采用副载波参考波形方法的GNSS双载波环多径抑制技术

为了实现对高阶二进制偏移载波(BOC)信号的无模糊和抗多径接收,将码相关参考波形的闸波设计思路应用于GNSS双载波环路接收方法的副载波锁相环。在副载波锁相环中引入设计的闸波参与信号的相干积分过程,使双载波环法具备抗多径性能且不需要额外引入相关器。对该设计方法的理论和具体实现进行阐述和分析,从副载波多径误差包络和跟踪精度两方面对改进的双载波环路方法性能进行评估。仿真结果显示,采用的算法与双载波环路法相比,可以降低BOC(1,1)信号81.1%的副载波多径误差包络面积以及BOC(14,2)信号75.1%的副载波多径误差包络面积。但是,改进的双载波环路法会带来-6 dB的相干积分后载噪比损失,降低跟踪精度。因此,在闸波参数设计上,需要谨慎选择以平衡算法的多径抑制和跟踪精度性能。综合来看,该方法适用于解决非弱信号条件下及多径环境下的高阶BOC信号接收问题。     

OFDM抗多径机理分析与系统仿真

抗多径衰落是正交频分复用(OFDM)系统的显著特点之一。具体分析了OFDM抗多径的机理,说明了两种不同情况下多径对信号频谱的影响,并提出了相应的减轻多径影响的方法。通过仿真分析验证了HiperLANType2标准规定的OFDM系统的抗多径性能,并提出了一些改善系统性能的方法。     

Humanitarian response to hurricane disasters: Coordinating flood‐risk mitigation with fundraising and relief operations

To alleviate flooding, caused by hurricanes, governments build structural barriers called levees. In addition, relief providers such as the nongovernmental organizations and charities raise funds, and procure and deliver supplies (food, water, and medicines) for humanitarian relief. The strategy for managing disasters must, therefore, weigh the costs and benefits of building levees as well as procuring and delivering supplies. We use a three‐stage decision making framework to study how the investment in levee capacity can be integrated with supply procurements, fundraising, and rapid response. One of our major findings is that a large fundraising cost discourages postdisaster funding, implying relatively large investments in levee and prepositioned supplies. That notwithstanding, a large social value (of saving life) can tilt the balance in favor of postdisaster funding. If the levee capacity increases, funding for predisaster procurement is reduced without affecting postdisaster funding. For a sufficiently large increase in capacity, however, postdisaster response becomes superfluous. We also find that hurricane uncertainty motivates levees with large capacity. In contrast, levee‐failures motivate levees with small capacity.     

级联结构的阵列天线导航接收机抗干扰及多径抑制方法

阵列信号处理技术因其能够提供空域分辨能力已被广泛应用于卫星导航接收机领域以实现抗干扰和多径抑制。根据干扰和多径信号对导航接收机基带处理影响的不同提出了一种以数字相关器为界线划分的抗干扰与多径抑制两级处理结构:第一级处理在解扩前估计阵列接收数据的空时协方差矩阵,根据干扰信号功率远大于导航信号及噪声的特点利用子空间投影技术实现抗干扰;第二级处理在解扩后进行空间平滑解相干处理,利用基于Householder变换的广义旁瓣相消技术进行波束形成以实现多径抑制。理论分析和仿真结果表明,该级联处理技术能够有效地压制强干扰,并显著减小多径信号对导航接收机伪码测量的影响。     

多径条件下的空时解相关接收机研究

通过建立了一般的空时CDMA系统模型 ,探讨多址干扰和多径效应的联合抑制问题。首先分析了系统的可辨识条件 ,然后利用多约束最优化准则研究了FIR空时解相关接收机设计和实现 ,并进一步分析了系统性能     

