激光器重复频率对多普勒激光雷达信噪比影响的模拟研究

多普勒激光雷达在风场测量和数值天气预报等方面有重要意义,而信噪比则是衡量系统测量精度的重要参数.以2007年中国海洋大学研制的我国第一台车载多普勒测风激光雷达系统为参考,模拟了在相同平均功率下,使用4种重复频率不同的激光器的激光雷达系统测量海面风场中的信噪比,并与汕尾测量的海面风场数据进行了对比,模拟数据和实测数据能够较好吻合,模拟结果证明使用低重复频率激光器的激光雷达系统能够获得较高的信噪比.而根据多普勒激光雷达业务测量要求,要求比较高的激光重复频率,通过模拟研究,建议选择激光器激光重复频率500～1000 Hz.     

Feature classification criterion for missing features mask estimation in robust speaker recognition

Currently, many speaker recognition applications must handle speech corrupted by environmental additive noise without having a priori knowledge about the characteristics of noise. Some previous works in speaker recognition have used the missing feature (MF) approach to compensate for noise. In most of those applications, the spectral reliability decision step is performed using the signal to noise ratio (SNR) criterion, which attempts to directly measure the relative signal to noise energy at each frequency. An alternative approach to spectral data reliability has been used with some success in the MF approach to speech recognition. Here, we compare the use of this new criterion with the SNR criterion for MF mask estimation in speaker recognition. The new reliability decision is based on the extraction and analysis of several spectro-temporal features from across the entire speech frame, but not across the time, which highlight the differences between spectral regions dominated by speech and by noise. We call it the feature classification (FC) criterion. It uses several spectral features to establish spectrogram reliability unlike SNR criterion that relies only in one feature: SNR. We evaluated our proposal through speaker verification experiments, in Ahumada speech database corrupted by different types of noise at various SNR levels. Experiments demonstrated that the FC criterion achieves considerably better recognition accuracy than the SNR criterion in the speaker verification tasks tested.     

Combined MR data acquisition of multicontrast images using variable acquisition parameters and K-space data sharing

A new technique to reduce clinical magnetic resonance imaging (MRI) scan time by varying acquisition parameters and sharing k-space data between images, is proposed. To improve data utilization, acquisition of multiple images of different contrast is combined into a single scan, with variable acquisition parameters including repetition time (TR), echo time (TE), and echo train length (ETL). This approach is thus referred to as a "combo acquisition." As a proof of concept, simulations of MRI experiments using spin echo (SE) and fast SE (FSE) sequences were performed based on Bloch equations. Predicted scan time reductions of 25%-50% were achieved for 2-contrast and 3-contrast combo acquisitions. Artifacts caused by nonuniform k-space data weighting were suppressed through semi-empirical optimization of parameter variation schemes and the phase encoding order. Optimization was assessed by minimizing three quantitative criteria: energy of the "residue point spread function (PSF)," energy of "residue profiles" across sharp tissue boundaries, and energy of "residue images." In addition, results were further evaluated by quantitatively analyzing the preservation of contrast, the PSF, and the signal-to-noise ratio. Finally, conspicuity of lesions was investigated for combo acquisitions in comparison with standard scans. Implications and challenges for the practical use of combo acquisitions are discussed.     

Real-time adaptive filtering for projection reconstruction MR fluoroscopy

Magnetic resonance (MR) imaging has faced a dramatic increase in real-time capabilities over the last years. However, the application of fast pulse sequences still suffers from low signal-to-noise ratios (SNRs), which can be the limiting factor for the actual acquisition speed. In MR fluoroscopy, filtering along the time and/or spatial domain can be applied to increase the image quality. In this paper, a projection-based noise filter is presented that significantly enhances the SNR in projection reconstruction (PR) fluoroscopy without apparent loss of resolution in the reconstructed images. In contrast to an imaged-based approach, this method allows a very efficient computational implementation. The filter algorithm was implemented on a digital signal processor and was applied to real-time processing during PR fluoroscopy. A quantitative analysis of the improvement in SNR and results for different fluoroscopic MR applications are given. Apart from MR fluoroscopy, the proposed technique has the potential to be applied to low dose computed tomography fluoroscopy.     

