Outage Capacity of the Fading Relay Channel in the Low-SNR Regime

In slow-fading scenarios, cooperation between nodes can increase the amount of diversity for communication. We study the performance limit in such scenarios by analyzing the outage capacity of slow fading relay channels. Our focus is on the low signal-to-noise ratio (SNR) and low outage probability regime, where the adverse impact of fading is greatest but so are the potential gains from cooperation. We showed that while the standard Amplify-Forward protocol performs very poorly in this regime, a modified version we called the Bursty Amplify-Forward protocol is optimal and achieves the outage capacity of the network. Moreover, this performance can be achieved without a priori channel knowledge at the receivers. In contrast, the Decode-Forward protocol is strictly suboptimal in this regime. Our results directly yield the outage capacity per unit energy of fading relay channels     

Optimal Constellations for the Low-SNR Noncoherent MIMO Block Rayleigh-Fading Channel

Reliable communication over the discrete-input/continuous-output noncoherent multiple-input multiple-output (MIMO) Rayleigh block-fading channel is considered when the signal-to-noise ratio (SNR) per degree of freedom is low. Two key problems are posed and solved to obtain the optimum discrete input. In both problems, the average and peak power per space–time slot of the input constellation are constrained. In the first one, the peak power to average power ratio (PPAPR) of the input constellation is held fixed, while in the second problem, the peak power is fixed independently of the average power. In the first PPAPR-constrained problem, the mutual information, which grows as $O({rm {SNR}}^{2})$, is maximized up to second order in SNR. In the second peak-constrained problem, where the mutual information behaves as $O({rm{SNR}})$, the structure of constellations that are optimal up to first order, or equivalently, that minimize energy per bit, are explicitly characterized. Furthermore, among constellations that are first-order optimal, those that maximize the mutual information up to second order, or equivalently, the wideband slope, are characterized. In both PPAPR-constrained and peak-constrained problems, the optimal constellations are obtained in closed form as solutions to nonconvex optimizations, and interestingly, they are found to be identical. Due to its special structure, the common solution is referred to as space–time orthogonal rank one modulation, or STORM. In both problems, it is seen that STORM provides a sharp characterization of the behavior of noncoherent MIMO capacity.      

How Good Is PSK for Peak-Limited Fading Channels in the Low-SNR Regime?

This paper investigates the achievable information rate of phase-shift keying (PSK) over frequency nonselective Rayleigh and Rician fading channels without channel state information (CSI). The fading process exhibits general temporal correlation characterized by its spectral density function. We consider both discrete-time and continuous-time channels, and find their asymptotics at low signal-to-noise ratio (SNR). Compared to known capacity upper bounds under peak constraints, these asymptotics lead to negligible rate loss in the low-SNR regime for slowly time-varying fading channels. We further specialize to case studies of Gauss-Markov and Clarke's fading models     

MIMO Channels in the Low-SNR Regime: Communication Rate, Error Exponent, and Signal Peakiness

We consider multiple-input multiple-output (MIMO) fading channels and characterize the reliability function in the low signal-to-noise (SNR) regime as a function of the number of transmit and receive antennas. For the case when the fading matrix H has independent entries, we show that the number of transmit antennas plays a key role in reducing the peakiness in the input signal required to achieve the optimal error exponent for a given communication rate. Further, by considering a correlated channel model, we show that the maximum performance gain (in terms of the error exponent and communication rate) is achieved when the entries of the channel fading matrix are fully correlated. The results we presented in this work in the low-SNR regime can also be applied to the infinite bandwidth regime     

基于DVB-S标准的低信噪比数字解调器

提出了一种基于DVB-S标准的全数字QPSK解调模块,阐述了数字解调模块的频率恢复与相位跟踪两个主要环路的算法与实现,并针对抗噪声要求提出了自主设计的频率矫正器,实验结论表明整个设计可正常工作在满足DVB-S标准要求的噪声环境中.     

