空时块码与波束形成结合方案及性能分析

Alamouti空时块码与波束形成相结合的方案只使用了两个功率均分的波束,在很多情况下并非最优。该文提出了新的空时块码与波束形成相结合的方案,新方案能够根据信道互相关矩阵的特征值,展开不超过发射端天线数量的波束,在满足空时块码发射天线数量等于波束数量的条件下,与任意码率空时块码结合。该文还利用Gauss-Chebyshev积分对线性调制下新方案的误符号率进行了性能分析,得出了简单的数值计算表达式。新方案由于同时获得了分集增益和波束形成增益,显著提高了系统的性能。     

Calculation of Doppler spectral power density functions

A volume integral method for the calculation of Doppler ultrasound spectral power density (spd) functions is described. Axisymmetric flow in a circular tube with a power law velocity profile is assumed. The spd function is regarded as a probability density function for scatterer velocity, and the assumptions under which this is justified are considered. It is shown that the spd function is independent of Doppler angle except in the presence of wall reflection effects. A coordinate system centered on the beam is used and this enables the integrals to be easily formulated for arbitrary beams. Irregularly shaped and nonuniform beams can be treated. For the common flow and beam patterns, which exhibit symmetry, the volume integrals can often be reduced to a single integral and evaluated directly. The method is applied and the spectra are calculated for various different cases. Results are obtained for uniform rectangular and circular insonating beams, and for nonuniform beams with Gaussian, jinc, and sinc profiles. The effects of narrow beams and wall reflection are shown. The method may be readily applied to other beam and flow patterns, and extension to more complicated situations is also discussed.     

Gabor's expansion of an aperture field in exponential elementarybeams

Gabor's expansion of an aperture field in a discrete two-dimensional superposition of Gaussian elementary beams is extended to incorporate exponential beams. The resultant expansion scheme has an advantage over the commonly used Gaussian beam representation in situations involving an exponential aperture distribution such as the modal field in a thin slab laser amplifier     

Design and Analysis of Broadband Beamspace Adaptive Arrays

A beamspace adaptive planar array for broadband beamforming is proposed based on the frequency invariant beamforming technique. A detailed analysis is provided for both the linear arrays and the planar arrays, where two conditions are imposed: the beams should exhibit a good frequency invariant property and be linearly independent. These two conditions are not independent and there is a tradeoff between them. To improve the interference cancellation capability of the array, we may need to sacrifice the frequency invariant property to some degree for more linearly independent beams. A sufficient condition for the linear independence of the beams is derived with a DFT matrix based design proposed. Simulation results verified our analysis and the proposed method.     

Pattern translation and rotation in uncorrelated sourcedistributions for multiple beam antenna design

In the uncorrelated scenario model, a continuous source distribution illuminates a receiver with waves bearing signals that are uncorrelated with respect to their angle of arrival. This model is used for multipath situations such as scintillating atmospheres, but also can be applied to optical beams for indoor communications, acoustic beamforming, and, of particular interest here, mobile communications. For diversity antennas operating in such a scenario, patterns that produce uncorrelated signals are required. An angular separation of directive beams in multipath scenarios acts to decorrelate the received signals. It is of interest to quantify the minimum angular spacing required of beams in order to provide a framework for the design and configuration of the antennas. The approach is to consider only the main lobe of the antenna pattern and to take it as a real function. This is reasonable as long as most of the energy is conveyed via the main beam. In practice, the sidelobe structure and nonuniformity in the phase of an actual pattern act to improve the situation in the sense that the decorrelation angles become smaller. The conditions for angular diversity result in a simple rule-of-thumb for the minimum beam separation requirement, which is essentially independent of the directivity. Finally, both scalar and vector elliptic beams rotated about their boresight axes are analyzed for the decorrelation rotation angle as a function of the ratio of the ellipticity. The resulting design curves offer a guideline to beam configuration for multipath scenarios     

Covariance of narrow-band noise in multiple beams of a phased-array system

Covariance of narrow-band noise among two simultaneous beams, looking toward different directions with identical power patterns, for a phased array system is formally analyzed. It is shown that the noise correlation coefficient as a function of the angle of separation between two beams is the same as the normalized beam power pattern. This result is independent of the way the beams are formed, in particular these conclusions for both the total power beam systems and the correlator beam systems are examined in detail.     

