雷达目标的散射中心建模研究

雷达目标总散射场可近似为若干散射中心贡献的叠加,而每一个散射中心都对应着特定的电磁散射机理,这些特定散射机理的散射场可表示成输入为雷达参数（频率、极化、视向角）和目标参数（几何、材料）的参数化模型.散射中心参数化模型在雷达回波实时模拟、半实物仿真、雷达目标识别等领域得到了广泛的关注.文中总结了雷达散射中心模型的发展历程,包括散射中心数学模型和散射中心参数提取方法,以及散射中心建模的难点问题和未来的研究方向.期望该文可为从事该方面研究的科研人员提供一些参考.     

Analysis and optimization of pilot symbol-assisted Rake receivers for DS-CDMA systems

The effect of imperfect channel estimation (CE) on the performance of pilot-symbol-assisted modulation (PSAM) and MRC Rake reception over time- or frequency-selective fading channels with either a uniform power delay profile (UPDP) or a nonuniform power delay profile (NPDP) is investigated. For time-selective channels, a Wiener filter or linear minimum mean square error (LMMSE) filter for CE is considered, and a closed-form asymptotic expression for the mean square error (MSE) when the number of pilots used for CE approaches infinity is derived. In high signal-to-noise ratio (SNR), the MSE becomes independent of the channel Doppler spectrum. A characteristic function method is used to derive new closed-form expressions for the bit error rate (BER) of Rake receivers in UPDP and NPDP channels. The results are extended to two-dimensional (2-D) Rake receivers. The pilot-symbol spacing and pilot-to-data power ratio are optimized by minimizing the BER. For UPDP channels, elegant results are obtained in the asymptotic case. Furthermore, robust spacing design criteria are derived for the maximum Doppler frequency.     

多带隙声子晶体低频带隙特性研究

利用有限元法计算了一种多带隙声子晶体的特征频率,传递损失和位移矢量,理论推导了反共振产生条件。研究结果表明,这种声子晶体在低频范围内具有多条平直带隙,带隙的耦合影响因素少,具有比一般局域共振声子晶体和Bragg 散射声子晶体更宽的带隙。此外,利用原点反共振模型分析计算了这种多带隙声子晶体的材料和结构参数对带隙的影响。研究表明,在低于2 500 Hz时,多带隙声子晶体的带隙占90%以上,为低频多带隙声子晶体的研究作了探索。     

Availability Evaluation of Ground-to-Air Hybrid FSO/RF Links

Recently, a hybrid architecture that utilizes the complementary nature of free-space optics (FSO) and radio frequency (RF) links with respect to their individual weather sensitivities was proposed to significantly increase availability for terrestrial broadband links. Based on this architecture, we developed a channel model integrating both the RF and FSO channels. Using the model and cloud distribution data obtained from the International Satellite Cloud Climatology Project, availability of an airborne hybrid FSO/RF link is evaluated. From the results, we conclude that if the FSO link alone is used, availability is greatly hampered by clouds due to attenuation and temporal dispersion. Contrarily, the RF signals are relatively immune to cloud influence, thus improving the hybrid link availability significantly. Furthermore, because of the significant temporal dispersion caused by multiple scattering of cloud particles, availability of FSO links can be improved by using frequency division schemes, though far from compensating for losses incurred by clouds.     

周期量级激光脉冲中里德堡原子光电离的不对称性

采用全量子非微扰散射理论,研究了周期量级微波频率激光脉冲中高激发里德堡态铷原子光电离的不对称现象。发现了光电子角分布的不对称性,这一不对称性由电离过程中不同跃迁通道之间的干涉引起。讨论了光电子角分布的不对称性对载波-包络（Carrier-Envelo?pe , CE）相位、脉冲宽度及光电子能量的依赖性,发现光电子角分布的不对称性随CE相位、脉冲宽度及光电子能量的变化有显著变化。这一研究证实了最近的实验观测,为微波频率光学范畴内CE相位的测量及光电子角分布的控制提供了一种有效的方法。     