一种利用窄波束成形提高短波信道相干带宽的方法

短波通信信道具有频率选择衰落特性,相干带宽是描述单载波系统平坦衰落信道带宽的重要参数。提高相干带宽有利于系统使用更大的带宽来实现高数据速率传输并且有助于减少接收机的复杂度。该文提出一种利用发射端窄波束成形在空间上将发射能量集中于主路径且抑制其它路径信号来提高短波信道相干带宽的方法,并利用自适应技术来建立及保持点对点之间的窄波束通信。理论研究表明,该方法能有效地提高信道相干带宽。该文以中距离短波通信为例,计算了使用不同发射天线数时的相干带宽值,计算结果证实了该方法的有效性。     

利用局部直线段模糊投票的低SNR海天线提取方法

提出了一种采用“假设-检验”策略的海天线提取新方法。在假设阶段,提出了一种新的直线段Hough变换检测疑似海天线。首先,以局部直线段为整体估计直线参数,理论推导证明此时估计误差方差远小于各边缘点独立估计时误差方差的平均值。其次,针对残余估计误差,设计了局部直线段模糊投票策略,实现全局直线段的投票聚类。最后,检测峰值点形成疑似海天线假设。在检验阶段,提出了三类新特征描述海天线与海杂波的属性差异,并采用SVM分类器识别海天线。该方法提取低信噪比海天线准确,识别正确率高,仿真和实测数据的实验结果验证了所提方法的有效性。      

高动态低信噪比下扩频信号捕获算法研究

分析了高动态对扩频信号捕获的影响因素,针对低信噪比、高动态的双重条件对捕获时间的严格要求,提出了一种基于二次捕获的高动态低信噪比下扩频信号快速捕获方法,能够缩短捕获时间,快速进行扩频伪码相位、载波多普勒频率以及载波多普勒频率变化率的捕获.     

UWS脉冲雷达回波低信杂比识别检测算法研究

UWS脉冲雷达属于全新体制的雷达系统,低信杂比信号检测是系统关键性技术之一。现有雷达采用的相关检测理论,不适于UWS脉冲雷达系统。通过对脉冲雷达接收信号的时域特征分析,提出波形特征矢量提取及双门限识别检测理论,给出了详细的理论分析和实际算法的实验效果。基于识别检测思想,将信号检测与特征识别进行有机结合,有效减少了数据处理步骤,对于波形保真、快速处理等具有较大优势,为远距离UWS脉冲雷达的工程化实现提供了一定的理论基础。     

Design in the Power-Limited Scaling Regime

Technology scaling has entered a new era, where chip performance is constrained by power dissipation. Power limits vary with the application domain; however, they dictate the choices of technology and architecture and necessitate implementation techniques that tradeoff performance for power savings. This paper examines technology options in the power-limited-scaling regime and reviews sensitivity-based analysis that can be used for the optimal selection of optimal architectures and circuit implementations to achieve the best performance under power constraints. These tradeoffs are examined in the context of power minimization at the technology, circuit, logic, and architecture levels, both at the design and run times.     

A New Probability Density Function Enhancing Packet Detection Analysis for Low-SNR Links

Packet detection is the first task that a receiver has to perform in a random access communication scheme. The implementation and evaluation of different packet detection methods depend on the probability density functions (PDFs) of the decision variables and their construction process. This paper provides a new PDF that enables the analysis of one method, due to Schmidl and Cox (SC), in packet detection. The new PDF enables determination of the detection threshold and performance evaluation. The new PDF also avoids the Gaussian assumption in the low-signal-to-noise-ratio (SNR) regime that is typical in the ultrawideband systems and converges to Gaussian for high SNR. Comparison with other packet detection methods shows that the SC method is both robust to channel multipath and provides a good tradeoff between performance and signal processing complexity     

Low-SNR limit of the Cramer-Rao bound for estimating the time delayof a PSK, QAM, or PAM waveform

In this letter we consider the Cramer-Rao bound (CRB) for the estimation of the time delay of a noisy linearly modulated signal with random data symbols and random carrier phase. Because of the presence of the nuisance parameters (i.e., data symbols and carrier phase), a closed-form expression of this CRB is very hard to obtain for arbitrary PSK, QAM or PAM constellations and a band-limited transmit pulse. Instead, we derive a simple expression for the limit of the CRB at low signal-to-noise ratio (SNR), which is a relevant benchmark for timing recovery algorithms operating at small E_s/N₀     

Coherence bandwidth in turbulence

Two statistics — coherency and spectrum of amplitude difference — are used to estimate bandwidth limitations for radio wave propagation in a turbulent atmosphere. The statistics are used to estimate the available bandwidth for radio frequencies in close proximity with the 60 GHz molecular oxygen resonant line. Predicted results of .01 % for the rms differential amplitude variations are obtained for a fluctuation frequency passband from 10^?5 Hz to 10^?1 Hz for a radio frequency passband of 5 GHz centered at 60 GHz and a 1 km path length.     