GNSS信号功率增强对调零抗干扰接收机的性能影响

由于卫星端信号发射功率小、信号传播损耗大,到达接收机的卫星导航信号极其微弱,容易被干扰。为有效应对电磁干扰的威胁,在系统层面和用户层面同时采取抗干扰措施将是一种很好的选择。建立调零抗干扰接收机抗干扰能力分析模型,并定量分析信号功率增强对调零抗干扰接收机抗干扰性能的提升效果。结果表明：信号功率每增强10 dB,接收机抗干扰能力可提升3～4 dB,但从提高接收机非极限条件下的接收性能(载噪比、测距定位精度等)的角度,信号功率增强15～20 dB时,接收性能最优。研究成果可为卫星功率增强量的优化设计以及终端抗干扰设计提供参考。     

非平稳噪声背景下DOA分步估计方法研究

针对相干和非相干信源同时存在的情况,结合斜投影理论和互相关矢量Toeplitz矩阵重构(CVTR)的方法,提出一种新的信源DOA分步估计方法。该方法把相干信源和非相干信源分开分辨,对相干信源用CVTR方法来恢复为满秩,避免了常规平滑算法阵列孔径损失大、运算量大的缺点;用斜投影算子而非差分方法对信源进行分离,不受相关矩阵须为Toeplitz结构的限制,使得算法可适用于任意阵列结构形式。这种分步分辨思路可有效增强信源过载能力,同时在互相关矢量Toeplitz矩阵重构过程中,可把非平稳噪声协方差矩阵转换成白噪声结构,使得算法对非平稳噪声有较好的适应能力。     

Relative performance of novel blast wave mitigation system to conventional system based on mitigation percent criteria

Recent researches focused on developing robust blast load mitigation systems due to the threats of terrorist attacks. One of the main embraced strategies is the structural systems that use mitigation techniques. They are developed from a combination of structural elements and described herein as conventional systems. Among the promising techniques is that redirect the waves propagation through hollow tubes. The blast wave propagation through tubes provides an efficient system since it combines many blast wave phenomena, such as reflection, diffraction, and interaction. In this research, a novel blast load mitigation system, employed as a protection fence, is developed using a technique similar to the technique of the bent tube in manipulating the shock-wave. The relative performance of the novel system to the conventional system is evaluated based on mitigation percent criteria. Performances of both systems are calculated through numerical simulation. The proposed novel system proved to satisfy high performance in mitigating the generated blast waves from charges weight up to 500 kg TNT at relatively small standoff distances (5 m and 8 m). It mitigates at least 94％ of the blast waves, which means that only 6％ of that blast impulse is considered as the applied load on the targeted structure.     

Mitigation effects on the reflected overpressure of blast shock with water surrounding an explosive in a confined space

The mitigation of blast shock with water has broad application prospects. Understanding the mitigation effects on the reflected overpressure of the explosion shock with water surrounding an explosive in a confined space is of great significance for military explosives safety applications. To estimate the effects of the parameters on the reflected overpressure of blasted shock wave, a series of experiments were carried out in confined containers with spherical explosives immersed in a certain thickness of water, and numerical simulations were conducted to explore the corresponding mechanisms. The results reveal that the reflected overpressure is abnormally aggravated at a small scaled distance. This aggravation is due to the high impulse of the bulk accelerated water shell converted from the explosion. With increasing scaled distance, the energy will be gradually dissipated. The mitigation effects will appear with the dispersed water phase front impacting at a larger scaled distance, except in the case of a dense water phase state. A critical scaled distance range of 0.7—0.8 m/kg1/3 for effective mitigation was found. It is suggested that the scaled distance of space walls should be larger than the critical value for a certain water-to-explosive weight ratio range (5—20).     

多径条件下米波OFDM-MIMO雷达波束性能及特性分析

针对米波正交频分多路复用-多输入多输出(Orthogonal Frequency Division Multiplexing-Multiple Input Multiple Output, OFDM-MIMO)雷达波束形成问题,建立了米波条件下频分复用MIMO雷达阵列信号模型,理论导出了该体制下多径特性定量分析的理论框架、直达波和多径回波模型及其联合发射-接收双程方向图增益闭合表达式。提出了利用米波OFDM-MIMO雷达阵列双程方向图具有距离-角度二维分辨能力抑制多径算法。所提方法能够有效抑制阵列法线方向多径回波,同时有效抑制输出信干噪比周期性损失,且随频偏带宽的增大抑制能力进一步提升。