Polynomial spline-approximation of Clarke's model

We investigate polynomial spline approximation of stationary random processes on a uniform grid applied to Clarke's model of time variations of path amplitudes in multipath fading channels with Doppler scattering. The integral mean square error (MSE) for optimal and interpolation splines is presented as a series of spectral moments. The optimal splines outperform the interpolation splines; however, as the sampling factor increases, the optimal and interpolation splines of even order tend to provide the same accuracy. To build such splines, the process to be approximated needs to be known for all time, which is impractical. Local splines, on the other hand, may be used where the process is known only over a finite interval. We first consider local splines with quasioptimal spline coefficients. Then, we derive optimal spline coefficients and investigate the error for different sets of samples used for calculating the spline coefficients. In practice, approximation with a low processing delay is of interest; we investigate local spline extrapolation with a zero-processing delay. The results of our investigation show that local spline approximation is attractive for implementation from viewpoints of both low processing delay and small approximation error; the error can be very close to the minimum error provided by optimal splines. Thus, local splines can be effectively used for channel estimation in multipath fast fading channels.     

基于非均匀阵列的特征波束发射分集方案

在发射端将特征波束形成与空时分集技术相结合,在保持分集阶数的同时可以获得阵列增益.通过对该混合系统误码率性能的分析,提出了基于非均匀阵的技术方案.它可以在弱相关信道中保持较高的阵列增益,在强相关环境下使分集支路的平衡度得到改善.仿真结果表明,在相关度不同的多种信道环境中,采用非均匀阵可获得比均匀阵更好的误码率性能.     

Statistical characterization of non-WSSUS mobile radio channels

In this paper, statistical characterizations of mobile radio channels that do not satisfy the assumption of wide-sense stationary uncorrelated scattering (WSSUS) are discussed, most importantly the local scattering function. The framework presented is particularly suited for doubly underspread non-WSSUS channels. Application examples and measurement results illustrate its practical usefulness.     

风轮机雷达散射特性仿真及微多普勒特征分析

风轮机复杂的电磁散射特性,会对其附近的空管通信、导航和监视等电子设备产生严重影响.研究风轮机的电磁散射特性,可为风轮机杂波检测和抑制提供理论依据,对保证空中交通安全具有重要的意义.论文首先基于风轮机散射点叠加的理论,考虑了雷达入射波到风轮机叶片和桅杆的初始相位以及入射波方位角和俯仰角对回波的影响,将单基地回波模型扩展到双基地模型.同时,在散射点叠加模型的基础上,提出了基于混合模型的风轮机散射特性分析.混合模型结合了散射点叠加模型和电磁仿真软件FEKO的优点,考虑了电磁波在叶片和桅杆上的反射系数等因素对回波的影响,可以实现任意观测点处的电磁散射特性计算及其微多普勒特征的分析.最后,分别对散射点叠加模型、FEKO以及混合模型的风轮机电磁散射特性分析方法进行了对比分析,给出了各自的优缺点及其适用场合.     

Decoupled estimation of DOA and angular spread for a spatiallydistributed source

In mobile communications, local scattering in the vicinity of the mobile results in angular spreading as seen from a base station antenna array. In this paper, we consider the problem of estimating the parameters [direction-of-arrival (DOA) and angular spread] of a spatially distributed source, using a uniform linear array (ULA). A two-step procedure enabling decoupling the estimation of DOA from that of the angular spread is proposed. This method combines a covariance matching algorithm with the use of the extended invariance principle (EXIP). More exactly, the first step makes use of an unstructured model for the part of the covariance matrix that depends on the angular spread. Then, the solution is refined by invoking EXIP. Instead of a 2-D search, the proposed scheme requires two successive 1-D searches. Additionally, the DOA estimate is robust to mismodeling the spatial distribution of the scatterers. A statistical analysis is carried out, and a formula for the asymptotic variance of the estimates is derived. Numerical examples illustrate the performance of the method     

On channel estimation in cell-free massive MIMO for spatially correlated channels with correlated shadowing under Rician fading

