空地通信信道的统计特性研究

空地通信信道的主要统计特性包括多径衰落特性、多普勒功率谱和时延功率谱。将空地通信信道视为“双线模型”,推导了用信号平均接收功率和莱斯因子表示的接收信号包络的概率密度函数;假设接收端多径分量的到达角服从某范围内高斯分布,给出了接收信号的多普勒功率谱表达式及其示意图;考虑地球曲率对空地通信的影响,建立了信道的时延模型,并给出了接收信号的时延功率谱。     

天线主瓣照射区多普勒谱分布规律研究

为了研究雷达高度表相对于地面运动时天线主瓣照射区内多普勒谱分布规律,建立了大地回波宽多普勒带模型,利用基于等多普勒带、等距离环的网格映像法将波束照射区划分为若干矩形散射网格,采用等多普勒带内各矩形网格回波功率叠加的方法对回波多普勒功率谱进行仿真,克服了基于极坐标积分法计算繁琐且不利于后续工程实现的缺点,仿真结果与理论值相符,表明模型合理,最后分析得出了雷达高度表在不同高度及速度下多普勒谱分布规律,为后续雷达高度表的回波模拟提供了理论支撑.     

宽带双基地MIMO雷达的参数联合估计的新方法

提出了一种基于分数阶功率谱的宽带双基地MIMO雷达中参数联合估计的新方法。在许多情况下,根据窄带信号模型对宽带回波信号中的参数进行估计是不合适的,因此提出一个新的宽带回波信号模型对运动目标参数进行估计。根据分数阶功率谱(FPSD)的峰值点对多普勒频移因子和时延参数进行联合估计。并依据分数阶功率谱的峰值点构造2个子阵,提出的FPSD-MUSIC算法和FPSD-ESPRIT算法实现了对收发角的联合估计。接下来推导了该信号模型中参数估计的克拉美罗界。仿真实验验证了算法的有效性。     

时变多普勒衰落信道硬件模拟的FPGA实现

针对不同散射环境下衰落信道具有不同的多普勒功率谱形状,提出了一种改进的复谐波叠加模型用于时变多普勒衰落信道的模拟,仿真分析了该方法输出信道衰落的幅值及相位连续性,并据此设计了基于FPGA硬件平台的时变多普勒衰落信道模拟器。硬件实测结果表明,该模拟器输出的时变多普勒功率谱与理论仿真非常吻合,可用于实际中多普勒功率谱实时变化场景的模拟。     

改进的一维时变海面模型及其分数阶功率谱研究

该文针对经典双尺度海面模型不能有效分析高海况和低掠射角条件下的非Bragg散射问题,在模型中引入调频率描述多普勒频率的变化,提出了一种更符合工程实际的1维时变海面散射模型。然后,通过计算改进模型的海面平均散射功率,得到了时变海面的角度散射特性。其次,研究了改进模型的分数阶功率谱(FPS)特性,得出在分数阶Fourier变换(FRFT)域,海面散射信号的功率谱由多分量冲激信号组成,其在FRFT域的位置可用于估计模型的频率参数。最后,采用X波段实测海面回波数据对改进模型进行验证,并讨论了入射波长和变换角对FPS的影响,仿真结果表明该模型适合分析和提取时变海面回波的频率变化及多普勒频移。     

受限多普勒功率谱建模方法研究

为了准确反映复杂环境下移动通信节点间的无线信道传播特性,本文研究了具有受限多普勒功率谱的信道仿真模型,讨论了三种建模方法:单路滤波法、双路滤波法、改进拼接法的原理和仿真实现方法,对比了典型模型生成信道随机过程的自相关函数、多普勒谱的算法复杂度和内存占用度。研究和仿真结果表明:在信道具有受限多普勒功率谱时,改进拼接法的自相关函数的均方根误差明显另外两种方法,同时还能避免设计数字滤波器和希尔伯特变换器,但是改进拼接法的算法复杂度远远大于其他两种模型,复杂度最小的是单路滤波法,所以在实际仿真中需要看情况取舍。论文所做比较有利于受限多普勒谱建模工作的模型选取,并为更加复杂的多普勒功率谱建模提供参考。      

无线瑞利衰落信道的自回归模型研究

在不同多普勒谱下分析自回归(AR)模型参数与系统极点位置、幅度响应关系,采用极点位置累试法对模型参数进行修正,改善了仿真功率谱,在通带衰落方面不满足理论要求的缺陷,提高了模型对瑞利衰落信道功率谱的仿真精度。仿真结果表明,通过累试法调整系统极点分布,可以提高仿真结果与理论值的匹配度,方法简单且精度较高,降低了仿真分析的难度。     

