Low Complexity Polynomial Expansion Multiuser Detector for CDMA Systems

The polynomial expansion (PE) multiuser detector can iteratively approximate the linear decorrelating and MMSE multiuser detectors. This is a very promising approach since the complexity of the PE detector is considerably less than that of the decorrelating and MMSE detectors. The concept of the PE multiuser detector is a weighted matrix polynomial for which optimal weighting has been suggested in the literature. Unfortunately, the optimal weights apply only to a specific correlation matrix. As soon as the correlation between the users changes (and hence also the correlation matrix), a new set of optimal weights has to be calculated. The calculation of these weights is computationally very intense. In this paper, an approach is presented where the weights are predefined and apply for all matrices. Furthermore, a normalization factor is needed to ensure convergence. It will be shown how this factor has to be chosen to obtain the optimal convergence speed. For the optimal normalization factor the minimum and maximum eigenvalues are needed. A low complexity and accurate method to estimate these eigenvalues is derived which can be applied to all correlation matrices and therefore to any code division multiple access (CDMA) scenario. Hence, the matrix-dependent optimal normalization factor can easily be calculated and ensures a good bit error rate (BER) performance, even if the correlation between the users changes quickly (e.g., in time-variant channels). Furthermore, it will be shown how to enhance the PE detector such that it has a high near-far resistance. Additionally, for the first time it can be proven that the approximation error of the proposed detector diminishes exponentially with the number of iterations. Finally, simulations verify the fast convergence of the proposed PE detector and its flexible usage in a variety of scenarios.     

Information analysis of single photon emission computed tomography with count losses

An analysis is presented of the information transfer from emitter-space to detector-space in single photon emission computed tomography (SPECT) systems. The analysis takes into account the fact that count loss side information is generally not available at the detector. Side information corresponds to the number gamma-rays lost deleted due to lack of interaction with the detector data. It is shown that the information transfer depends on the structure of the likelihood function of the emitter locations associated with the detector data. This likelihood function is the average of a set of ideal-detection likelihood functions, each matched to a particular set of possible deleted gamma-ray paths. A lower bound is derived for the information gain due to incorporating the count loss side information at the detector. This is shown to be significant when the mean emission rate is small or when the gamma-ray deletion probability is strongly dependent on emitter location. Numerical evaluations of the mutual information, with and without side information, associated with information-optimal apertures and uniform parallel-hole collimators are presented.     

Characterization of pinhole SPECT acquisition geometry

A method is presented to estimate the acquisition geometry of a pinhole single photon emission computed tomography (SPECT) camera with a circular detector orbit. This information is needed for the reconstruction of tomographic images. The calibration uses the point source projection locations of a tomographic acquisition of three point sources located at known distances from each other. It is shown that this simple phantom provides the necessary and sufficient information for the proposed calibration method. The knowledge of two of the distances between the point sources proves to be essential. The geometry is estimated by fitting analytically calculated projections to the measured ones, using a simple least squares Powell algorithm. Some mild a priori knowledge is used to constrain the solutions of the fit. Several of the geometrical parameters are however highly correlated. The effect of these correlations on the reconstructed images is evaluated in simulation studies and related to the estimation accuracy. The highly correlated detector tilt and electrical shift are shown to be the critical parameters for accurate image reconstruction. The performance of the algorithm is finally demonstrated by phantom measurements. The method is based on a single SPECT scan of a simple calibration phantom, executed immediately after the actual SPECT acquisition. The method is also applicable to cone-beam SPECT and X-ray CT.     

一种红外目标模拟器的大气传输校准方法研究

红外辐射在大气中传输会在大气分子、气溶胶粒子的吸收和散射以及大气自身辐射的影响下发生变化,导致红外辐射测量精度的降低。为消除大气在红外目标模拟器校准中的影响,在基于恒定标准源的宽动态红外辐射测量方法的基础上,提出了一种红外目标模拟器的大气传输校准方法。在水平均匀大气近距离的红外目标模拟器校准中,利用卷积神经网络的数据分析能力建立了不同波段、不同温度、不同距离下的大气透过率和大气程辐射的动态模型,将探测器输出电压作为基于编码器-解码器结构的卷积神经网络的输入,按照训练流程对网络进行训练,在实验环境下预测了大气传输对红外辐射的影响。所建模型能够反映大气透过率和大气程辐射的动态变化规律,并通过红外辐射反演对提出的方法进行了验证。实验结果表明：基于编码器-解码器结构的卷积神经网络算法能够较好地预测大气透过率和大气程辐射,在三个波段下的平均误差为3.078 3%、3.818 6%、5.345 2%,低于传统方法,降低了大气透过率和大气程辐射的影响,从而减小了红外辐射的测量误差,提高了校准精度。     