Analysis of UWB transmitted-reference communication systems in dense multipath channels

Transmitted-reference (TR) signaling, in conjunction with an autocorrelation receiver (AcR), offers a low-complexity alternative to Rake reception. Due to its simplicity, there is renewed interest in TR signaling for ultrawide bandwidth (UWB) systems. To assess the performance of these systems, we develop an analytical framework based on the sampling expansion approach. In particular, we derive closed-form expression for the bit-error probability (BEP) of TR signaling with AcR that can be used to exploit multipath diversity inherent in wideband channels. We further extend our analysis to the BEP derivation of modified AcR with noise averaging. Our methodology does not require the Gaussian approximation and is applicable for any fading scenario, provided that the correlator output signal-to-noise ratio (SNR) can be characterized in terms of a characteristic function. We show that the validity of the conventional Gaussian approximation depends on the time-bandwidth product and the number of transmitted pulses per symbol. Our results enable the derivation of a computationally simple lower bound on the BEP of TR signaling with AcR. This lower bound allows us to obtain the SNR penalty associated with an AcR, as compared with All-Rake and Partial-Rake receivers.     

High Diversity Transceiver for Low Power Differentially Encoded OFDM System

In this work, we investigate differentially encoded blind transceiver design in low signal‐to‐noise ratio (SNR) regimes for orthogonal frequency‐division multiplexing (OFDM) signaling. Owing to the fact that acquisition of channel state information is not viable for short coherence times or in low SNR regimes, we propose a time‐spread frequency‐encoded method under OFDM modulation. The repetition (spreading) of differentially encoded symbols allows us to achieve a target energy per bit to noise ratio and higher diversity. Based on the channel order, we optimize subcarrier assignment for spreading (along time) to achieve frequency diversity of an OFDM modulated signal. We present the performance of our proposed transceiver design and investigate the impact of Doppler frequency on the performance of the proposed differentially encoded transceiver design. To further improve reliability of the decoded data, we employ capacity‐achieving low‐density parity‐check forward error correction encoding to the information bits.     

一种基于距离-多普勒二维联合的群目标分辨方法

编队飞机由于间距很小,常规低分辨雷达无法直接从距离和方位上分辨架次.本文提出一种基于距离—多普勒二维联合处理的群目标分辨方法.发射线性调频脉冲信号,将观测时间内的若干次回波在距离维和多普勒维分别转换为单频信号的叠加,再联合距离－多普勒二维信息估计协方差矩阵,用超分辨信源数估计算法——新的改进盖氏圆盘法分辨目标架次.这种方法可以直接地给出判决结果,适用于各种飞行状态、低信噪比环境或波束驻留时间较短的情况,而且和重频参差工作模式相兼容,尤其适合中、高重频工作方式.     

隐含训练序列信道估计中的功率分配

与传统时分发送训练序列的信道估计算法相比,隐含训练序列信道估计算法将训练序列与信息序列直接相加后通过天线发送,从而节约了信道带宽。然而,在天线发送总功率一定时,训练序列的功率越大,信息序列的功率便越小,从而导致信道均衡器的信噪比减小。本文研究了基于MIMO系统的隐含训练序列信道估计算法,分析了信道均衡器信噪比与训练序列功率的关系,并根据均衡器信噪比最大原则推导出训练序列与信息序列的最佳功率分配。分析和仿真结果表明:在训练序列的最佳功率点上,信道均衡器的信噪比最高;随着接收天线信噪比的增加,训练序列的最佳功率增大。     

Performance of a Multi-hop UWB Transmitted Reference System using Decode-and-Forward Relays

