构造雷达关联成像随机辐射场的方法分析EI北大核心CSCD

关联成像与传统的成像不同,是光学成像领域中一种新的成像体制,实现了探测与成像的分离。关联成像在雷达成像中拥有广阔的应用前景,为雷达成像的发展提供了新的思路。阐述了实现雷达关联成像中随机辐射场的几种方法,并重点分析其中一种模型。通过改变模型参数,分析了辐射场的变化趋势,研究了信号带宽和目标散射点间距对图像分辨率的影响,研究结果对雷达关联成像系统参数的设置具有一定的参考作用。     

天基平台宽谱段成像光学系统设计北大核心CSCD

针对用于地球静止轨道卫星的遥感面阵快照式成像光谱仪传输数据量过大引起的数据传输困难、信号采集处理时间长的问题,利用地球静止轨道平台可以长期驻留固定区域上空的特点,提出采用压缩感知的大口径宽谱段快照式光谱仪方案,对其光学系统结构进行设计,并对相关参数进行了计算。物镜采用同轴三反式无焦系统,用分色片对系统分光,经过对各系统进行优化处理,最终获得了幅宽为400 km×400 km,可见光地面像元分辨率为50 m、中波红外地面像元分辨率为400 m、长波红外地面像元分辨率为625 m的光学系统。该设计中,可见光路在78.125 lp/mm的MTF高于0.455,中波红外的光谱分辨率为光路在33.3 lp/mm处的MTF高于0.518,长波红外光路在20.8 lp/mm处的MTF高于0.498;可见光光谱分辨率为20 nm、中波红外的光谱分辨率为50 nm、长波红外的光谱分辨率为150 nm;可见光路二级光谱小于0.05 mm,设计结果具有良好的成像质量,各部分光学系统成像质量接近衍射极限,设计结果满足应用和指标需求。     

视场拼接复眼成像系统结构及装调方法EI北大核心CSCD

提出了以高分辨率、小视场的子眼镜头为中心,曲面阵列化排布多个分辨率较低、视场大的边缘阵列子眼镜头的人工仿生复眼的结构形式.基于物方视场空间应该保证无缝拼接并尽量缩小重合区域的原则,分析并得到子眼镜头在X方向和Y方向上视场角与复眼系统总体视场角的数学关系式,推导出了边缘阵列子眼镜头的周期阵列数n的数学模型,确定了子眼镜头在曲面上的阵列排布方式.依据曲面阵列排布形式设计了子眼镜头曲面固定本体,提出了利用自带光源的自准直经纬仪及计算机图像处理技术的装调方法.在实际装配和调整过程中,完成了中心子眼镜头和边缘第一阵列子眼镜头的安装,利用该系统采集图像数据,结果表明:物方视场空间的实际重合区域与理论设计一致.     

基于可编程LED阵列照明的透射体视显微镜EI北大核心CSCD

为了实现对待测样品高质量的体视显微成像,提出了基于可编程发光二极管(LED)阵列照明的透射体视显微镜。该显微镜光学系统使用可编程LED阵列作为照明双光源,照明孔径、角度以及波长均可自由选择,通过改变可编程LED阵列上红蓝圆形光源的半径r和圆心间距d分别实现对体视显微镜焦深和体视角的灵活可控。实验结果表明,该显微镜系统简便,观察者根据自身需要选择合适的焦深和体视角可有效解决因人眼瞳距个体差异大引起单一体视角出现的双像不能重合等问题,实现对待测样品的直接体视显微观看。使用数值孔径NA=0.1(4×)的显微物镜,在最佳参数r=2,d=3时可清晰观测到待测样品的层次关系以及相对位置等三维信息。     

一种电磁干扰源成像多分辨率分区算法北大核心CSCD

在利用抛物反射面对电磁干扰源成像过程中,由于系统衍射受限导致干扰源成像模糊,分辨率低,难以分辨,由于不同频率不同区域干扰源所成图像分辨率不同,具有分区域多分辨率的特征,采用已有超分辨算法难以提高分辨率。利用Mean Shift算法,在原有算法基础上改进使其能够适应多分辨率的电磁干扰源成像,在图像分割的基础上对多分辨率图像进行分块抽离,并采用基于L_R迭代的盲反卷积算法分别对各区域进行分辨率的提高,仿真结果表明算法能够适应对干扰源的多分辨率电磁成像并提高分辨率。     