厄米-高斯光束通过光阑-透镜系统的聚焦特性

使用Collins公式研究了厄米高斯光束通过光阑透镜分离光学系统的聚焦特性。利用数值计算详细分析和说明了偶数模厄米高斯光束的轴上光强分布和焦移。分析得出,选择恰当的光束和光学系统参数可引起焦移相反或无焦移。特别是对于TEM22模厄米高斯光束,在几何焦面两边存在两个相等的轴上光强极大点,故存在两个符号相反的焦移。     

Limiting current of helical electron beams in gyrotrons

Potential depression (PD) and limiting current (LC) of helical electron beams (HEB) are investigated. Nonneutral electron beams and two velocity distributions: uniform and Rayleigh distribution are taken into account. For Rayleigh distribution electron reflection from magnetic mirror is taken into account. It is shown that velocity spread may lead to substantial reduction of LC for real values of the mean pitch-factors of helical trajectories. PD and LC are an unmonotonous functions of the velocity spread for Rayleigh distribution. A connection of PD with the change of gyrofrequency and with the beam broadening in gyrotron operating space is retraced.     

具有双应变梁的硅微机械倾角传感器结构设计

提出了一种能抗侧向耦合干扰的微机械硅倾角传感器的敏感结构设计 ,它由两个不同大小的质量块和连接它们的两个相互平行的硅应变梁组成。文中通过推导给出了硅应变梁上所受的应变与倾角及位置的解析关系 ,并计算了在两个不同质量块作用下硅梁上的应力分布 ,最后提出了制作该结构的工艺设计     

指数响应的强非局域介质中正交偏振双厄米高斯光束传输特性

运用变分法研究了具有e指数响应的强非局域介质中两束正交偏振、中心重合的厄米高斯光束的传输特性,得到了光束各参量的演化方程和一个临界功率Pc。当两光束以临界功率入射时,可以形成厄米高斯型空间光孤子;当两光束以总功率2Pc入射,但两光束入射功率不相等时,两光束可以形成厄米高斯型呼吸子。通过数值模拟研究发现,低阶光束时,可以得到近似孤子解;当光束阶数大于3阶时,则得不到孤子解。比较变分解与数值解,结果表明在光束阶数小于3阶时,变分解较好的反映了具有e指数响应的强非局域介质中两束正交偏振、中心重合的厄米高斯光束的传输特性。     

非线性自聚焦和发散锥型梯度折射率棒中的两正交偏振光束

研究两正交偏振高斯光束经克尔非线性自聚焦和发散锥型梯度折射率棒的传播特性,求解了两光束无量纲束宽参量的耦合微分方程,给出了光束在棒中的两种传播方式。     

Electron-Beam Two-Stream Instability in Quantum-Effect Structures

A dispersion relation for a one-dimensional multistream system of electron beams in a quantum-effect structure is derived. For two identical counterstreaming electron beams, the roots of the dispersion relation are found. In the quantum case, two-stream instability is shown to have no threshold, due to a high degree of the dispersion relation.     

