Improvement in signal-to-noise ratio of Rayleigh backscatteringmeasurement using optical low coherence reflectometry

I have calculated the signal-to-noise ratio (S/N) of fluctuations remaining in Rayleigh backscattered signal distributions obtained after performing moving and frequency averaging with an optical low coherence reflectometer (OLCR). From the calculation, I obtained the number of measurements needed for each averaging that achieves the required S/N with the minimum sampling data. Specifically, I derived the numbers of measurements numerically for Gaussian and Lorentzian light spectra, respectively. The result was successfully applied to diagnosis on long silica-based waveguides by using an OLCR with a high-power narrow-band light source     

An enhanced first-order sigma-delta modulator with a controllable signal-to-noise ratio

This paper presents an enhanced first-order sigma-delta modulator. The proposed modulator, derived using the Al-Alaoui operator, outperforms the conventional modulator. A comparison is drawn showing that the conventional sigma-delta modulator is a special case of the enhanced sigma-delta modulator. The paper includes an analytical derivation as well as extensive simulations revealing the superiority of the proposed modulator reaching optimal performance.     

干涉法测量中的信噪比

本文根据现代随机介质中波的传播理论,从二阶互相干函数出发导出了干涉图信噪比的分析表达式,对不同情况下的结果作了讨论,表明信噪比决定于光束的波长与宽度,传播距离和湍流强度等因素。在一定条件下信噪比将趋向于一个常数,可以用强度起伏的饱和效应来解释这个现象。     

Optimization of directivity and signal-to-noise ratio of an arbitrary antenna array

In this paper, a unified formulation is made for the optimization of directivity and signal-to-noise ratio of an arbitrary array, with or without a constraint on the array Q-factor. When there is a constraint, the solution is reduced to that of a polynomial; when there is no constraint, the solution is given in a very simple form. First, it is shown that for a given array geometry there exists a finite permissible range of the Q-factor and this range reduces to zero for large spacings. Second, a detailed comparison between four well-known excitations (uniform, Hansen-Woodyard, optimum cophasal, and optimum) is made and the main results are as follows, 1) The Hansen-Woodyard excitation yields a directivity higher than that of the uniform only when the element spacing is somewhat smaller than a half wavelength (λ/2), but at the price of much higher Q. On the other hand, it is much lower than that of the optimum excitation. 2) For spacing less than λ/2, the optimum excitation is strongly tapered toward the ends of the array and approximately antiphasa (i.e., 0, 180° 0, 180°, ...); whereas for spacing greater than or equal to λ/2, it is nearly uniform and cophasal. 3) For large spacings, the directivity of uniform excitation is nearly optimum. For small spacings, the optimum directivity becomes much higher than all others, but is always associated with an enormously large Q-factor. Therefore in this case a constraint of the Q-factor is important. 4) Hansen-Woodyard and uniform arrays have the interesting property that their sensitivity factors are independent of spacing. The optimization of signal-to-noise ratio is also demonstrated. In particular, the result shows that although an improvement in gain over the uniform excitation is very difficult to realize in practice, a substantial improvement in signal-to-noise ratio is entirely practical. Other numerical results and some extensions of the theory to aperture antennas are also included.     

Architectures with improved signal-to-noise ratio in photonicsystems with fiber-loop delay lines

In a class of photonic systems, the intensity of the noise component caused by crosstalk from internal signals is investigated. The relevant systems are those that use loops of optical fiber as reentrant delay-line memory elements. Three architectural variations are shown to improve the ratio of the intensity of an internal signal to the intensity of this noise component. These variations include providing one extra memory element, minimizing the number of switches in a memory element that a photonic signal must traverse, and designing large systems hierarchically from smaller ones. These architectures, and the detailed formulas used to evaluate them, pertain specifically to a photonic time-slot interchanger, but the concepts are believed to generalize to similar photonic systems with applications in computing and signal processing     

Encoding with frames in MRI and analysis of the signal-to-noise ratio

Recently, many new encoding techniques have been suggested for magnetic resonance imaging and an important image reconstruction problem has been raised in order to fully exploit the advantages promised by these new encoding techniques. In terms of frames in an L2 (R) space, we treat these encoding techniques in a unified perspective and propose a solution for this image reconstruction problem. We first develop a matrix form of frame theory and apply it to numerically construct the duals of the encoding functions, which are the transverse excitation profiles generated by radio-frequency pulses along a linear magnetic field gradient. An analysis of the signal-to-noise ratio (SNR) is also presented. Simulations have been carried out and they show that our image reconstruction algorithm minimizes the mean square error between the original and reconstructed images. The SNRs evaluated from simulations agree with our theoretical predictions.     