基于显微物镜的表面等离子体共振EI北大核心CSCD

利用高数值孔径显微物镜(油浸)并采用Kretschmann结构在52.3nm金膜上激发了表面等离子体共振.从麦克斯韦方程和菲涅尔公式出发,建立了相应的数学模型,并对物镜后焦面的光强分布进行了数值模拟.由图像传感器采集了后焦面的光强分布,通过计算机处理并显示.与理论结果对比,实验得到的共振角与理论值相符.对数值模拟和检测结果的分析为金膜表面介质的表面等离子体共振成像应用和折射率检测提供了理论与实验依据.     

星载扫描层析临边成像光谱仪光学设计EI北大核心CSCD

星载扫描层析临边成像光谱仪已成为空间大气遥感领域的迫切需求,根据临边宽水平覆盖轻小型星载扫描层析临边成像光谱仪的研究目标,采用一维扫描镜、离轴抛物面望远系统和非球面光栅光谱成像系统匹配,设计了一个视场0.75°×0.017°、相对孔径1/1.8、工作波段280～800 nm的星载扫描层析临边成像光谱仪光学系统,利用光学设计软件CODE V进行了光线追迹和优化设计,光谱成像系统不同波长的点列图直径的均方根（RMS）值均小于34 mm,光谱分辨率0.84 nm,满足的指标要求（≤1.2 nm）,扫描层析临边成像光谱仪全系统在空间方向各波长在特征频率处的光学传递函数均达到0.57以上,能够满足成像质量要求,适合空间大气遥感应用。     

光场强度分布测量中CCD噪声的校正北大核心CSCD

为了有效抑制CCD测量噪声带给激光对比度分布测量的影响,基于对比度测量模型,利用CCD测量噪声的先验知识及其输出观测值,给出了CCD噪声的校正方法。与现有计算方法的比较表明,该方法可以有效抑制CCD测量噪声带给测量的影响,其扩展不确定度从0.6%降低至0.3%,降低了测量不确定度,提高了测量的置信度。     

基于光时分复用技术的高速成像系统北大核心CSCD

提出了一种基于光时分复用技术的高速成像系统。飞秒激光器中心波长1557nm,脉冲宽度90fs,对USAF-1951分辨率板线性扫描成像,扫描频率为38.88 MHz。在连续时间序列编码放大显微成像技术的基础上,运用光时分复用技术,复制光脉冲信号并携带检测物体相同的空间信息。原光脉冲和复制光脉冲以相同的采样率分别采样,通过相应的数据处理将两次采样数据整合在一起还原图像。实验结果表明,与传统的超快成像方法相比,成像系统利用10GHz的数字采样设备可以达到20GHz的采样率,采样点数是传统超快成像方法的两倍。该方法有效克服了成像系统采样率不足的问题,提高了成像系统的空间分辨率。与此同时,该系统算法复杂程度不高,有利于进一步促进超高速成像技术的发展。     

超高斯波束圆口径天线近场辐射特性北大核心CSCD

采用口径场理论分析了不同孔束比(孔径与束腰之比)的超高斯分布圆形口径天线近场辐射特性,得到了聚焦条件下焦点位置的近场增益解析式。并对不同观测位置点天线的辐射特性进行了仿真,结果表明:当观测点小于焦距的0.3倍时,且波束阶数大于4时,天线的辐射图样与孔束比无关;当观测点位于焦点时,设定孔束比,不同阶数的波束分布的辐射图样明显不同,但当孔束比为4以上时,辐射图样基本保持稳定,而且高阶波束没有旁瓣出现。     