低轨星座的跳波束资源调度策略

跳波束技术作为一种高通量卫星资源分配方法,能够根据地面业务需求灵活配置星上资源,提高星上资源利用率。本文将跳波束技术应用到低轨星座场景中,针对用户业务先验未知和先验已知两种情况,考虑波束间的共信道干扰,基于迭代算法和凸优化制定了适用于低轨星座的两种跳波束资源调度策略。与传统策略对比,能达到更高的系统吞吐量,同时具有较好的时延性能,且适应用户业务的不均分布。     

激光束的分类

对激光光学中一个十分有意义的论题,即激光束的分类作了综述和分析。基于二阶矩方法和维格纳分布函数,激光束可以按照其空间对称性分为无像散光束、简单像散光束和复杂像散光束。另一方面按空间相干性激光束可分为部分空间相干光和完全空间相干光。对各类激光束的独立特征参数及其联系作了分析。     

A Double Wavelength Laser Doppler System to Investigate Skin Microcirculation

A new experimental laser Doppler setup has been designed to discriminate between total and superficial skin blood flow. This selectivity is based on the use of two wavelengths with different penetration depths into the skin. An argon ion and helium-neon laser are mounted on the same optical bench and are stabilized by an optical feedback loop. A single optical fiber directs the beams to the skin and collects the reflected light back to a photodetector, the signal of which is sampled and Fourier transformed to give a frequency power spectrum. Several models of light scattering by the skin are examined, and a single Lorentzian function is found to be the best fit for our experimental power spectra. Flow parameters have been thus measured for several in vitro and in vivo situations. In vitro calibration indicates that there is a constant ratio between the frequency responses at both wavelengths used. The decrease of this ratio encountered in in vivo measurements is attributed to different depths of investigated skin microcirculation according to the incoming wavelengths.     

非线性发散抛物型梯度折射率棒中的两正交偏振光束

研究非线性发散抛物型梯度折射率棒中两正交偏振高斯光束的传播特性，导出了棒内两光束的束宽半径和波前曲率半径的倒数。     

Numerical calculations of two-beam energy coupling gains inphotorefractive BaTiO3 crystal

Steady-state two-beam energy coupling gains are numerically obtained in photorefractive BaTiO₃ crystal. The energy coupling gains are calculated by changing the angle subtended by two incident beams and by rotating the crystal relative to the grating wave vector. Polarizations of the two incident beams are assumed to be (ordinary, ordinary), (extraordinary, extraordinary), (ordinary, extraordinary), or (extraordinary, ordinary). The wavelength of the beams is assumed to be 515 nm. The maximum coupling gain is found to be 4.8/cm when both beams are ordinary and 74.7/cm when the beams are extraordinary. It is also shown that energy can transfer between ordinary (extraordinary) and extraordinary (ordinary) beams. In this case the maximum energy coupling gain is found to be 9.2/cm     

光折变晶体中非相干光前向耦合及其对噪音栅擦除的研究

对光折变晶体 Ce:KNSBN中两束 e偏振简并非相干光的前向耦合进行了理论分析和实验研究 ,并通过非相干光耦合作用在实验上获得了亮环和暗环的衍射图样 ,从而获得了一种利用非相干光擦除来克服光折变晶体中扇形效应的简便方法。     

双光栅成像中的汇合级光能量

从光传播基本理论和光栅衍射性质出发,研究了双光栅成像效应中的光能量传递,得到了确定汇合级光强的表达式.结合双光栅成像方程,分析了两光栅的衍射性质与成像位置对汇合级光能量的影响.分析表明汇合级光强与两光栅具体的衍射效率和双光栅成像时的位置密切相关.测量了三组不同类型双光栅成像系统的汇合级光能量,实验结果和理论分析基本一致.     

A statistical description of stimulated Brillouin scattering beamcombination efficiency

The mutual coherence between two stimulated Brillouin signals produced by undepleted pump beams that are partially overlapped is studied. The mutual coherence of the stimulated-Brillouin-scattered beams determines the efficiency with which the two beams can be combined. The statistical properties of the mutual coherence are calculated. The mechanism for coupling of two stimulated-Brillouin-scattering outputs is four-wave mixing. Mutual coherence is found to depend on the pump beam's pulse duration, the medium's phonon lifetime, and the degree of overlap between the laser pump beams