利用线性相位模型提高干涉SAR图像相干性及改进相干性估计

本文提出一种对干涉合成孔径雷达(干涉SAR)处理中单视复图像(SLC)进行地形自适应预滤波的方法,提高从不同入射角获得的SAR图像之间的空间相干性,并同时改进相干性估计.通过Chirp-Z变换进行干涉图局部频率估计,从而提取干涉图线性相位模型,并利用其进行距离和方位向的地形自适应预滤波.由于滤波器参数随地形进行调整,其性能优于仅仅依赖轨道参数或平坦地球相位估计的固定带宽非自适应滤波器.文中对SIR-C/X-SAR的Etna火山干涉数据同时进行了距离和方位向地形自适应处理,利用相干系数直方图对该方法与固定带宽非自适应滤波方法的性能作了比较.最后,本文还应用线性相位模型改进相干性估计,得到了去除地形变化因素后的相干系数图.     

Bit-Interleaved Coded Modulation in the Wideband Regime

The wideband regime of bit-interleaved coded modulation (BICM) in Gaussian channels is studied. The Taylor expansion of the coded modulation capacity for generic signal constellations at low signal-to-noise ratio (SNR) is derived and used to determine the corresponding expansion for the BICM capacity. Simple formulas for the minimum energy per bit and the wideband slope are given. BICM is found to be suboptimal in the sense that its minimum energy per bit can be larger than the corresponding value for coded modulation schemes. The minimum energy per bit using standard Gray mapping on $M$-PAM or $M^2$ -QAM is given by a simple formula and shown to approach ${-}$ 0.34 dB as $M$ increases. Using the low SNR expansion, a general tradeoff between power and bandwidth in the wideband regime is used to show how a power loss can be traded off against a bandwidth gain.      

低信噪比缓动点目标的序贯检测算法的分析和改进

本文主要讨论的是低信噪比红外图像序列中缓动点目标的实时检测问题.在对传统序贯假设检测(正推法)进行了分析后,针对正推法的轨迹交叉和初始点多而引起的组合爆炸等问题,本文给出了一种基于逆推法的统计检测算法,并对如何利用目标的缓动特性和分段技术来提高信噪比,如何表示轨迹等进行了论述.实验结果表明,该算法有较好的性能.     

基于多视InSAR相干性估计误差模型的相干性迭代估计

文章全面分析了引起InSAR多视相干性估计误差的因素,建立了与区域增长相干性估计方法对应的多视InSAR相干性估计误差模型,提出了一种新的基于该误差模型的相干性迭代估计方法.该方法首先利用基于强度图像的区域长原理计算相干性,区域增长保证窗里的所有像素属于同一分布,从而能够消除不同分布样本导致的相干性估计误差;后利用所建立的多视相干性与真实相干性的非线性模型对所得的相干性估计值进行高斯牛顿迭代,迭代可以减少由于相性太低和估计样本太少导致的相干性估计误差,得到更加准确的相干性估计.     

Coherence resonance in chaotic electronic circuits

It is experimentally demonstrated that a chaotic electronic circuit exhibits the main features of coherence resonance. This phenomenon shows up as a quasi-periodic response when the circuit is subjected to an external white-noise source of the appropriate level     

光学相干层析成像中散斑噪声减小算法

干涉测量中的散斑噪声极大地限制了光学相干层析(OCT)成像中特征信息的提取,因此,如何减小散斑噪声成为OCT成像领域的一个重要问题.利用最小化Csiszar Ⅰ-散度的测量方法推出一种散斑最小化算法,不仅可以生成与测量数据一致的图像数据,而且可以推出图像的附加信息,展示了采用散斑最小化算法对人手指正常皮肤的OCT图像散斑噪声最小化处理结果,结果表明,此算法不仅可以极大地减小散斑噪声、提高信噪比,而且能保持清晰的边缘效果.     

Low-SNR limit of the Cramer-Rao bound for estimating the carrierphase and frequency of a PAM, PSK, or QAM waveform

We consider the Cramer-Rao bound (CRB) for the estimation of the carrier phase and frequency of a noisy linearly modulated signal with random data symbols. The observation vector consists of the matched filter output samples taken at the symbol rate, assuming known symbol timing. Because of the presence of the random data, the evaluation of this CRB is quite tedious. Instead, here we derive a simple closed-form expression for the limit of the CRB at low-signal-to-noise ratio (SNR), which holds for arbitrary PAM, PSK, and QAM constellations