密布式多输入多输出声呐阵列目标波达方向估计

针对低信噪比条件下多输入多输出声呐受对称噪声分量影响导致测向性能降低的情况,提出了一种基于协方差矩阵重构方法的波达方向估计算法。首先,将噪声场分为对称噪声和非对称噪声两部分,利用协方差矩阵虚部与对称信号无关的性质,去掉协方差矩阵的实部来降低对称噪声对目标波达方向估计精度的影响,采用降维转换方法和矩阵虚部置换原理重构协方差矩阵的实部,避免了双频谱的干扰。然后利用Toeplitz方法对重构的协方差矩阵进行解相干修正,通过奇异值分解获得噪声子空间,最后对目标的波达方向进行估计,可实现微弱信号的准确测向。理论分析和实验结果表明,该方法明显抑制了对称噪声,提高了目标的波达方向估计性能,具有运算速度快、自由度高和目标分辨力强的特点。     

Direction-of-arrival estimation for co-located multiple-input multiple-output radar using structural sparsity Bayesian learning

This paper addresses the direction of arrival(DOA) estimation problem for the co-located multiple-input multipleoutput(MIMO) radar with random arrays. The spatially distributed sparsity of the targets in the background makes compressive sensing(CS) desirable for DOA estimation. A spatial CS framework is presented, which links the DOA estimation problem to support recovery from a known over-complete dictionary. A modified statistical model is developed to accurately represent the intra-block correlation of the received signal. A structural sparsity Bayesian learning algorithm is proposed for the sparse recovery problem. The proposed algorithm, which exploits intra-signal correlation, is capable being applied to limited data support and low signal-to-noise ratio(SNR) scene. Furthermore, the proposed algorithm has less computation load compared to the classical Bayesian algorithm. Simulation results show that the proposed algorithm has a more accurate DOA estimation than the traditional multiple signal classification(MUSIC) algorithm and other CS recovery algorithms.     

双基地多输入多输出虚拟阵列的稳健低旁瓣波束优化技术

双基地多输入多输出(MIMO)系统需要对目标的波达角和波离角进行二维估计,针对高分辨算法中阵列协方差矩阵处理运算量大以及估计性能对阵列流形失配敏感的缺陷,提出了一种基于二阶锥规划的抗阵列流形失配的双基地MIMO虚拟阵列波束优化算法.该方法利用MIMO的波形分集技术与双基地的目标定位解算,通过波达角和波离角在双基地定位椭圆上的对应关系,将双基地MIMO的二维合成方向矢量转化为一维的虚拟阵列流形,并证明了双基地MIMO虚拟阵列的波束图等效为发射波束图在接收阵角度域上的响应和接收波束图的合成.在优化波束图旁瓣的同时,该算法兼顾了固定方向上的相干干扰抑制,并提高了双基地MIMO虚拟阵列波束优化抗阵列流形失配的能力.通过波束图和方位谱优化的仿真分析和水池试验,验证了算法的有效性.     

Weak signal direction of arrival estimation for colocated multiple-input multiple-output sonar array

In order to suppress the influence of symmetrical noise component on multiple-input multiple-output(MIMO)sonar's direction of arrival(DOA)estimation under the condition of low signal-to-noise ratio,we propose a DOA estimation algorithm based on covariance matrix reconstruction method.Firstly,the noise field can be decomposed into symmetrical noise field and asymmetrical noise field.We utilize symmetry property of colored noise matrix and the feature that the imaginary part of covariance matrix has no relation with the symmetry noise to remove the real part of covariance matrix.This operation helps to suppress the influence of colored noise on DOA estimation accuracy.Based on the principle of the imaginary matrix part displacement and the dimension reduction transformation method,the real part of covariance matrix is reconstructed,which helps to suppress the bilateral spectrum interference.Thereafter,Toeplitz method is applied for the covariance matrix decorrelation amendment,and a noise subspace is formed by singular value decomposition(SVD).Finally,we can estimate the DOA of target signals.Both theoretical analysis results and numerical simulation results verify the symmetrical noise suppression performance of this algorithm,and the estimation performance of target azimuth is improved obviously.This method has the characteristics of lower operational complexity,higher degrees of freedom and stronger target resolution.     

