相干光正交频分复用传输系统中的 无导频相位纠偏方法

提出了一种适用于16-QAM的新型相位纠偏方法,克服了基于M次方算法的传统相位纠错方法不适用于高阶正交振幅调制的缺点.该方法结合正交频分复用调制方式,无需导频即可完成相位盲估计,且计算量较少.本文基于112 Gb/s 1 040 km的相干光正交频分复用传输系统进行了算法验证,并与传统的4次方纠偏方法进行了比较,实验证明该算法适用于正交频分复用和16-QAM调制平台,相位纠偏结果显著优于基于导频的4次方算法.     

综合考虑局部和全局特性的电网关键节点识别

为有效识别网络中的关键节点,提出一种综合考虑网络局部和全局特性的节点重要性识别综合指标,依据此指标对加权标准测试系统IEEE39和IEEE118中的节点进行重要性排序,并将排序结果与基于介数法和点权法对节点重要性进行排序结果进行对比,基于结构的网络效能分析和基于动力学的失同步扩散时间、同步能力比较均表明,提出的基于综合指标的节点重要性排序更合理,优于基于介数和点权的节点重要性识别方法.     

对称三角线性调频水声通信信号的同步检测方法

同步捕获是水声通信中的重要环节。基于分数阶傅里叶变换(FRFT)可提高对称三角线性调频波(STLFM)的信号同步捕获精度。理论分析表明,对称三角线性调频波在同步窗口正确时,其分数阶傅里叶变换(FRFT)域上具有双尖峰对称性。利用FRFT域上两个尖峰的位置偏移可估计并矫正同步信号的多普勒频移与时间延迟,进而利用FRFT域上两个尖峰的峰值差可实现对同步跟踪环路的精细调整。水声传播仿真和五缘湾浅海信道测试结果表明,在相同时间带宽乘积情况下,所提同步方法的同步误差均值比线性调频(LFM)同步方法低1/3,同步误差均方误差上降低至1/5¹/10,具有更好的准确性和稳定性。     

Vibration analysis of defected and pristine triangular single-layer graphene nanosheets

This paper investigates the vibration behavior of pristine and defected triangular graphene sheets; which has recently attracted the attention of researchers and compare these two types in natural frequencies and sensitivity. Here, the molecular dynamics method has been employed to establish a virtual laboratory for this purpose. After measuring the different parameters obtained by the molecular dynamics approach, these data have been analyzed by using the frequency domain decomposition (FDD) method, and the dominant frequencies and mode shapes of the system have been extracted. By analyzing the vibration behaviors of pristine triangular graphene sheets in four cases (right angle of 45-90-45 configuration, right angle of 60-90-30 configuration, equilateral triangle and isosceles triangle), it has been demonstrated that the natural frequencies of these sheets are higher than the natural frequency of a square sheet, with the same number of atoms, by a minimum of 7.6% and maximum of 26.6%. Therefore, for increasing the resonance range of sensors based on 2D materials, non-rectangular structures, and especially the triangular structure, can be considered as viable candidates. Although the pristine and defective equilateral triangular sheets have the highest values of resonance, the sensitivity of defective (45,90,45) triangular sheet is more than other configurations and then, defective (45,90,45) sheet is the worst choice for sensor applications.     

行波管栅控电子枪热形变研究

行波管栅控电子枪各组件尤其是阴极和栅网的热形变对行波管电子光学性能有着较大的影响。基于有限元分析方法,利用热力场耦合对栅控电子枪热形变问题进行了研究,结果表明:阳极的总形变量最大,最高处达0.28 mm;控制栅网、阴影栅、阴极及部分支撑结构向着阳极方向发生形变,由温升引起的形变不会使控制栅网和阴影栅发生接触或交叉,而阴极形变量较大时,阴极与阴影栅则存在接触的危险,这对电子枪的工作性能有很大的影响,甚至会使阴影栅烧坏。降低电子截获率,适当增加控制栅网和阴影栅的厚度,使用热传导率高、线膨胀系数低的材料,可以减小栅控电子枪各组件的热形变。      

Separated Product Rules for High and Low Frequencies in Isotopic Molecules

Separation of high and low frequencies is a known approximation in the theory of molecular vibrations. Accordingly the Teller-Redlich product rule for isotopic frequencies splits into separated product rules. A theorem states that the approximate frequency ratios for the high and low frequencies when multiplied together give the exact ratio for the whole set of frequencies. The theorem is illustrated by numerical computations for H₂S₂/D₂S₂ and proved mathematically for special cases as well as the general case.     

Tailoring red-green-blue emission from Er3+, Eu3+ and Tb3+ doped Y2O3 nanocrystals produced via PVA-assisted sol-gel route

Y₂O₃ luminescent nanoparticles were synthesized via PVA-assisted sol-gel method and their structural and optical properties were investigated. Effects of rare earth (Er³⁺, Eu³⁺ and Tb³⁺) doping on luminescence properties of the produced nanophosphors have been investigated under NIR (800 nm) and UV (240–300 nm) excitation. Intense infrared to red and green emissions were observed and a weak blue upconverted luminescence was also detected. Moreover, it was observed that changing the doping ions, the color emitted by the samples could be modified and different combinations of UV excitation and doping produced effective white light emissions. The obtained results demonstrate that PVA-assisted sol-gel is an effective methodology for the synthesis of rare-earth doped Y₂O₃ nanophosphors.     

Frequency Downconverting Fiber Grating-Based Beamformers for High-Capacity Phased Arrays

A technique is presented to remove the beat noise limitation in multibeam beam formers using a simple all-optical microwave frequency downconversion technique prior to performing the true-time delay equalization in the optical domain. The frequency conversion concept enables a significant increase in beam-number capac ity to be achieved due to the elimination of beat noise limits, and also effectively removes the power penalty due to chromatic dispersion limitations of the chirped grating units in the beamformer. The Bragg grating requirements for the frequency converting beamforming network are analyzed and show that tanh-profile apodized gratings can meet the isolation, reflectivity, and narrow bandwidth requirements. For an X-band phased array, more than a twofold increase in beam capacity is shown through the use of the frequency conversion technique with the grating-based beamformer, and the resulting beamformer has the minimum number of optical interconnects with true-time delay operation.     

Sheath and plasma parameters in a magnetized plasma system

The variation of electron temperature and plasma density in a magnetized N₂ plasma is studied experimentally in presence of a grid placed at the middle of the system. Plasma leaks through the negatively biased grid from the source region into the diffused region. It is observed that the electron temperature increases with the magnetic field in the diffused region whereas it decreases in the source region of the system for a constant grid biasing voltage. Also, investigation is done to see the change of electron temperature with grid biasing voltage for a constant magnetic field. This is accompanied by the study of the variation of sheath structure across the grid for different magnetic field and grid biasing voltage as well. It reveals that with increasing magnetic field and negative grid biasing voltage, the sheath thickness expands.     

A comparison of a posteriori error estimators for mixed finite element discretizations by Raviart-Thomas elements

We consider mixed finite element discretizations of linear second order elliptic boundary value problems with respect to an adaptively generated hierarchy of possibly highly nonuniform simplicial triangulations. In particular, we present and analyze four different kinds of error estimators: a residual based estimator, a hierarchical one, error estimators relying on the solution of local subproblems and on a superconvergence result, respectively. Finally, we examine the relationship between the presented error estimators and compare their local components.