Simulation of ultra-small GaAs MESFET using quantum momentequations

Ultra-small MESFETs have characteristic lengths comparable to quantum lengths: wavelength, mean free path, etc. In a first attempt to incorporate these quantum lengths, the authors develop a model based upon a set of quantum moment equations obtained from the Wigner function equation-of-motion. Interesting time-dependent current oscillation behavior has been observed when a step voltage is applied to the initial steady state. The oscillation frequency is peaked around 500 GHz, which is related to the plasma response of the carriers in the channel. Quantum effects, such as barrier repulsion and penetration, have been demonstrated in the simulation. These effects modify the electron density distribution and current density distribution both in the channel and near the source. Modifications of the frequency spectrum of the oscillation current due to the quantum effects are obvious     

A Wigner function-based quantum ensemble Monte Carlo study of a resonant tunneling diode

We present results of resonant tunneling diode operation achieved from a particle-based quantum ensemble Monte Carlo (EMC) simulation that is based on the Wigner distribution function (WDF). Methods of including the Wigner potential into the EMC, to incorporate natural quantum phenomena, via a particle property we call the affinity are discussed. Dissipation is included via normal Monte Carlo procedures and the solution is coupled to a Poisson solver to achieve fully selfconsistent results.     

Applied bias slewing in transient Wigner function simulation ofresonant tunneling diodes

The Wigner function formulation of quantum mechanics has shown much promise as a basis for accurately modeling quantum electronic devices, especially under transient conditions. In this work, we demonstrate the importance of using a finite applied bias slew rate (as opposed to instantaneous switching) to better approximate experimental device conditions, and thus to produce more accurate transient Wigner function simulation results. We show that the use of instantaneous (and thus unphysical) switching can significantly impact simulation results and lead to incorrect conclusions about device operation. We also find that slewed switching can reduce the high computational demands of transient simulations. The resonant tunneling diode (RTD) is used as a test device, and simulation results are produced with SQUADS (Stanford QUAntum Device Simulator)     

A strongly correlated electron state at one-dimensional quantum points

The method of exact diagnoalization is applied to find the distribution of electrons over single-particle states at a 1D quantum point and the Fourier spectrum of the electron density. It is shown that the distribution function contains an unexpected signularity at the Fermi level. It is found that (i) the singularity results from the Wigner electron ordering and (ii) the Wigner ordering involves suppression of spatial Fourier harmonics of the electron density observed when the interaction potential grows over the entire range of wave vectors except for the double Fermi wave vector. It is demonstrated that the distribution function calculated within the Luttinger model also has a singularity at the Fermi level and this singularity is of an irregular shape.     

Bipolar quantum-transport modeling of carrier injection into aSCH-quantum-well laser

A quantum mechanical model of carrier transport in separate confinement heterostructure (SCH) quantum-well lasers based on the Wigner function is presented for the first time. In the simulation, the three quantum Liouville equations with respect to the Wigner functions defined for electron, heavy-hole, and light-hole are solved simultaneously with the Poisson's equation to consider self-consistency in potential, As a simulation model, InGaAsP-InGaAs SCH quantum-well lasers are considered. The carrier injection into single and double quantum-well lasers is simulated. It is demonstrated that the amount of electrons and holes injected into the single quantum-well active layer is not equal in general if the quantum transport is considered. In the double quantum-well structure, the bottleneck phenomenon of heavy-hole injection into the second well and the quite different shape of heavy-hole density profiles in the two wells are simulated     

Quantum Transport Simulation of Silicon-Nanowire Transistors Based on Direct Solution Approach of the Wigner Transport Equation

In this paper, we present a self-consistent and 3D quantum simulator for Si-nanowire transistors based on the Wigner function model and multidimensional Schrodinger-Poisson algorithm. To achieve a sufficient numerical accuracy for calculating subthreshold current, we introduced a third-order differencing scheme for discretizing the drift term in the Wigner transport equation. By comparing with semiclassical Boltzmann and nonequilibrium Green's function approaches, the validity of the present simulator is confirmed. We also demonstrate that the source-drain tunneling is a critical physical phenomenon related to a scaling limit of nanowire devices, and the semiclassical simulation measurably underestimates a minimum gate length.     

Fast computation of the ambiguity function and the Wignerdistribution on arbitrary line segments

By using the fractional Fourier transformation of the time-domain signals, closed-form expressions for the projections of their auto or cross ambiguity functions are derived. Based on a similar formulation for the projections of the auto and cross Wigner distributions and the well known two-dimensional (2-D) Fourier transformation relationship between the ambiguity and Wigner domains, closed-form expressions are obtained for the slices of both the Wigner distribution and the ambiguity function. By using discretization of the obtained analytical expressions, efficient algorithms are proposed to compute uniformly spaced samples of the Wigner distribution and the ambiguity function located on arbitrary line segments. With repeated use of the proposed algorithms, samples in the Wigner or ambiguity domains can be computed on non-Cartesian sampling grids, such as polar grids     

基于Radon-Wigner变换的LFM扩谱信号的检测

提出了基于Radon-Wigner变换的线性调频扩谱信号的检测方法。根据LFM扩谱信号的特点,首先采用倒谱法估计出信号的码速率,然后截取一个码元的信号来提取码内的细微特征,通过检测由RWT生成的变换域,可以准确提取信号的调频斜率、载频等参数。理论和仿真实验表明,该方法有较强的抑制噪声能力。     

混响背景中目标回波检测和参数估计的一种新方法

本文研究了基于Wigner-Ville分布(WVD)和Hough变换(HT)联合技术抗混响的方法.给出了简要的理论介绍、具体的实现方法和相应的处理结果.仿真研究表明,在混响背景中,WVD-HT联合的方法能比较有效地完成目标回波的检测和参数估计.     

