OPAST子空间跟踪数据压缩半盲多用户检测

基于子空间方法的最小均方误差半盲多用户检测的计算核心是对信号子空间的特征值与特征向量的同时跟踪.仅跟踪计算信号子空间特征向量的子空间跟踪算法不能直接应用于这种检测方法.利用数据压缩技术,提出一种只需跟踪计算信号子空间正交规范基的自适应数据压缩半盲多用户检测.将著名的正交投影逼近子空间跟踪(OPAST)算法应用于这种数据压缩半盲多用户检测,发现OPAST算法具有自然的数据压缩结构,在几乎不增加运算量的情况下即可实现数据压缩半盲多用户检测.仿真实验表明:基于OPAST算法的数据压缩半盲多用户检测具有良好的检测性能.     

适用于放大自发辐射平滑光束的脉宽压缩方法

为了增加靶上的功率密度和耦合效率,基于饱和增益开关原理,发展了一种适用于放大自发辐射平滑光束的脉宽压缩方法。利用该方法,在赛格德大学的一台放电泵浦KrF准分子激光器上进行了实验,将该激光器输出的脉宽为14.5 ns的放大自发辐射光束压缩到了7.5 ns,并且保持了光束原有的均匀性。     

The effect of a non-aligned unit on double-screen frequency-selective surface transmission characteristics

Double-screen frequency-selective surfaces (FSSs) can bring about a better flattened effect and a rapidly declining edge. They are therefore an effective means to achieve outer-zone stealth of the radar cabin to detect radar waves. In this article, a double-screen wide-bandpass FSS structure is designed and the transmission characteristics of the units under alignment and non-alignment are simulated by means of the spectral domain approach. Meanwhile, the experimental parts fabricated by vacuum evaporation and lithography are tested in a microwave chamber. The results show that the aligned unit structure has good incident angle stability and can achieve high transmittance when the bandwidth is 3.3 GHz, and the transmission loss is less than 1 dB. When the units have a non-aligned structure, the bandwidth decreases and transmission loss increases with increasing incident angle.     

Influence of pumping laser bandwidth on the quantum fluctuation chracteristic of a non-degenerate optical parametric amplifier

Usually the quantum fluctuation characteristic of a non-degenerate optical parametric amplifier is analysed under the assumption of monochromatic pumping. However, in experiments, a driving beam with finite bandwidth is used to obtain the non-degenerate signal and idler beam amplifications. On account of that, we derive an analytical solution for the non-degenerate optical parametric amplification system with finite bandwidth laser pumping, and evaluate the associated quantum fluctuation. Finally, the application of the V1 criterion to bipartite entanglement is discussed.     

Hot deformation behavior and constitutive modeling of VCN200 low alloy steel

The hot working behavior of VCN200 medium carbon low alloy steel was analyzed by performing hot compression tests in temperature range of 850–1150 °C and at strain rates of 0.001–1 s⁻¹. Flow curves were typical of dynamic recrystallization during hot working over temperature range of 900–1150 °C and strain rates of 0.001–1 s⁻¹. However, at lower temperatures no indication of flow softening was observed. The constitutive analysis using the hyperbolic sine function was performed and the value of apparent activation energy for the hot deformation determined to be about 435 kJ/mol. The flow curves up to the peak were successfully modeled using a dynamic recovery model. All the factors in this model were defined in terms of the Zener–Hollomon parameter. A modified form of Avrami equation was used to estimate the fractional softening due to the dynamic recrystallization for any given strain in flow curve. Using this model, the flow curves were successfully predicted and generalized to different deformation conditions.     

