随机共振在认知无线电频谱感知中应用的最新研究进展

频谱感知是认知无线电网络的一项关键技术.低信噪比(SNR)环境下频谱检测的性能会大幅降低,而随机共振(SR)能有效提高信号信噪比,所以将其应用到频谱感知中,能增强认知用户对主用户(PU)的检测性能.首先介绍了随机共振在认知无线电频谱感知中应用的最新研究进展,包括随机共振在本地感知中(如能量检测、协方差矩阵频谱感知、循环平稳特征检测)及协作感知中的应用,然后指出了随机共振在认知无线电频谱感知中还有待解决的问题,并提出了下一步的研究方向.     

认知无线电中基于阵列天线和协方差矩阵的频谱感知算法

该文提出一种基于阵列天线和协方差矩阵的频谱感知算法,该算法能够在噪声不确定性的条件下进行盲频谱感知。该算法在协方差矩阵的基础上,构建新的检测统计量,推导判决门限,对检测统计量与判决门限进行比较进而做出最终判决;在主用户信号到达方向与认知用户接收天线法线方向不一致的情况下,为使认知用户能完全接收主用户信号,利用了阵列天线技术。仿真结果表明,与Zeng等人(2009)提出的绝对值协方差矩阵频谱感知算法(Covariance Absolute Value Spectrum Sensing, CAVSS)相比,该算法判决门限的计算方法更加准确;在相同条件下,该算法的检测概率高于CAVSS。     

Ad Hoc认知网络中基于梯度算法的协作压缩频谱感知方法

ad hoc认知网络(CRAHNs)比起传统ad hoc网络有很多优势,如分布式多跳结构及动态变化的网络拓扑等。频谱感知是认知无线电技术的基础,采用压缩感知方法以低于奈奎斯特采样率的速率来获得较好的重构信号,同时可减轻数字处理设备的压力。本文使用多跳模型,每个认知用户感知的频谱不仅包括主用户的主频谱,也包括认知用户自身收到其他认知用户干扰的更新频谱。由于具有动态特性的ad hoc网络缺少融合中心,所以本文中提出一种基于梯度的分布式协作感知算法。该算法基于能量检测,具有可靠的感知性能和较少的能量消耗,并能快速收敛。仿真实验验证了算法的有效性。      

主用户活跃性对新的认知无线电系统容量的影响

在机会频谱接入认知无线电系统中,认知用户只有在通过感知确定信道空闲时才可以接入授权信道,因此频谱感知对于系统性能影响非常重要。本文提出了基于新帧结构的四元频谱感知模型,考虑主用户活跃性对认知网络吞吐量的影响,采用可同时最大化感知和数据传输时间的新帧结构模型,不需要考虑感知和吞吐量的均衡。理论分析新模型下感知时间,主用户活跃性,目标检测概率,主用户接收信噪比对系统吞吐量的影响,并与传统模型进行对比。     

一种基于随机共振增强的协方差矩阵频谱感知算法

为了在避免对主用户系统产生有害干扰的同时 提高频谱利用效率,要求认知无线电系统的频谱感知算法能在极低的信噪比下快速检测出主用户信号。由于可以避免能量检测面临的噪声不确定性问题,基于协方差矩阵的检测算法是一种有效的盲频谱感知算法。为了进一步提高极低信噪比下的性能,本文提出了一种基于随机共振的协方差矩阵频谱感知算法。该算法通过在接收信号中加入优化的特定信号,利用随机共振原理,增大有无主用户信号下的检测统计量概率分布函数的分离度,提高频谱感知的性能。仿真结果表明,相对于现有的协方差矩阵频谱感知算法 ,在相同的虚警概率下,所提算法可以显著提高极低信噪比下的检测概率,同时大幅度缩减检测时间。      