An alternative derivation for the signal-to-noise ratio of a SSMAsystem

We present an alternative derivation for the signal-to-noise ratio (SNR) of a phase-coded spread-spectrum multiple access (SSMA) system by use of ensemble-average approach. We show that our formula can be explicitly expressed in terms of system parameters and the expression for the SNR derived by Pursley (1977) is just a special case when the user number is very large     

Enhancing the signal-to-noise ratio of ICA-based extracted ERPs

When decomposing single trial electroencephalography it is a challenge to incorporate prior physiological knowledge. Here, we develop a method that uses prior information about the phase-locking property of event-related potentials in a regularization framework to bias a blind source separation algorithm toward an improved separation of single-trial phase-locked responses in terms of an increased signal-to-noise ratio. In particular, we suggest a transformation of the data, using weighted average of the single trial and trial-averaged response, that redirects the focus of source separation methods onto the subspace of event-related potentials. The practical benefit with respect to an improved separation of such components from ongoing background activity and extraneous noise is first illustrated on artificial data and finally verified in a real-world application of extracting single-trial somatosensory evoked potentials from multichannel EEG-recordings.     

Degradation of signal-to-noise ratio due to amplitude distortion

The effect of filtering on the signal-to-noise ratio (SNR) of a coherently demodulated band-limited signal is determined in the presence of worst-case amplitude ripple. The problem is formulated as an optimization in the Hilbert space L₂. The form of the worst-case amplitude ripple is specified, and the degradation in the SNR is derived in closed form. It is shown that, when the maximum passband amplitude ripple is 2δ (peak-to-peak), the SNR is degraded by at most (1-δ²), even when the ripple is unknown or uncompensated. For example, an SNR loss of less than 0.01 dB due to amplitude ripple can be assured by keeping the amplitude ripple under 0.42 dB     

Multiple integration method for a high signal-to-noise ratio readout integrated circuit

A multiple integration method is reported that greatly improves the signal-to-noise ratio (SNR) for applications with a high-resolution infrared (IR) focal plane array. The signal from each pixel is repeatedly sampled into an integration capacitor and then output and summed into an outside memory that continues for n read cycles during each period of a frame. This method increases the effective capacity of the charge integration and improves sensitivity. Because a low-noise function block and high-speed operation of the readout circuit is required, a new concept is proposed that enables the readout circuit to perform digitization by a voltage skimming method. The readout circuit was fabricated using a 0.6-/spl mu/m CMOS process for a 64/spl times/64 midwavelength IR HgCdTe detector array. The readout circuit effectively increases the charge storage capacity to 2.4/spl times/10/sup 8/ electrons and then provides a greatly improved SNR by a factor of approximately 3.     

荧光法成像系统信噪比的实验研究

在药物荧光成像系统中较好的荧光像的信噪比和分辨本领,可最大限度地克服肿瘤诊断中出现的假阳性,降低误诊率。理论分析并近似计算了用蓝光作为诊断早期鼻咽癌的激发光源时,信号和背景光的亮度分别为4.5×10-2lm·m-2·sr-1和877lm·m-2·sr-1。结果表明,为了能从背景中分辨出信号,窄带栅滤光片的荧光透过率应尽可能大,而背景光要截止到原来的4.3×10-5的量级。根据有色玻璃的吸收特性,实验时适当增加光在玻璃中的行程和带宽,使信号和背景光的透过率之比增大,从而得到较好的荧光像的信噪比和分辨本领,提高了图像目视探测的灵敏度。     

VSNR: a wavelet-based visual signal-to-noise ratio for natural images.