Sparse finite element methods for operator equations with stochastic data

Let A: V → V′ be a strongly elliptic operator on a d-dimensional manifold D (polyhedra or boundaries of polyhedra are also allowed). An operator equation Au = f with stochastic data f is considered. The goal of the computation is the mean field and higher moments of the solution. We discretize the mean field problem using a FEM with hierarchical basis and N degrees of freedom. We present a Monte-Carlo algorithm and a deterministic algorithm for the approximation of the moment for k⩾1. The key tool in both algorithms is a “sparse tensor product” space for the approximation of with O(N(log N) ^k−1) degrees of freedom, instead of N ^k degrees of freedom for the full tensor product FEM space. A sparse Monte-Carlo FEM with M samples (i.e., deterministic solver) is proved to yield approximations to with a work of O(M N(log N) ^k−1) operations. The solutions are shown to converge with the optimal rates with respect to the Finite Element degrees of freedom N and the number M of samples. The deterministic FEM is based on deterministic equations for in D ^k ⊂ ℝ^kd. Their Galerkin approximation using sparse tensor products of the FE spaces in D allows approximation of with O(N(log N) ^k−1) degrees of freedom converging at an optimal rate (up to logs). For nonlocal operators wavelet compression of the operators is used. The linear systems are solved iteratively with multilevel preconditioning. This yields an approximation for with at most O(N (log N) ^k+1) operations. This work was supported under IHP Network “Breaking Complexity” by the Swiss BBW under grant No. 02.0418     

准相位匹配倍频系统的带宽性质研究

根据准相位匹配倍频原理,讨论了实现宽带二次谐波转换需要满足的条件.以周期性极化铌酸锂晶体和掺杂氧化镁（7mol%）周期性极化铌酸锂（7mol%MgO-PPLN）晶体为例,分别比较了0型（e+e→e）和Ⅰ型（o+o→e）两种准相位匹配情况下宽带高效二次谐波转换的特性.研究表明,对于7mol%MgO-PPLN晶体,同时满足准相位匹配条件和群速度匹配条件的中心波长和带宽向短波方向移动,且0型（e+e→e）准相位匹配情况下可以获得更大的带宽.     

Calculation of time-varying equivalent inductance and resistance of helical flux compression generators using the 2D filamentary method and dynamic matrix concept in the frequency domain

In this paper we present an approach to calculate the time-varying equivalent inductance and resistance of helical flux compression generators accurately. This approach is based on the 2D filamentary method and uses the dynamic matrix concept. By using this method, it is possible to consider both the effects of the armature presence and frequency on the inductance and resistance of the HFCGs. The latter effect has not been discussed in previous open literature. The validation of the presented method has been verified by measurement results.     

Automatic detection of bioacoustics impulses based on kurtosis under weak signal to noise ratio

Passive acoustic monitoring (PAM) of marine mammal vocalizations has been efficiently used in a wide set of applications ranging from marine wildlife surveys to risk mitigation of military sonar emissions. The primary use of PAM is for detecting bioemissions, a good proportion of which are impulse sounds or clicks. A click detection algorithm based on kurtosis estimation is proposed as a general automatic click detector. The detector works under the assumption that click trains are embedded in stochastic but Gaussian noise. Under this assumption, kurtosis is used as a statistical test for detection. The algorithm explores acoustic sequences with the optimal frequency bandwidth for focusing on impulse sounds. The detector is successfully applied to field observations, and operates under weak signal to noise ratios and in presence of stochastic background noise. The algorithm adapts to varying click center frequency. Kurtosis appears as a promising approach to detect click trains, alone or in combination with other clicks detector, and to isolate individual clicks.     

Coded excitation for ultrasound tissue harmonic imaging

Coded excitation can improve the signal-to-noise ratio (SNR) in ultrasound tissue harmonic imaging (THI). However, it could suffer from the increased sidelobe artifact caused by incomplete pulse compression due to the spectral overlap between the fundamental and harmonic components of ultrasound signal after nonlinear propagation in tissues. In this paper, three coded tissue harmonic imaging (CTHI) techniques based on bandpass filtering, power modulation and pulse inversion (i.e., CTHI-BF, CTHI-PM, and CTHI-PI) were evaluated by measuring the peak range sidelobe level (PRSL) with varying frequency bandwidths. From simulation and in vitro studies, the CTHI-PI outperforms the CTHI-BF and CTHI-PM methods in terms of the PRSL, e.g., −43.5 dB vs. −24.8 dB and −23.0 dB, respectively.