Is space-time symmetry a suitable generalization of parity-time symmetry?

We discuss space-time symmetric Hamiltonian operators of the form H=H₀+igH^′$H=H_0+ig{H}^{\prime }$, where H₀$H_0$ is Hermitian and g$g$ real. H₀$H_0$ is invariant under the unitary operations of a point group G$G$ while H^′$H^{\prime }$ is invariant under transformation by elements of a subgroup G^′$G^{\prime }$ of G$G$. If G$G$ exhibits irreducible representations of dimension greater than unity, then it is possible that H$H$ has complex eigenvalues for sufficiently small nonzero values of g$g$. In the particular case that H$H$ is parity-time symmetric then it appears to exhibit real eigenvalues for all 0<g<g_c$0<g<g_c$, where g_c$g_c$ is the exceptional point closest to the origin. Point-group symmetry and perturbation theory enable one to predict whether H$H$ may exhibit real or complex eigenvalues for g>0$g>0$. We illustrate the main theoretical results and conclusions of this paper by means of two- and three-dimensional Hamiltonians exhibiting a variety of different point-group symmetries.     

An approach to suppress short-range clutter for non-side looking airborne radar

When the Airborne Early Warning (AEW) radar transmits medial or high Pulse Repetitive Frequency (PRF) signal, the range ambiguity occurs. The clutter of short-range clutter has serious range dependence problem for non-Side Looking Airborne Radar (non-SLAR). As a result, the clutter plus noise covariance matrix can not be estimated correctly, and the performance of clutter suppression obtained by Space-Time Adaptive Processing (STAP) degrades greatly. The uniform linear array has not elevation degrees; therefore, the short-range clutter can not be suppressed directly. A short-range clutter suppression method is proposed. The method first estimate the elevation angles of the am-biguous short-range gate, then eliminates short-range clutter by space time interpolation and adds moving target protection in the procedure. This method can suppress the short-range clutter well. Simulation results show the validity of the method.     

Theoretical analysis on the energy density of a laser field measured along an inertial world line of the space-time

Some mathematical relationships, in both the time and frequency domains, are presented in relation to the energy density and dipole-moment matrix element associated with a linearly polarized laser field evaluated on an inertial world line of the Minkowski space-time. As final result, we arrive at a formula for the minimum value of the time interval along which the field is evaluated.     

地球范围内相对论效应下的时间转移与同步

对于星载时间保持系统来说,相对论效应对时钟测量精度的影响不可忽视。该文运用相对论理论研究地球范围内大尺度时空转移引起的时钟测量误差问题。分析计算了星载时钟测量相对论效应误差组成和量级,验证了频率调整法和实时补偿法消除相对论误差的有效性。研究了普遍类型的地球轨道卫星、航空器以及月球探测器与地面时钟的转移与同步,提出在星载时钟频率调整分辨率不足时,可采取定步长时钟频率调整、带误差限的时钟频率调整或"跳秒"的形式控制时钟测量相对论效应长期误差,提出在无法实时获得航天器位置和速度时,可以根据互相关法预估周期误差。仿真验证了这些方法能有效地减小误差,为未来深空探测自主导航过程中时钟与地面时间的同步提供了方案。     

自适应机载雷达空-时处理性能的改善

在机载预警雷达中，对低速目标的检测是很重要的，却又是非常困难的．本文提出了一种时域预处理的时-空二维自适应处理新方法，与先前提出的TSA滤波法[1]相比，新方法稍增加一些运算量，但在低速日年检测性能上有较大的改善。     

多天线系统及其在OFDM系统中的应用

利用配置多个天线实现高速可靠的通信是当今无线通信领域研究的热点之一。本文介绍了具有多个天线系统的几种不同构成方式，从系统误码率和频带利用率两个方面讨论了不同构成方式对系统的改善。同时，对多天线技术应用于正交频分复用(OFDM)系统的性能进行了分析，并给出了频率选择性瑞利衰落信道中最优译码算法下的系统性能比较。仿真结果表明，多天线OFDM系统既保持了OFDM抗频率选择性衰落的特点，又能够改善系统的误码率性能，提高频带利用率。     

Properties of spacetimes with magnetic dipoles

The properties of an asymptotically flat spacetime with a mass and a magnetic dipole are studied. We focus on the breaking of spherical symmetry by the magnetic dipole. We investigate the geometry of the spacetime through the trajectories of null geodesics and the embedding diagrams of various hypersurfaces, included constant t and r surfaces and the infinite red shift surface. We find that the effects of the magnetic dipole on the spacetime are larger at the poles than on the equator.     

Parameter Estimates and Exact Variations for Stochastic Heat Equations Driven by Space-Time White Noise

Abstract

In this article we calculate the exact quadratic variation in space and quartic variation in time for the solutions to a one dimensional stochastic heat equation driven by a multiplicative space-time white noise. We use the knowledge of exact variations to estimate the drift parameter appearing in the equation.     

Alamouti空时分组码的MATLAB仿真与FPGA实现

在简单介绍Alamouti空时分组码的基础上,推导了Alamouti码的理论误码率并进行MATLAB仿真,仿真结果表明Alamouti码可以为系统提供良好的分集增益。最后利用FPGA器件并通过Verilog语言描述实现了Alamouti码的编码过程,整个过程在ALTERA公司的QUARTUSⅡ软件结合第三方工具MODELSIM上进行编译仿真,测试结果表明编码器的设计在速度和资源利用上都达到了很好的状态,满足设计要求。     

Spin 2 Field Equation in Expanding Universe

The spin 2 field equations are separated in the Robertson-Walker space-time by the Newman-Penrose formalism and by using a null tetrad frame previously considered. The angular and radial separated equations are integrated by generalizing and improving results relative to the massless case. The separated time equations are governed by two coupled linear differential equations that depend on the cosmological background. They are solved and studied for some models of cosmological expansion such as the linear and exponential expansion and the matter dominated and radiative expansion of the standard cosmology.