基于随机矩阵理论的DET合作频谱感知算法

针对认知无线电系统中的频谱感知问题,该文采用随机矩阵理论(Random Matrix Theory, RMT)对多认知用户(Secondary User, SU)接收信号采样协方差矩阵的最大特征值的分布特性进行了分析和研究,提出了一种新的基于双特征值判决门限(Double Eigenvalue Threshold, DET)的合作频谱感知算法。由该算法感知性能的理论分析可知:DET合作感知算法无需主用户(Primary User, PU)发射机信号的先验知识,也不需要预先知道信道背景噪声功率。仿真结果表明,与传统的频谱感知方法相比,该方法只需较少的认知用户就能获得较高的感知性能,并且对噪声的不确定性具有较强的鲁棒性。     

无线认知传感器网络特征模板匹配算法研究

频谱感知是实现认知无线电应用、构建WCSN(无线认知传感器网)的核心技术,也是保护主用户免受有害干扰、提高WCSN自身频谱资源利用率的重要前提。当前的频谱感知技术主要有匹配滤波器检测、特征模板匹配和能量检测。文章给出WCSN频谱感知的假设检验,分析主用户信号主特征向量的求解方法,描述PCA(主成分分析)框架下检测算法的实现流程。频谱感知实验结果表明,PCA方法比对应线性方法性能优越。     

协作频谱感知在认知网络中的应用

认知无线电能够灵敏感知周围环境的变化,通过频谱感知功能发现频谱空洞,使得认知无线电能够与周围通信环境相适应。在认知无线电网络中,认知用户（次用户）必须连续地监控频谱,检测主用户是否存在。如果主用户存在,次用户必须尽快地退出该频段,从而不影响主用户。在文中采用一种协作机制,即两用户进行协作频谱感知,从而提高主用户的检测率,减少了检测时间,并且得到捷变增益。我们用Matlab给出仿真结果。     

联合循环平稳特征PCA与RVM的频谱感知

针对无线信道环境中低信噪比情况下主用户信号检测率较低的问题,提出了一种基于循环平稳特征主成分分析(PCA)与相关向量机(RVM)的认知网络频谱感知算法。该算法结合了主成分分析算法与相关向量机分类方法,应用于解决认知网络频谱感知问题。首先对信号循环平稳特征参数进行特征提取,通过主成分分析进行降维提取信号主成分,生成训练样本和待测样本,并完成对相关向量机的训练,再采用训练完成的相关向量机算法分别对有无主用户情况下的信号进行分类检测,最后获得主用户信号存在性的感知判断。仿真实验表明,与人工神经网络、支持向量机和最大最小特征值算法相比较,所提算法在低信噪比情况下具有较高的分类检测性能,检测率最大可提高61.6%,有效地实现了对主用户信号的感知。     

小样本图像处理方法赋能的宽带频谱感知

针对强噪声环境下频谱感知方法计算复杂度高、难以获取大量标注样本、检测准确率低等问题,该文提出由图像去噪和图像分类思想驱动的频谱感知方法(IDCSS)。首先,对感知用户的接收信号进行时频变换,将无线电数值信号转换为图像。强噪声环境下感知用户接收信号图像与噪声图像相关度高,因此搭建生成对抗网络(GAN)来增加低信噪比下接收信号样本的数量,提高图像的质量。在生成器中,利用残差-长短时记忆网络取代生成网络U-Net结构中的跳跃连接,对图像进行去噪、提取感知用户接收信号图像的多尺度特征、建立基于熵的损失函数来构建网络的抗噪能力;在判决器中,设计适用无线电图像信号的多维度判决器来增强生成图像的质量、保留低信噪比感知用户信号的图像细节。最后利用分类器识别频谱占用状态。仿真结果表明,与现有频谱感知算法相比,所提算法具有较好的检测性能。     

Development of the readout system for waveguide multilayer optical card

Waveguide multilayer optical card （WMOC） is a novel storage device of three-dimensional optical information. An advanced readout system fitting for the WMOC is introduced in this paper. The hardware mainly consists of the light source for reading, WMOC, motorized stages addressing unit, microscope imaging unit, CCD detecting unit and PC controlling ＆ processing unit. The movement of the precision motorized stage is controlled by the computer through Visual Basic （VB） language in software. A control panel is also designed to get the layer address and the page address through which the position of the motorized stages can be changed. The WMOC readout system is easy to manage and the readout result is directly displayed on computer monitor.     