This work addresses channel estimation (CE) in the uplink phase for a cell-free massive multiple-input multiple-output system operating under the time division duplex protocol. Considering that, channels are spatially correlated under the Rician fading model, where the investigated model is composed of two components: deterministic and nondeterministic, with the deterministic component describing the line-of-sight paths and the nondeterministic component describing the non-line-of-sight paths. Additionally, we dealt with correlated shadow fading that represents the most realistic situation. On the other hand, this work introduces a dynamic cooperation cluster framework in which the user is not served with the whole network ( i.e., all access points [APs]) but only the APs that present the best channel conditions regarding that user. In other words, this work proposes partial CE for each user because only APs with the best channel conditions are allowed to compute channel estimates. Consequently, we proposed partial channel estimators that perform the CE process with low complexity, namely, a partial minimum mean square error estimator and a partial element-wise minimum mean square error estimator. In addition, a simple pilot assignment technique is proposed in order to reduce interference signals so that each user experiences low interference from other users. Furthermore, the computational complexity required by each estimator is derived, where it is represented by the number of complex multiplications that each estimator requires in each consistency block. Theoretical and simulated results are provided, where the performance of each estimator is evaluated and analyzed using the normalized mean-square error metric.     

草原环境地表电磁散射的矩量法研究

为了满足草原及其附近目标雷达探测和遥感的需要,选用高斯型分布表示草和土壤表面的高度起伏情况,选取Topp 模型表示土壤层介电常数,采用双弥散模型表示草的介电常数,运用矩量法研究了草原环境地表的电磁散射特性,得到了散射系数的角分布曲线,讨论了入射角、入射波频率、草含水量、土壤湿度、土壤层和草层表面相关长度及均方根高度、草层厚度对散射系数的影响。结果表明,散射系数随散射角振荡地变化,在镜反射方向处有明显的散射增强效应;入射角和入射波频率对散射系数的影响较大,草含水量、土壤层和草层表面起伏相关长度和均方根高度对散射系数影响较小且较为复杂,土壤湿度、草层厚度对散射系数几乎没有影响。     

Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃ perovskite films deposited under different yet optimized conditions using X‐ray scattering and cathodoluminescence spectroscopy, respectively. X‐ray scattering shows that a CH₃NH₃PbI₃ film involving spin coating of CH₃NH₃I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH₃NH₃PbI₃ film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH₃NH₃PbBr₃, while the crystallinity decreases when the HBr solution is used in a single‐step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH₂)₂PbBr₃ devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices.     

快速分析理想导体电磁散射及辐射的新近似模型

本文提出一种快速分析理想导体电磁散射及辐射问题的新近似模型.应用这种方法,一个理想导体通常被分成两个区域,在靠近源或者入射波照射区域,利用矩量法求解,而另一个区域的电流则通过近似方法获得.与现有的混合方法相比,新模型给出了更好的结果和物理解释.无论是二维还是三维情况,数值模拟结果都验证了该方法的准确性.     

Channel‐estimate‐based frequency‐domain equalization (CE‐FDE) for broadband single‐carrier transmission

A channel‐estimate‐based frequency‐domain equalization (CE‐FDE) scheme for wireless broadband single‐carrier communications over time‐varying frequency‐selective fading channels is proposed. Adaptive updating of the FDE coefficients are based on the timely estimate of channel impulse response (CIR) to avoid error propagation that is a major source of performance degradation in adaptive equalizers using least mean square (LMS) or recursive least square (RLS) algorithms. Various time‐domain and frequency‐domain techniques for initial channel estimation and adaptive updating are discussed and evaluated in terms of performance and complexity. Performance of uncoded and coded systems using the proposed CE‐FDE with diversity combining in different time‐varying, multi‐path fading channels is evaluated. Analytical and simulation results show the good performance of the proposed scheme suitable for broadband wireless communications. For channels with high‐Doppler frequency, diversity combining substantially improves the system performance. For channels with sparse multi‐path propagation, a tap‐selection strategy used with the CE‐FDE systems can significantly reduce the complexity without sacrificing the performance. Copyright © 2004 John Wiley & Sons, Ltd.     