Shannon information capacity of time reversal wideband multiple-input multiple-output system based on correlated statistical channels

Utilizing channel reciprocity, time reversal(TR) technique increases the signal-to-noise ratio(SNR) at the receiver with very low transmitter complexity in complex multipath environment. Present research works about TR multiple-input multiple-output(MIMO) communication all focus on the system implementation and network building. The aim of this work is to analyze the influence of antenna coupling on the capacity of wideband TR MIMO system, which is a realistic question in designing a practical communication system. It turns out that antenna coupling stabilizes the capacity in a small variation range with statistical wideband channel response. Meanwhile, antenna coupling only causes a slight detriment to the channel capacity in a wideband TR MIMO system. Comparatively, uncorrelated stochastic channels without coupling exhibit a wider range of random capacity distribution which greatly depends on the statistical channel. The conclusions drawn from information difference entropy theory provide a guideline for designing better high-performance wideband TR MIMO communication systems.     

基于长电偶极子和大磁圆环的新型电磁矢量传感器双基地多输入多输出雷达角度和极化参数联合估计

针对常规双基地电磁矢量传感器多输入多输出(multiple-input multiple-output,MIMO)雷达中短电偶极子(L/λ<0.1)和小磁环((2πR/λ)<0.1)辐射效率不足问题,本文根据实际应用中电磁矢量传感器的有效长度来设计新型的发射电磁矢量传感器阵列和接收电磁矢量传感器阵列.首先,通过平行因子...     

Adaptive multiple-input multiple-output mode switching for indoor visible light communication system with orthogonal frequency division multiplexing modulation

We propose and experimentally demonstrate an adaptive multiple-input multiple-output(MIMO) mode switching scheme for an indoor visible light communication system combined with orthogonal frequency division multiplexing modulation.Only requiring 1 bit feedback from the receiver,the MIMO mode at the transmitter switches between spatial multiplexing and transmit diversity adapting to the channel correlation.In such a way,we can take advantage of both spatial multiplexing and transmit diversity,where the spatial multiplexing benefits for its multiplexing gain in the low channel correlation environment,and the transmit diversity is robust to high channel correlation.Experimental results validate the performance improvement over the pure spatial multiplexing or transmit diversity system.With the increasing of the channel correlation,the measured bit error rates of the proposed system are below the 7% pre-forward error correction(pre-FEC) limit of 3.8 × 10~(-3)when the transmitted data rate is 50 Mb/s,and below the 20% pre-FEC threshold of 5.0 × 10~(-2) when the transmitted data rate is raised up to 100 Mb/s.     

X波段低副瓣波导缝隙阵列天线特性

:根据Stevenson等效电路法设计了两个X波段低副瓣波导缝隙阵列天线。通过建立多个缝隙时的谐振长度提取模型,计算了存在缝隙间互耦时的谐振长度。两个阵列天线测试结果和仿真结果吻合良好,线阵中心频率实测增益为17.83 dB,其仿真结果为18.2 dB,实测副瓣为-28.12 dB,其仿真结果为-29.97 dB;平面阵列中心频率实测增益为27 dB,其仿真结果为27.9 dB, 实测H面副瓣为-27.2 dB,其仿真结果为-29.9 dB,实测E面副瓣为-22 dB,其仿真结果为-22 dB。     

X波段低副瓣波导缝隙阵列天线特性

根据Stevenson等效电路法设计了两个X波段低副瓣波导缝隙阵列天线。通过建立多个缝隙时的谐振长度提取模型,计算了存在缝隙间互耦时的谐振长度。两个阵列天线测试结果和仿真结果吻合良好,线阵中心频率实测增益为17.83dB,其仿真结果为18.2dB,实测副瓣为-28.12dB,其仿真结果为-29.97dB;平面阵列中心频率实测增益为27dB,其仿真结果为27.9dB,实测H面副瓣为-27.2dB,其仿真结果为-29.9dB,实测E面副瓣为-22dB,其仿真结果为-22dB。     

A phased array echoplanar imaging system for fMRI

A fully parallel, simultaneous sampling phased array receiver system for a clinical echoplanar imaging system is described and evaluated for BOLD activation and relative cerebral blood volume (rCBV) experiments. A 4-coil array curved around the occipital lobe improved SNR by factors of 1.5 in the visual cortex and 3.1 in the visual association cortex relative to a 13-cm diameter surface coil, improving the statistical significance and coverage of visual activation maps. A 4-coil bilateral array increased SNR throughout the head relative to a quadrature head coil by up to a factor of 5 in much of the cortex, with proportional improvement in the SNR of rCBV maps.