WHT-Based Detection and Parameter Estimation of a Single LFM Interference Signal in Spread Spectrum Systems

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基于互Wigner-Ville分布的SAR运动目标检测

本文针对机载合成孔径雷达(SAR)运动目标回波信号的特点,提出了基于互Wigner-Ville分布(XWVD)的机载SAR运动目标检测方法,分析了它在单目标和多目标情况下的检测性能.实验证明,该方法可以在较低的信噪比下实现目标的检测,其性能大大优于常用的Wigner-Ville分布方法.为了抑制多个目标之间交叉项的影响,本文还提出了一种基于逐次消去思想的互Wigner-Ville分布检测方法,它可以解决强度相差较大的多目标检测问题.仿真实验的结果证明了算法的有效性.     

超短波通信系统的物理层仿真精确建模方法研究

在通信系统仿真设计中,物理层建模通常使用经验模型公式,无法精确地刻画仿真场景及配置的实际情况.为了提高仿真模型的有效性,针对超短波通信系统的特点以及物理层信道模型和地形因素对超短波通信的影响,本文设计了一种结合OMNET++通信模型、Simulink信道模型和WI模型的超短波通信仿真系统,通过与物理层经验模型公式进行对比实验,验证了物理层的建模精度对性能结果具有较大影响,并对性能结果影响因素进行了分析.     

线性共振力作用下振幅衰减模型中激发相干态的退相干

采用热纠缠态表象求解密度算符主方程的方法, 结合有序算符积分技术,求解了线性共振力 作用下振幅衰减模型中密度算符主方程,导出了主方程解的算符无限和表示。利用密度算 符无限和表示, 导出了激发相干态在振幅衰减模型中的演化规律。利用数值计算方法,描绘了Wigner函数的 演化曲线,以及 负部体积随耗散时间的演化曲线。通过对Wigner函数和负部体积随耗散时间的演化曲线分 析, 讨论了耗散和线性共振力对Wigener函数的影响。研究结果表明,随耗散时间的延长,激发 相干态的Wigener函数的负性减弱,负部体积减小;线性共振力作用,对Wigener函数的负性 没有影响。     

Analysis and filtering using the optimally smoothed Wignerdistribution

The authors consider the analysis and filtering of a deterministic signal with slowly time-varying spectra using the optimally smoothed Wigner distribution (OSWD). They compare this mixed time-frequency representation (MTFR) to other MTFRs such as the spectrogram, the short-time Fourier transform (STFT), and the Wigner and pseudo-Wigner distributions. The authors propose an approach to designing linear time-varying filters for slowly time-varying signals which is based on the concept of local nonstationarity cancellation and show that it is equivalent to masking the optimal STFT. The performance of the filter in suppressing white noise and in decomposing a slowly time-varying signal into its components is studied and compared to the performance of the techniques based on the STFT     

基于摄谱术的光信号门控可调时频分析模型

在光信号分析中运用时频分析方法,不但描述了信号频谱随时间的变化,而且显示了信号的能量密度.本文研究摄谱术并讨论魏格纳分布函数和光的时相关物理谱的联系.建立光信号门控可调时频分析模型,并利用LabVIEW进行仿真,分析了在不同门控时延条件下,不同速率的原始信号叠加噪声后的时频分布,对比了无噪声和时延的原始信号谱图,给出了不同条件下光信号较优的时解和频解.由仿真试验结果得出了基于摄谱术的门控可调时频分析模型是有效的光信号分析手段这一重要结论.     

Multiobjective neural network for image reconstruction

The authors propose a multiobjective neural network model and algorithm for image reconstruction from projections. This model combines the Hopfield model and multiobjective decision making approach. A weighted sum optimisation based neural network algorithm is developed. The dynamic process of the net is based on minimisation of a weighted sum energy function and Euler's iteration and this algorithm is applied to image reconstruction from computer-generated noisy projections and Siemens Somaton DR scanner data, respectively. Reconstructions based on this method are shown to be superior to those based on conventional iterative reconstruction algorithms such as MART (multiplicate algebraic reconstruction technique) and convolution from the point of view of accuracy of reconstruction. Computer simulation using the multiobjective method shows a significant improvement in image quality and convergence behaviour over conventional algorithms     

介观RLC电路的量子化及其在有限温度下的量子效应

利用正则化变换,给出了介观RLC电路体系的量子化方案.借助数值计算的方法,研究了体系热真空态的Wigner函数随温度变化的规律,同时借助量子算符及其Weyl-Wigner对应研究了体系中电荷及自感磁通量在热真空态下的量子效应.结果表明,低温下热真空态的Wigner函数为一稳定的波包,随温度升高,波包逐渐扩散;体系中电荷及自感磁通量在热真空态下的的量子涨落除与电路参数相关外,还与温度及时间密切相关.     

Coherence and localization in AC-driven quantum dots

We investigate the effects of an AC field on the dynamics of interacting electrons in quantum dots (QDs). We first consider a double QD containing two electrons. By mapping the system to an effective lattice model and employing Floquet theory we analyze the dynamics of the two-electron wavefunctions in the singlet space. This system presents a richer phenomenology than the case of a single electron in a double QD, displaying two distinct regimes of behaviour depending on whether the applied field strength or the inter-electron Coulomb repulsion is dominant. We then study the dynamics of two electrons in a 2D square QD in the limit of very low density, where the Coulomb interaction causes the electrons to become highly localized, forming a ‘Wigner molecule’. Using the same techniques we investigate how the AC field drives the dynamics of the Wigner states, and show how the parameters of the effective model may be measured in experiment.