In this paper, end-to-end average bit error rate (ABER) of a multi-hop decode-and-forward relay system is evaluated using ultra-wideband transmitted reference (TR) receiver over a multi-path fading channel. Distribution of individual hop signal to noise ratio (SNR) is approximated by a log-normal distribution and corresponding ABER is evaluated by Gauss-Hermite Quadrature rule. These individual hop ABERs are then used to find the end-to-end ABER performance analytically which is faster than the simulation method available in the literatures. Performances of three variants of non-coherent TR receivers: simple transmitted reference, average transmitted reference and differential transmitted reference (DTR) receivers are compared assuming same total transmit power for a fixed end-to-end distance in line-of-sight (LOS) and non line-of-sight (NLOS) channel model. It is observed that the end-to-end ABER performance using DTR receivers is the best and multi-hop relaying is also most effective using these receivers to improve the performance. It is also found that the laws of diminishing returns hold for performance gain in average transmit SNR per bit at a particular ABER in LOS channel i.e. performance improvement in terms of transmit SNR is not equal as the number of hop increases. It decreases with increase in hop number but the reverse trend can be found in NLOS channel.     

日光对532nm激光成像性能影响实验分析

532 nm激光成像雷达在成像过程中容易受到日光干扰而降低图像信噪比。利用532 nm条纹管激光雷达对复杂场景(建筑物群)进行昼夜成像实验,并采用直方图统计法分析同一场景分别在白昼和夜晚的激光强度像和距离像的像素值分布,进而分析日光对激光成像性能影响。实验结果表明:日照光会提高激光强度像的亮度以及背景/目标的对比度,同时也增加了距离像的空天背景噪声;夜晚激光距离像信噪比高,但目标强度像对比度不佳。     

Enhanced SNR estimates from direct-sequence spread-spectrum demodulator statistics

Demodulator statistics are used in a binary directsequence spread-spectrum receiver to improve the performance of a well-known signal-to-noise ratio (SNR) estimate for ratios of the binary symbol energy to one-sided noise density that are below approximately 7 dB. The new estimate requires no pilot symbols, training sequences, data symbol decisions, or channel measurements, and it can be obtained prior to decoding.     

Security aware hybrid precoding based peak‐to‐average power ratio reduction Approaches for telecommunication systems

In wireless telecommunication, one of the modulation approaches used is the orthogonal frequency division multiplexing (OFDM). Moreover, the high peak‐to‐average power ratio (PAPR) is the one notorious demerit in OFDM systems. Hence, the high power amplifier (HPA) is used in its linear region. Otherwise, the bit error rate (BER) will be increased. Several approaches are proposed in the wireless communications for reducing the PAPR issue. In this paper, we propose a hybrid Discrete Hartley Matrix Transform (DHMT) precoding using both selected mapping (SLM) and partial transmit sequence (PTS) PAPR reduction strategies. For the multicarrier modulation process, instead of Inverse Fast Fourier transform (IFFT) operation, the DHMT operation is used because of its low computational complexity. Based on multi‐chaotic, the time‐frequency domain encryption (TFDE) approach is adopted for physical layer security to confirm the security in data transmission. For enhancing the physical layer security, the proposed encryption system generates chaotic sequences based on Logistic maps and Lozi in the frequency and time domains together. In this study, the implementation of the DHMT‐based OFDM system is processed to reduce the maximum PAPR. Implementation is performed on the MATLAB platform, and the performances are calculated using complementary cumulative distribution function (CCDF), BER regards to signal‐to‐noise ratio (SNR), and the outputs are compared based on the computation time. However, compared with the existing models, the proposed model produced better PAPR minimization regarding SNR.     

