth of ??20 m, and actually measured 3D roughness spectra in the Doppler frequency range of ±5 Hz, we have calculated the frequency-angle reverberation spectra for an acoustic background-radiation signal of 1.5 kHz. The obtained angle and frequency reverberation characteristics are compared with the results of acoustic measurements conducted using linear horizontal receiver antennas.     

基于不同散射机制特征的海杂波时变多普勒谱模型

海杂波的多普勒谱建模对采用多普勒处理技术的雷达进行有效的海杂波抑制和目标检测具有重要的意义.本文分别考虑Bragg,白冠和破碎波三种散射机制对应的多普勒谱分量的特征,对三种谱分量的频移和展宽进行分离,并引入附加速度频移量,提出了基于不同散射机制特征的雷达海杂波时变多普勒谱模型.该模型假设谱强度为受观测时间区间影响的随机变量,能够同时适用于平均多普勒谱与短时多普勒谱建模.通过分别对黄海海域实测的岸基P,S波段海杂波平均多普勒谱与短时多普勒谱建模测试,结果表明该模型相对传统模型的建模精度更高,尤其体现在短时谱的观测时间较长和平均谱形式较为复杂的情况下,建模误差显著减小.     

Microwave Doppler spectra of sea return at small incidence angles: specular point scattering contribution

It is well known that the sea return echo contains contributions from at least two scattering mechanisms. In addition to the resonant Bragg scattering, the specular point scattering plays an important role as the incidence angle becomes smaller (≤20o). Here, in combination with the Kirchhoff integral equation of scattering field and the stationary phase approximation, analytical expressions for Doppler shift and spectral bandwidth of specular point scattering, which are insensitive to the polarization state, are derived theoretically. For comparison, the simulated results related to the two-scale method (TSM) and the method of moment (MOM) are also presented. It is found that the Doppler shift and the spectral bandwidth given by TSM are insufficient at small incidence angles. However, a comparison between the analytical results and the numerical simulations by MOM in the backscatter configuration shows that our proposed formulas are valid for the specular point scattering case. In this work, the dependences of the predicted results on incidence angle, radar frequency, and wind speed are also discussed. The obtained conclusions seem promising for a better understanding of the Doppler spectra of the specular point scattering fields from time-varying sea surfaces.     

Correlation functions and power spectra of Doppler response signals in ultrasonic medical applications

Ultrasound Doppler methods are widely used in clinical practice as prospective investigational tool to study the vascular system and soft biological tissues. Meanwhile, the most general relationship between the power Doppler spectra, spectral characteristics of the scattering fluctuations and the probing ultrasound field parameters for some clinical implementations are still unexplored. Based upon the continuum model of scattering inhomogeneities, a set of the closed-form expressions for the correlation functions and the spectra of Doppler response of soft tissues and blood have been derived. The influence of the correlation among inhomogeneities and the diffusion processes on the Doppler power spectra formed by stationary flows have been examined. Computer simulations of Doppler spectra were performed for different values of correlation radius and diffusion coefficient. With simulation results the effects of the correlation among inhomogeneities and the diffusion processes on the spectral width and mean frequency are established and discussed in respect to turbulent flows. Closed-form expressions for correlation functions and Doppler spectra for the vibrational sonoelastography technique for visualizing malignant tumors in tissues have been derived. Based on the peculiarities of the obtained Doppler spectra, it is shown that the differentiation of soft tissues with respect to the amplitude value of constrained oscillations is feasible. The expressions were derived for the cases of non-stationary accelerated blood movement. It has been found that the frequency dependence reveals solely at a finite time of observation and depends on the initial phase of the accelerated movement.     

油膜覆盖的非线性海面电磁散射多普勒谱特性研究

当海面上方漂浮油膜时,海面的毛细波成分将因油膜的阻尼作用而被破坏.本文采用PM谱,基于Marangoni阻尼效应,建立油膜覆盖的一维Creamer非线性海面模型,并简单分析了油膜的阻尼作用对海面轮廓的影响.在此基础上,利用迭代物理光学方法研究了L波段下该模型的后向散射回波的多普勒谱特性,通过与基于线性模型的海面散射回波多普勒谱对比发现,在大中入射角下,非线性海面散射回波与线性海面多普勒谱的差异不可忽略,说明采用Creamer非线性理论建立海面几何模型的必要性.研究发现,油膜覆盖海面的散射回波的多普勒频移及展宽与干净海面雷达回波的多普勒特性具有明显差异,这表明海面上漂浮的油膜对雷达散射回波的多普勒特性具有显著的影响.数值结果重点分析了入射角、油膜参数以及风速对油膜覆盖海面散射回波多普勒谱展宽和频移的影响规律.     

双尺度动态分形粗糙海面的电磁散射及多普勒谱研究

利用更为符合海面实际散射场计算的双尺度法分析并计算了一维动态分形粗糙海面的后向散射截面，同时与微扰法有关结果进行了比较.研究了动态分形海面后向散射信号的多普勒谱分形特征，分析了不同极化状态下后向散射截面和多普勒谱谱宽随入射角及分维的变化规律，最后详细讨论了多普勒谱中心频率与海面Bragg谐振频率间的关系. 关键词： 双尺度模型 分形海面 电磁散射 多普勒谱     

双扩展信道中的时延-多普勒匹配滤波

本文研究了时延和多普勒在信号和信道分析中的重要作用,论述了信号和信道的时延—多普勒性质,并进行了表征。信号的模糊度函数可用于指导发射波形设计,而广义平稳非相关散射条件下信道的散射函数则可用于指导信道估计。在此基础上,提出了发?收联合时延?多普勒匹配滤波框架,得出了发射?处理波形的互模糊度函数与信道散射函数在时延?多普勒平面内的重叠面积与系统性能成正比的结论。计算机仿真结果验证了本文方法的有效性。