码宽对水声信号功率谱的影响分析

通信信号的功率谱分布情况影响着通信系统的传输效率,码宽对功率谱分布的影响较大。针对四相相移键控、二进制频移键控等典型水声通信信号,采用现代功率谱估计方法中基于参数模型的Burg算法,对其功率谱分布进行了估计。重点研究了不同码宽条件下,Burg算法估计信号功率谱的基本特征和分布规律,对码宽对功率谱的影响进行了归纳总结,为扩频通信和典型水声通信信号的设计提供了技术支持。     

真实场景下机载脉冲压缩雷达地杂波模拟

真实场景下的地杂波模拟是雷达系统射频仿真的重要内容,文章假设机载脉冲压缩雷达的地面背景为平坦的中等干度裸地,深入研究了真实场景下宽带雷达地杂波的实时性重构算法。文章第一部分利用小微扰法和Weibull分布模型研究了背景的后向散射特性;第二部分利用子带分割原理计算了归一化的宽带地杂波功率谱;第三部分首先在不考虑多普勒效应的情况下通过推导证明无多普勒频移的宽带地杂波功率谱等于归一化的宽带地杂波功率谱与发射信号功率谱的乘积;第四部分提出了一种包括多普勒调制算法在内的基于杂波频域特征的宽带地杂波模拟算法,并且给出了仿真结果,证明了算法的有效性。     

多普勒频率检测与噪声干扰信号

对ＰＤ雷达采用ＰＤ噪声干扰好。噪声干扰要覆盖一定数量的多普勒滤波器，传统的噪声调制已调信号的功率谱密度按正态规律分布，这对远离中心的多普勒滤波器干扰不利，采用“马鞍”型或均匀型功率谱分布要好。通过与ＰＤ雷达进行的对抗试验得到了证明。     

Spectral properties of signal fading and Doppler spectra distribution in urban mobile communication links

In this work, we continue the analysis of a probabilistic approach and the corresponding stochastic multi‐parametric model of wave propagation, in built‐up areas with randomly distributed buildings. We have concentrated on the spectral properties of signal strength spatial variations and on Doppler spread spectrum distribution of signal power. The analysis is based on a unified stochastic approach of radio wave propagation above the built‐up terrain with applications to mobile communications. We analyze the signal power spectrum of spatial frequencies and the signal power distribution in the Doppler domain for moving vehicles, taking into account a Doppler shift proportional to the vehicle antenna speed relative to the base station. The comparison between the theoretical prediction and experimental data was motivated by the proposed stochastic model and other existing statistical models to verify the signal power distribution in the Doppler domain for various urban environments and terminal heights with respect to building rooftops. New effects of terrain features on signal spectrum are obtained, examined and compared with existing models. Copyright © 2006 John Wiley & Sons, Ltd.     

Analysis of pulsed wave Doppler ultrasound spectra obtained from amodel intracoronary catheter

Abnormal arterial blood flow patterns have been implicated in the evolution of various vascular disease processes. Intravascular ultrasound techniques using the pulsed wave Doppler catheter offer the opportunity to characterize these abnormal flow patterns. The authors have developed a mathematical model that predicts the first two moments of the Doppler spectrum obtained using a Doppler catheter based on the distribution of ultrasonic beam power and velocity profile of fluid flow with an arbitrary distribution of flow disturbances. A scaled-up, in vitro experimental arterial system was used to confirm the validity of the model. Comparison of the predicted first two moments of the Doppler spectrum to the experimental values in this system demonstrated that the distribution of beam power significantly affects the magnitude of the first two moments. Additionally, both velocity gradient and velocity fluctuation broadening effects play prominent roles in determining the magnitude of the second moment. These phenomena must therefore be considered when evaluating in vivo Doppler spectra used for the characterization of abnormal flow patterns     

三维移动-移动场景非平稳MIMO信道空时相关性

提出一种三维移动-移动场景的非平稳多输入多输出(multi-input multi-output, MIMO)信道模型,该模型改进了不同信道状态之间衰落相位的连续性,可保证输出多普勒频移的正确性。同时,针对该新模型研究了非平稳衰落特性引起的时变空时相关性,并针对收发端信号角度谱服从通用Von Mises Fisher(VMF)分布情况,详细推导了归一化空域互相关函数和时域自相关函数的表达式。最后,针对低速和高速移动两种场景的仿真结果表明,获得的理论结果与数值仿真具有很好的拟合度, 但能极大地减低计算复杂性,进而有效提高了分析和仿真复杂动态场景下MIMO 通信系统的效率。     

Statistical cellular Gaussian scatter density channel model employing a directional antenna for mobile environments

The statistical Gaussian scatter density model (GSDM) is used to evaluate the effects on the Doppler power density spectrum (PDS) when a directional antenna at base station (BS) is employed. We derive the angle of arrival (AOA) probability density function (pdf ) of the multipath components at the mobile station (MS). The AOA pdf at the MS and the PDS, are compared with those for the Clarke's model and the macrocell circular model. We demonstrate that the circular model overestimates the probability of scatterers with AOA near to the line of sight (LoS), resulting in a poorer estimation of the PDS unlike the Gaussian model.     