Nanoimprint Lithography -A Next Generation High Volume Lithography Technique

IntroductionNanoimprint Lithography is a well-acknowl-edged low cost, high resolution, large area pattern-ing process. It includes the most promising methods,high-pressure hot embossing lithography (HEL) [2],UV-cured imprinting (UV-NIL) [3] and micro contactprinting (m-CP, MCP) [4]. Curing of the imprintedstructures is either done by subsequent UV-lightexposure in the case of UV-NIL or by cooling downbelow the glass transition temperature of the ther-moplastic material in case of HEL…     

Collinear phase-matching study of terahertz-wave generation via difference frequency mixed in GaAs and Inp

The collinearly phase-matching condition of terahertz-wave generation via difference frequency mixed in GaAs and InP is theoretically studied. In collinear phase-matching, the optimum phase-matching wave hands of these two crystals are calculated. The optimum phase-matching wave bands in GaAs and lnP are 0.95-1.38μm and 0.7-0.96μm respectively. The influence of the wavelength choice of the pump wave on the coherent length in THz-wave tuning is also discussed. The influence of the temperature alteration on the phase-matching and the temperature tuning properties in GaAs crystal are calculated and analyzed. It can serve for the following experiments as a theoretical evidence and a reference as well.     

Effects of the alloy compositions on the phonon-polaritons in ternary mixed crystals

Composition dependence of bulk and surface phonon-polaritons in ternary mixed crystals are studied in the framework of the modified random-element-isodisplacement model and the Bom-Huang approximation. The numerical results for Several Ⅱ - Ⅵ and Ⅲ- Ⅴ compound systems are performed, and the polariton frequencies as functions of the compositions for ternary mixed crystals AlxGa1-xAs, GaPxAS1-x, ZnSxSe1-x, GaAsxSb1-x, GaxIn1-xP, and ZnxCd1-xS as examples are given and discussed. The results show that the dependence of the energies of two branches of bulk phonon-polaritons which have phonon-like characteristics, and surface phonon-polaritons on the compositions of ternary mixed crystals are nonlinear and different from those of the corresponding binary systems.     

Influence of PVK ： NPB hole transporting layer on the characteristics of organic light-emitting devices

A doping system consisting of NPB and PVK is employed as a composite hole transporting layer （CHTL）. By adjusting the component ratio of the doping system, a series of devices with different concentration proportion of PVK ： NPB are constracted. The result shows that doping concentration of NPB enhances the competence of hole transporting ability, and modifies the recombination region of charge as well as affects the surface morphology of doped film. Optimum device with a maximum brightness of 7852 cd/m^2 and a power efficiency of 1.75 lm/W has been obtained by choosing a concentration proportion of PVK ： NPB at 1：3.     

New insert layer structure OLEDs

An insert layer structure organic electroluminescent device（OLED） based on a new luminescent material （Zn（salen）） is fabricated. The configuration of the device is ITO/CuPc/NPD/Zn（salen）/Liq/LiF/A1/CuPc/NPD/Zn（salen）/Liq/LiF/A1. Effective insert electrode layers comprising LiF（1nm）/Al（5 nm） are used as a single semitransparent mirror, and bilayer cathode LiF（1 nm）/A1（100 nm） is used as a reflecting mirror. The two mirrors form a Fabry-Perot microcavity and two emissive units. The maximum brightness and luminous efficiency reach 674 cd/m^2 and 2.652 cd/A, respectively, which are 2.1 and 3.7 times higher than the conventional device, respectively. The superior brightness and luminous efficiency over conventional single-unit devices are attributed to microcavity effect.     