This paper presents an efficient metric for quantifying the visual fidelity of natural images based on near-threshold and suprathreshold properties of human vision. The proposed metric, the visual signal-to-noise ratio (VSNR), operates via a two-stage approach. In the first stage, contrast thresholds for detection of distortions in the presence of natural images are computed via wavelet-based models of visual masking and visual summation in order to determine whether the distortions in the distorted image are visible. If the distortions are below the threshold of detection, the distorted image is deemed to be of perfect visual fidelity (VSNR = infinity) and no further analysis is required. If the distortions are suprathreshold, a second stage is applied which operates based on the low-level visual property of perceived contrast, and the mid-level visual property of global precedence. These two properties are modeled as Euclidean distances in distortion-contrast space of a multiscale wavelet decomposition, and VSNR is computed based on a simple linear sum of these distances. The proposed VSNR metric is generally competitive with current metrics of visual fidelity; it is efficient both in terms of its low computational complexity and in terms of its low memory requirements; and it operates based on physical luminances and visual angle (rather than on digital pixel values and pixel-based dimensions) to accommodate different viewing conditions.     

Evaluation of signal-to-noise and distortion ratio degradation in nonlinear systems

This paper presents a new figure-of-merit to evaluate signal-to-noise and distortion (SINAD) ratio degradation in nonlinear systems, herein referred to as the noise and distortion figure (NDF). In order to obtain a mathematical formula for this NDF, the best linear approximation calculation is presented for memoryless and dynamic nonlinear systems, which can be modeled by a finite Volterra series. To the best of the authors' knowledge, this is the first time such an attempt of calculating the NDF for a nonlinear and dynamic system is made. NDF results are discussed in both types of systems by means of numerical simulations of systems up to the third order.     

Prediction of the standard deviation of the signal-to-noise ratio in a data-transmission system with orthogonal subcarrier frequencies

Expressions for prediction of the standard deviation of the signal-to-noise ratio (SNR) in a wireless data-transmission network in which orthogonal frequency division multiple access (OFDMA) technology of data transmission is used are obtained. It is shown that the proposed prediction is more accurate than the predictions based on the mean value.     

调制相位差对TDLAS信噪比的影响

用可调谐二极管激光吸收光谱技术检测气体时,不同的调制相位差结果不同。为了提高谐波检测灵敏度,采用同时考虑频率调制与振幅调制的方法分析和提取二次谐波信号,推导了相位差与二次谐波信号光电流和残余幅度调制噪声光电流的函数关系表达式,通过模拟实验分别取得了二次谐波信号和残余幅度调制噪声随相位差的变化趋势,并重点分析了相位差对信噪比的影响。结果表明,振幅调整和频率调制之间的相位差对信噪比的影响很大,与典型值/2时相比,最大降幅为59.18%;调制相位差是继残余幅度调制之外,影响信噪比的一个重要因素。     

低信噪比下雷达回波谱矩快速估计方法

针对在低信噪比下机载气象雷达回波多普勒参数（谱矩）估计不准确的问题，本文在气象目标的雷达回波频谱服从高斯分布的基础上，给出了一种利用协方差矩阵分解的快速参数化谱矩估计算法。通过理论分析，推导出雷达回波的协方差矩阵具有范德蒙结构特性，进而将用于谱矩估计的代价函数转化为类傅里叶变换结构，然后进一步通过快速傅里叶变换和高斯加权滑窗计算代价函数，实现快速的谱矩估计。仿真实验结果表明，该方法在信噪比低于5 dB时仍可以有效估计雷达回波的谱矩参数，同时运算复杂度大大降低，而且在谱宽值较大情况下仍能保持较好的估计性能。      

硅光电开关应用之一——隔离比和信噪比的测量

根据微微秒脉冲作用半导体的光电特性,我们已成功地研制了几种开关元件,它的上升时间、光电灵敏度、抖动等,优越于其他开关元件。 利用硅光电开关,对锁模选脉冲开关的隔离比和激光信噪比进行测量,显示了测量方法简便、可靠和灵敏度高的优点,并且能够测出微小光信号的具体能量值。 报告分为三部分。首先介绍硅开关结构及光电特性的简单计算和测量。算得在微小光能～10微焦耳照射时,可流过电流达～10安培,而实验测得硅开关在10微焦耳光能下,电导通已接近饱和,证实了硅开关的光电灵敏特性。实验中详细测得其光电特性曲线,同时拍得开