Clinical DT-MRI estimation, smoothing, and fiber tracking with log-Euclidean metrics

Diffusion tensor magnetic resonance imaging (DT-MRI or DTI) is an imaging modality that is gaining importance in clinical applications. However, in a clinical environment, data have to be acquired rapidly, often at the expense of the image quality. This often results in DTI datasets that are not suitable for complex postprocessing like fiber tracking. We propose a new variational framework to improve the estimation of DT-MRI in this clinical context. Most of the existing estimation methods rely on a log-Gaussian noise (Gaussian noise on the image logarithms), or a Gaussian noise, that do not reflect the Rician nature of the noise in MR images with a low signal-to-noise ratio (SNR). With these methods, the Rician noise induces a shrinking effect: the tensor volume is underestimated when other noise models are used for the estimation. In this paper, we propose a maximum likelihood strategy that fully exploits the assumption of a Rician noise. To further reduce the influence of the noise, we optimally exploit the spatial correlation by coupling the estimation with an anisotropic prior previously proposed on the spatial regularity of the tensor field itself, which results in a maximum a posteriori estimation. Optimizing such a nonlinear criterion requires adapted tools for tensor computing. We show that Riemannian metrics for tensors, and more specifically the log-Euclidean metrics, are a good candidate and that this criterion can be efficiently optimized. Experiments on synthetic data show that our method correctly handles the shrinking effect even with very low SNR, and that the positive definiteness of tensors is always ensured. Results on real clinical data demonstrate the truthfulness of the proposed approach and show promising improvements of fiber tracking in the brain and the spinal cord.     

Wavelet-Domain Medical Image Denoising Using Bivariate Laplacian Mixture Model

In this paper, we proposed novel noise reduction algorithms that can be used to enhance image quality in various medical imaging modalities such as magnetic resonance and multidetector computed tomography. The noisy captured 3-D data are first transformed by discrete complex wavelet transform. Using a nonlinear function, we model the data as the sum of the clean data plus additive Gaussian or Rayleigh noise. We use a mixture of bivariate Laplacian probability density functions for the clean data in the transformed domain. The MAP and minimum mean-squared error (MMSE) estimators allow us to efficiently reduce the noise. The employed prior distribution is mixture and bivariate, and thus accurately characterizes the heavy-tail distribution of clean images and exploits the interscale properties of wavelets coefficients. In addition, we estimate the parameters of the model using local information; as a result, the proposed denoising algorithms are spatially adaptive, i.e., the intrascale dependency of wavelets is also well exploited in the enhancement process. The proposed approach results in significant noise reduction while the introduced distortions are not noticeable as a result of accurate statistical modeling. The obtained shrinkage functions have closed form, are simple in implementation, and efficiently enhances data. Our experiments on CT images show that among our derived shrinkage functions usually BiLapGausMAP produces images with higher peak SNR. However, BiLapGausMMSE is preferred especially for CT images, which have high SNRs. Furthermore, BiLapRayMAP yields better noise reduction performance for low SNR MR datasets such as high-resolution whole heart imaging while BiLapGauMAP results in better performance in MR data with higher intrinsic SNR such as functional cine data.      

微脉冲差分吸收激光雷达CO2探测性能研究

为了研究差分吸收激光雷达低空CO2浓度探测效率,分析了探测系统的原理与信噪比,采用近红外微脉冲激光雷达进行了仿真实验,当激光脉冲能量为20μJ、重复频率为10kHz时,推算出信噪比、所需积累时间与探测距离的关系。在不考虑背景噪声情况下(夜间工作),可进行4.5km以下CO2气体浓度的探测;考虑背景噪声情况下(白天工作),只能探测2.4km以下的CO2气体浓度。结果表明,选择夜间探测和更好的探测器件可以提高探测距离。     