An application of univariate marginal distribution algorithm in MIMO communication systems

The paper discusses a sequence detector based on univariate marginal distribution algorithm (UMDA) that jointly estimates the symbols transmitted in a multiple input multiple output (MIMO) communication system. While an optimal maximum likelihood detection using an exhaustive search method is prohibitively complex, it has been shown that sphere decoder (SD) achieves the optimal bit error rate (BER) performance with polynomial time complexity for smaller array sizes. However, the worst‐case complexity of SD is exponential in the problem dimensions, this brings in question its practical implementation for larger number of spatial layers and for higher‐order signal constellation. The proposed detector shows promising results for this overly difficult and complicated operating environment, confirmed through simulation results. A performance comparison of the UMDA detector with SD is presented for higher‐order complex MIMO architectures with limited average transmit power. The proposed detector achieves substantial performance gain for higher‐order systems attaining a near optimal BER performance with reduced computational complexity as compared with SD. Copyright © 2009 John Wiley & Sons, Ltd.     

GF5B热红外通道星上定标与验证

以GF5B卫星发射前实验室定标为基础,采用星上0级黑体定标数据,建立了适用于GF5B热红外通道的星上绝对辐射定标模型。通过对2022年01月12日星上黑体定标数据进行处理,获得成像仪热红外通道的绝对辐射定标系数。对星上定标系统精度进行分析,并采用地面同步烟台浮标数据对定标结果进行精度验证,结果表明,在轨后星上定标系统的绝对定标精度为0.9 K;星地验证结果显示B11和B12通道亮温的偏差分别为0.33、0.77 K。说明基于星上黑体的定标方法具有较好的精度,定标结果可靠,可满足遥感数据定量化应用的需要,为实时准确获取热红外通道定标系数提供了方法借鉴。     

Hadamard变换成像光谱仪实验室辐射定标方法

Hadamard变换成像光谱仪采用多通道探测数字变换技术实现光谱成像。主要介绍了基于数字微镜阵列器件的Hadamard变换成像光谱仪的工作原理与仪器结构,研究设计了一套适用于该Hadamard变换成像光谱仪的实验室辐射定标方案。用远距点光源光路进行CMOS探测器像元响应不均匀性修正,获得相对定标精度达到4.6%;采用太阳模拟光源和均匀平行光路,用光谱辐射度计实现标准辐射亮度的传递进行光谱辐射定标,绝对定标精度达到8.92%。通过实物成像,Hadamard变换成像光谱仪的实验室辐射定标方法精确、实用。     

Efficient parameter determination of code acquisition detector for CDMA wireless communication

The optimal procedure minimising the mean acquisition time is proposed to determine the parameter values of the code acquisition detector in code division multiple access (CDMA) wireless communication. The optimal relationships among the threshold (vth), the number of correlation samples (N) and the number of noncoherent accumulations (L) are obtained for the code acquisition detector. A suboptimal solution is presented to obtain the integer solution and is shown to reduce the code synchronisation time and the computational complexity of the CDMA receiver.     

基于加权最小二乘法的红外辐射定标

随着红外物理与红外技术的深入发展,目标的辐射特性测量受到了广泛的重视,红外探测器的辐射定标直接决定了辐射特性测量精度,为此文中建立了辐射定标系统,对3～5μm的红外焦平面阵列相机进行了绝对辐射定标实验。文中给出了辐射定标的数学模型,制定了定标的流程并进行了相应的辐射定标实验;针对红外标准源和环境波动性的影响,提出了加权最小二乘法的辐射定标优化方法;通过外场辐射特性测量实验表明,该方法的反演误差优于4%,提高了辐射定标的精度,结果满足实际工程的需要。     

背景辐射对HgCdTe中波叠层光导器件噪声的影响

研究了HgCdTe中波叠层光导器件在不同背景辐射条件下的性能变化,设计了增加冷光栏和使用不同温度的黑体对器件进行辐照的两种改变背景辐射的实验方案.结果表明,随着背景辐射的减小,器件的测量噪声亦减小.利用非平衡载流子和器件有效寿命理论对器件的产生复合噪声进行了计算,计算结果与实验结果在随背景辐射变化的趋势上相似.进一步的噪声频谱测量表明,1/f噪声是叠层器件噪声随背景辐射变化的主要原因;而叠层结构中存在的边缘接触不对称MIS结构增大了背景辐射变化对1/f噪声的影响.     