Analysis of error performance on Turbo coded FDPIM

Due to variable symbol length of digital pulse interval modulation（DPIM）, it is difficult to analyze the error performances of Turbo coded DPIM. To solve this problem, a fixed-length digital pulse interval modulation（FDPIM） method is provided. The FDPIM modulation structure is introduced. The packet error rates of uncoded FDPIM are analyzed and compared with that of DPIM. Bit error rates of Turbo coded FDPIM are simulated based on three kinds of analytical models under weak turbulence channel. The results show that packet error rate of uncoded FDPIM is inferior to that of uncoded DPIM. However, FDPIM is easy to be implemented and easy to be combined, with Turbo code for soft-decision because of its fixed length. Besides, the introduction of Turbo code in this modulation can decrease the average power about 10 dBm, which means that it can improve the error performance of the system effectively.     

A method for calculating the center position of collimated light

It is a key problem to accurately calculate beam spots＇ center of measuring the warp by using a collimated laser. A new method, named double geometrical center method （DGCM）, is put forward for the first time. In this method, a plane wave perpendicularly irradiates an aperture stop, and a charge couple device （CCD） is employed to receive the diffraction-beam spots, then the geometrical centers of the fast and the second diffraction-beam spots are calculated respectively, and their mean value is regarded as the center of datum beam. In face of such adverse instances as laser intension distributing defectively, part of the image being saturated, this method can still work well. What＇s more, this method can detect whether an unacceptable error exits in the courses of image receiving, processing and calculating. The experimental results indicate the precision of this method is high.     

Preparation and Characterization of TaN ALD Precursors

High purity organic-tantalum precursors for thin film ALD TaN were synthesized and characterized.Vapor pressure and thermal stability of these precursors were studied.From the vapor pressure analysis,it was found that TBTEMT has a higher vapor pressure than any other published liquid TaN precursor,including TBTDET,TAITMATA,and IPTDET.Thermal stability of the alkyl groups on the precursors was investigated using a 1H NMR technique.The results indicated that the tertbutylimino group is the most stable group on TBTDET and TBTEMT as compared to the dialkylamido groups.Thermal stability of TaN precursors decreased in the following order:TBTDET ＞ PDMAT ＞ TBTEMT.In conclusion,precursor vapor pressure and thermal stability were tuned by making slight variations in the ligand sphere around the metal center.     

Curved crystal spectrometer for the measurement of X-ray lines from laser-produced plasmas

In order to diagnose the laser-produced plasmas, a focusing curved crystal spectrometer has been developed for measuring the X-ray lines radiated from a laser-produced plasmas. The design is based on the fact that the ray emitted from a source located at one focus of an ellipse will converge on the other focus by the reflection of the elliptical surface. The focal length and the eccentricity of the ellipse are 1350 mm and 0.9586, respectively. The spectrometer can be used to measure the X- ray lines in the wavelength range of 0.2-0.37 nm, and a LiF crystal （200） （2d = 0.4027 nm） is used as dispersive element covering Bragg angle from 30° to 67.5°. The spectrometer was tested on Shengnang- Ⅱ which can deliver laser energy of 60-80 J/pulse and the laser wavelength is 0.35 μm. Photographs of spectra including the 1 s2p ^1P1-1s^2 ^1S0 resonance line（w）, the 1s2p ^3P2-1s^2 1S0 magnetic quadrupole line（x）, the 1s2p ^3P1-1 s^2 ^1S0 intercombination lines（y）, the 1 s2p ^3S~1-1 s^2 ^1S0 forbidden line（z） in helium-like Ti Ⅹ Ⅺ and the 1 s2s2p ^2P3/2-1 s622s ^2S1/2 line（q） in lithium-like Ti Ⅹ Ⅹhave been recorded with a X-ray CCD camera. The experimental result shows that the wavelength resolution（λ/△ 2） is above 1000 and the elliptical crystal spectrometer is suitable for X-ray spectroscopy.