工作参数对激光诱导土壤等离子体光谱特性的影响

采用波长1064 nm的调Q脉冲Nd:YAG激光器和多通道小型光纤光栅光谱仪,搭建了一套激光诱导击穿光谱分析系统。选择土壤中常见元素AlⅡ(422.68 nm)作为分析线,详细研究激光能量和采样延迟对激光诱导土壤等离子体光谱特性的影响。在相同激光能量下,随着采样延迟时间增加时,信号强度、背景强度、噪声都将变小,而SNR则呈现先增大后减小的趋势;在相同采样延迟时间下,增加激光能量,信号强度、噪声也将增强,而背景强度和信噪比的变化则呈现先增加后减小的趋势。对于某一定的激光能量, 存在一个与之相对应最佳采样延迟时间,随着激光能量增加时,最佳延迟时间也会增大。综合考虑采样延迟时间和激光能量对激光诱导等离子体光谱信噪比的影响,给出了系统的最优化工作参数是激光能量120 mJ、最佳采样延迟时间1.5 μs。     

多通道TDICCD卫星遥感相机图像噪声分析与处理

为了解决多路时间延迟积分电荷耦合器件(TDICCD)拼接遥感相机成像的噪声问题,提高载荷焦面电箱的成像信噪比和成像质量,通过分析噪声现象,发现多路TDICCD成像噪声的主要原因有两个:一是多通道TDICCD未能实现同时刻开启成像,使得成像电路电源出现高频纹波噪声且多通道噪声相混叠,这样对各通道CCD模拟信号造成干扰;二是电路中的DC-DC电源开关噪声扰动影响了CCD有效视频信号的采集。从工程研制实际出发,采取CCD通道之间共用统一的系统时钟,相同系统复位等措施对多TDICCD成像电路系统进行改进,抑制通道间成像串扰的发生,通过对主要干扰源DC-DC电源进行滤波处理与TDICCD端多点接地等方式,抑制电源噪声对TDICCD成像的干扰。对改进后的多通道TDICCD成像电路系统进行成像和信噪比测试。实验结果表明,采取的措施有效地去除了TDICCD成像噪声,相机信噪比提高了2 dB,且外场成像质量高,可满足实际工程的需求。     

光学相干层析系统的信噪比分析及优化

为提高光学相干层析(OCT)系统的信噪比(SNR),改进系统的探测灵敏度,保证系统的成像质量,从理论上详细分析了光学相干层析成像系统中的主要噪声源,建立了系统噪声的理论模型,分析了光学相干层析成像系统中的各个组成单元对系统信噪比的影响.建立了一套实用型的光学相干层析成像系统,对该探测系统中的噪声进行了测量,得到系统噪声的实验模型.然后对理论分析的结果进行一定的修正,并对实验系统进行优化,得到了16μm的纵向分辨率,-90 dB的探测灵敏度.     

激光主动成像图像降噪方法

针对激光主动成像图像的特点,并考虑到实时性的要求,提出了一种基于中值滤波与提升小波变换级联的图像降噪方法。首先在空域中对图像进行快速中值滤波,然后对含噪图像进行提升小波分解,最后在小波域内对高频子带进行阈值降噪处理。采用信噪比（SNR）、运行时间T和图像灰度曲面图作为图像降噪效果的评估,将该算法与传统小波降噪、传统小波与中值滤波结合降噪等进行对比实验。实验结果表明,该算法能有效滤除激光图像中的噪声,提高图像的信噪比,且运算速度更快,同时满足了工程应用中对降噪效果和实时性的要求。     

Time hopped transmitted reference with multiple autocorrelation sampling for ultra wideband radio

Transmitted reference (TR) schemes for ultra wideband (UWB) eliminate the need for channel estimation, reducing receiver complexity at the cost of reduced performance. This letter proposes a transmitted reference (TR) scheme with multiple autocorrelation sampling detection. The proposed receiver captures the energy in the received signal's autocorrelation side lobes, outperforming the traditional TR scheme for the operational range of signal to noise ratio (SNR) values. Time hoping, as well as the use of bandwidth efficient signaling with favorable spectral characteristics, facilitate significant improvement in system capacity in comparison to a similar scheme using orthogonal chirp signals in multipath spread channels.