Antenna pattern scattering by rectangular cylinders

The uniform geometrical theory of diffraction (UTD) and the complex source point method are combined to predict two-dimensional (2-D) scattering by rectangular cylinders for E and H-polarized beam sources. Arrays of such beam solutions are also used to model propagation of radio beams into the shadow regions of buildings for local extended sources. Direct numerical verification of the results is obtained for square cylinders of moderate width     

基于导频信号的LTE系统高移动性信道估计算法

在长期演进(Long Term Evolution, LTE)系统中,精确的信道估计是均衡和相干解调的前提。在高速移动信道下,由于LTE下行链路离散非均匀的导频模式,传统的时间方向信道估计方法性能较差。为了提高信道估计精度,首先利用离散傅里叶变换(Discrete Fourier Transform, DFT)的信道估计方法提高导频处的估计精度。然后根据LTE系统的导频结构,设计了两种时间方向信道估计方法:1) 当信道的统计特性未知时,把不等间隔的导频分布转换为等间隔的导频分布,并利用三阶样条内插估计出数据处的信道频域值;2) 当信道的统计特性已知时,基于最小均方误差准则,设计高性能的自适应内插器,以准确获得信道估计所需系数。仿真和分析表明,相对于传统的线性内插算法,这两种信道估计算法利用更多的导频信息,可大大提高信道估计的精度,同时提升系统的解调性能。      

MSE-based regularization approach to direction estimation ofcoherent narrowband signals using linear prediction

This paper addresses the problem of directions of arrival (DOAs) estimation of coherent narrowband signals impinging on a uniform linear array (ULA) when the number of signals is unknown. By using an overdetermined linear prediction (LP) model with a subarray scheme, the DOAs of coherent signals can be estimated from the zeros of the corresponding prediction polynomial. Although the corrected least squares (CLS) technique can be used to improve the accuracy of the LP parameters estimated from the noisy array data, the inversion of the resulting matrix in the CLS estimation is ill-conditioned, and then, the CLS estimation becomes unstable. To combat this numerical instability, we introduce multiple regularization parameters into the CLS estimation and show that determining the number of coherent signals is closely related to the truncation of the eigenvalues. An analytical expression of the mean square error (MSE) of the estimated LP parameters is derived, and it is clarified that the number of signals can be determined by comparing the optimal regularization parameters with the corresponding eigenvalues. An iterative regularization algorithm is developed for estimating directions without any a priori knowledge, where the number of coherent signals and the noise variance are estimated from the noise-corrupted received data simultaneously     

BER performance of a uniform circular array versus a uniform linear array in a mobile radio environment

In this letter, we present a comparison between the bit error rate (BER) performance of a uniform circular array (UCA) and a uniform linear array (ULA) assuming quadrature phase-shift keying (QPSK) and maximal-ratio-combining (MRC) in a mobile radio communication environment. The results are based on analysis, assuming a flat Rayleigh fading channel with omni-directional antennnas and considering the azimuthal plane only. The analytical BER is derived as a function of the spatial fading correlation for both types of antenna arrays. Results show that for similar aperture sizes the UCA outperforms the ULA when considering all angles-of-arrival. However, there is considerable variability over central angle-of-arrival (AOA) for low-to-moderate angle spreads. For angles-of-arrival concentrated near the broadside of the linear array, the ULA typically performs as well as or better than the UCA. A truncated Gaussian AOA (AOA) distribution is assumed to model spatial correlation and the numerical results focus on four element arrays.     

关键词检测系统中基于音素网格的置信度计算

该文提出了一种基于音素网格的置信度计算方法。与传统的基于整个声学模型的置信度不同的是,这种方法在解码器生成的音素网格上计算关键词的置信度,从而具有更好的拒识能力。另外,针对两种置信度取值范围的不同,该文采用权重因子的方法综合利用两种置信度,取得了较好的效果。在自然对话的电话数据测试中,与传统的置信度计算方式相比,混和置信度的FOM(Figure Of Merit)值相对提高了17.0%。