Revised Radiometric Calibration Technique for LANDSAT-4 Thematic Mapper Data

A technique for the radiometric correction of Landsat-4 Thematic Mapper (TM) data was proposed by the Canada Centre for Remote Sensing (CCRS) in 1982, and two reports defining the method and discussing preliminary results were presented by CCRS at the Landsat-4 Scientific Characterization Early Results Symposium [1] and [2]. Subsequent detailed observations of raw image data, raw radiometric calibration data, and background measurements extracted from the raw data stream on High Density Tape have highlighted in the proposed method, major shortcomings, which if left uncorrected, can cause severe radiometric striping in the output product. Observations presented here show that there are random fluctuations in the background level for spectral band 1 of magnitudes ranging from 2 to 3.5 digital numbers (DN), depending on detector number. Similar variability is observed in all the other reflective bands, but with smaller magnitude in the range 0.5 to 2.5 DN. More significantly, it is shown how measurements of the dc background level can be correlated with variations in both image data background and calibration samples. The effect on both raw data and on data corrected using the earlier proposed technique is explained, and the correction required for these factors as a function of individual scan line number for each detector is described. It is shown how the revised technique, which includes corrections for a line-dependent offset in addition to the scene-dependent gain and offset, can be incorporated into an operational environment.     

基于VIIRS传感器的场地反射率测量精度的验证

反射率基法是目前最常用、最有效的可见光近红外通道在轨辐射定标方法。在该方法中,准确的场地反射率测量是得到高精度定标结果的重要前提之一。利用双光谱仪法进行了野外地物反射率时间序列测量,避免了用传统测量方法可能出现的辐射条件不一致问题。用中心区和高反区场地反射率测量值对VIIRS传感器的表观反射率进行了模拟,并将VIIRS可见光近红外9个通道(I1、I2、M1～M7)的卫星观测表观反射率值与6S辐射传输模型模拟值进行了对比。除M6通道外(水汽吸收),相对偏差均在3%以内,验证了场地反射率测量的准确性。     

基于低温辐射计高精度光辐射定标结果(1999和2003年)的对比研究

对1999年和2003年采用不同测量光路情况下可见光波段低温辐射计高精度光辐射定标测量结果及其不确定度进行了比较。结果表明，2003年低温辐射计激光功率测量不确定度比1999年的要小；2003年硅陷阱探测器光谱响应率测量不确定度和1999年的在同一水平上；陷阱探测器绝对光谱响应率变化在1．20%以内。这些结果证实陷阱探测器稳定性优良，光路改造合理。     

红外热像仪高精度测温标定技术

红外热像仪能够监测目标温度从而起到事故预警与位置确认、大规模人体表面温度筛查等作用。由于环境温度变化与红外辐射吸收产生的温度漂移现象,目前大部分测温红外热像仪需要采用黑体进行实时校正,但是基于黑体的红外热像仪架设场景固定,便携性差。针对以上问题提出了一种无黑体式红外热像仪测温标定和温度补偿技术,通过对红外测温原理进行推导,采用多黑体标定获得目标温度与辐射量的先验关系,针对探测器内部结构引起温度漂移,由牛顿冷却定律进行非线性建模实现实时温度补偿。通过实验验证,所提出的测温标定技术可将测温平均相对误差长时稳定保持在0.9%以内,相比于标定前平均相对误差有效降低64%,从而实现小型化红外热像仪便携、实时、稳定高精度测温。     

溯源于低温辐射计的高精度遥感器辐射定标技术

低温辐射计作为拥有10-4不确定度的辐射初级标准已经开始运用于遥感器辐射定标领域。总结了低温辐射计的高精度初级标准传递至星载遥感器的定标过程,详细介绍了标准传递链中各级标准的不确定度。通过VXR、TXR在NASA的EOS计划中的应用及国内辐亮度探测器对FY-2号的实际定标,展现溯源于低温辐射计的定标技术广阔的应用前景。     