地空衰落信道建模与仿真

在对地空信道角度统计特性分析的基础上,得到非直视分量通用多普勒功率谱和天线空间相关系数数学表达式。将地空信道小尺度衰落和大尺度衰落结合起来,用Nakagami衰落来建模地空信道小尺度衰落。将Zheng模型仿真法和MSEM仿真法分别用来模拟地空小尺度衰落和大尺度衰落模型。仿真结果表明,信号经过信道后幅度和频率都将发生显著的变化。     

Wideband HAP-MIMO Channels: A 3-D Modeling and Simulation Approach

High-altitude platforms (HAPs) are considered as an alternative technology to provide future generation broadband wireless communications services. This paper proposes a three-dimensional (3-D) geometry-based reference model for wideband HAP multiple-input–multiple-output (MIMO) channels. The statistical properties of the channel are analytically studied in terms of the elevation angle of the platform, the antenna arrays configuration, and the angular, the Doppler and the delay spread. Specifically, the space-time-frequency correlation function (STFCF), the space-Doppler power spectrum, and the power space-delay spectrum are derived for a 3-D non-isotropic scattering environment. Finally, a sum-of-sinusoids statistical simulation model for wideband HAP-MIMO channels is proposed. The results show that the simulation model accurately and efficiently reproduces the STFCF of the reference model. The proposed models provide a convenient framework for the characterization, analysis, test, and design of wideband HAP-MIMO communications systems with line-of-sight and non-line-of-sight links.     

Parameter Estimation Based on Fractional Power Spectrum Density in Bistatic MIMO Radar System Under Impulsive Noise Environment

This paper takes an Alpha-stable distribution as the noise model to solve the parameter estimation problem of bistatic multiple-input multiple-output (MIMO) radar system in the impulsive noise environment. For a moving target, its echo often contains a time-varying Doppler frequency. Furthermore, the echo signal may be corrupted by a non-Gaussian noise. It causes the conventional algorithms and signal models degenerating severely in this case. Thus, this paper proposes a new signal model and a novel method for parameter estimation in bistatic MIMO radar system in the impulsive noise environment. It combines the fractional lower-order statistics (FLOS) and fractional power spectrum density (FPSD), for suppressing the impulse noise and estimating parameters of the target in fractional Fourier transform domain. Firstly, a new signal array model is constructed based on the \(\alpha \)-stable distribution model. Secondly, Doppler parameters are jointly estimated by peak searching of the FLOS–FPSD. Furthermore, two modified algorithms are proposed for the estimation of direction-of-departure and direction-of-arrival (DOA), including the fractional power spectrum density based on MUSIC algorithm (FLOS–FPSD–MUSIC) and the fractional lower-order ambiguity function based on ESPRIT algorithm (FLOS–FPSD–ESPRIT). Simulation results are presented to verity the effectiveness of the proposed method.     

Parameter estimation based on fractional power spectrum under alpha-stable distribution noise environment in wideband bistatic MIMO radar system

This paper takes the alpha-stable distribution as the noise model and works on the parameter estimation problem of wideband bistatic Multiple-Input Multiple-Output (MIMO) radar system in the impulsive noise environment. In many applications, it is not appropriate to approximate the wideband signal by the narrowband model. Furthermore, the echo signal may be corrupted by the non-Gaussian noise. The conventional algorithms degenerate severely in the impulsive noise environment. Thus, this paper proposes a new wideband signal model and a novel method in wideband bistatic MIMO radar system. It combines the fractional lower order statistics and fractional power spectrum, for suppressing the impulse noise and estimating parameters of the target. Firstly, a new signal array model is proposed under the alpha-stable distribution noise model. Secondly, Doppler stretch and time delay are jointly estimated by peak searching of the FLOS-FPSD. Furthermore, two modified algorithms are proposed for the estimation of the direction-of-departure and direction-of-arrival, including the fractional power spectrum density based on MUSIC algorithm (FLOS-FPSD-MUSIC) and the fractional lower-order ambiguity function based on ESPRIT algorithm (FLOS-FPSD-ESPRIT). Simulation results are presented to verity the effectiveness of the proposed method.