Some results for the fast MMSE-SIC detection in spatially multiplexed MIMO systems

We study the implementation algorithm for the MMSE-SIC detection with optimal detection order in a VBLAST system. We develop the LDL^H decomposition based algorithm with backward substitution (LDBABS), which is shown to require as many multiplications as two other most efficient algorithms. We then propose a modified LDBABS algorithm that utilizes the scheme of incomplete LDL^H decomposition together with the initial ordering information. The modified LDBABS algorithm is shown to require averagely fewer multiplications than any other implementation algorithm under the independent identically distributed Rayleigh fading channel. It is known that the real-valued MMSE-SIC detector with optimal detection order outperforms its complex-valued counterpart by approximately 1 dB. We finally propose a fast algorithm for the real-valued MMSE-SIC detector with optimal detection order. The number of real-valued multiplications required by this fast algorithm is approximately 1.57 times that required by the complex-valued LDBABS algorithm.     

Optimal signaling and detector design for power-constrained binary communications systems over non-gaussian channels

In this letter, joint optimization of signal structures and detectors is studied for binary communications systems under average power constraints in the presence of additive non-Gaussian noise. First, it is observed that the optimal signal for each symbol can be characterized by a discrete random variable with at most two mass points. Then, optimization over all possible two mass point signals and corresponding maximum a posteriori probability (MAP) decision rules are considered. It is shown that the optimization problem can be simplified into an optimization over a number of signal parameters instead of functions, which can be solved via global optimization techniques, such as particle swarm optimization. Finally, the improvements that can be obtained via the joint design of the signaling and the detector are illustrated via an example.     

Automatic nonrigid calibration of image registration for real time MR-guided HIFU ablations of mobile organs

Real time magnetic resonance imaging (MRI) is rapidly gaining importance in interventional therapies. An accurate motion estimation is required for mobile targets and can be conveniently addressed using an image registration algorithm. Since the adaptation of the control parameters of the algorithm depends on the application (targeted organ, location of the tumor, slice orientation, etc.), typically an individual calibration is required. However, the assessment of the estimated motion accuracy is difficult since the real target motion is unknown. In this paper, existing criteria based only on anatomical image similarity are demonstrated to be inadequate. A new criterion is introduced, which is based on the local magnetic field distribution. The proposed criterion was used to assess, during a preparative calibration step, the optimal configuration of an image registration algorithm derived from the Horn and Schunck method. The accuracy of the proposed method was evaluated in a moving phantom experiment, which allows the comparison with the known motion pattern and to an established criterion based on anatomical images. The usefulness of the method for the calibration of optical-flow based algorithms was also demonstrated in vivo under conditions similar to thermo-ablation for the abdomen of twelve volunteers. In average over all volunteers, a resulting displacement error of 1.5 mm was obtained (largest observed error equal to 4-5 mm) using a criterion based on anatomical image similarity. A better average accuracy of 1 mm was achieved using the proposed criterion (largest observed error equal to 2 mm). In both kidney and liver, the proposed criterion was shown to provide motion field accuracy in the range of the best achievable.     

On sequential sign detection of a constant signal

In fixed sample size signal detection, the sign detector is easier to implement and requires much less knowledge of the noise density than does the optimum fixed sample size detector. These advantages accrue at a cost of more observations needed to achieve the same performance. By utilizing the sign detector sequentially, it is shown that considerable saving in the (average) number of observations is obtained over the corresponding optimal fixed sample size detector. The asymptotic relative efficiencies, as the signal-to-noise ratio and both probabilities of error approach zero, of the sequential sign detector compared to fixed sample size detectors are derived. The possibility of truncating the test to avoid excessively large numbers of observations is considered.     

Log-normal分布杂波背景下有序统计恒虚警检测器性能分析

该文证明了在形状参数已知的均匀对数-正态(Log-normal)杂波背景下有序统计恒虚警(OS-CFAR)检测器能保持恒虚警的特性,导出了该背景下OS-CFAR检测器对非起伏目标的检测性能表达式和平均判决阈值(ADT)表达式。最后用数值方法讨论了最佳序值的选取以及OS-CFAR检测器在取不同参考单元数时相对于理想CFAR的信杂比损失。