A novel timing synchronization for CO-OFDM systems using chirp signals

A fast and efficient timing offset correction is an essential function in the coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. The timing synchronization schemes for OFDM systems in wireless communications can be used for CO-OFDM systems. However, the performance of these schemes is reduced due to the chromatic dispersion. To fulfill the requirement of the CO-OFDM systems, a novel symbol timing synchronization scheme based on chirp signals has been proposed. The simulation results show that the proposed method is robust in CO-OFDM systems which is suffering from noise.     

均匀度估计的噪声源盲分离算法研究

盲信号分离技术是源输入未知时识别噪声源的一种有力手段。源的独立性常是应用盲分离算法的一个基本要求。但由于概率密度函数一般未知且估计繁琐,传统盲分离算法对源信号的独立性较难检验。为此,从信号独立性分析出发,理论上推导了随机变量的独立性和其概率分布函数的联合分布之间的关系,提出了一种独立性度量指标均匀度的估计算法,并给出了相应的盲分离算法。利用电机和海水泵的振动信号分离试验对方法进行了验证,并与现有的半熵盲分离算法进行比较,结果表明在分离效果和计算时间方面均优于现有的方法。充分说明了本文算法的有效性。      

相干光正交频分复用传输系统中的 无导频相位纠偏方法

提出了一种适用于16-QAM的新型相位纠偏方法,克服了基于M次方算法的传统相位纠错方法不适用于高阶正交振幅调制的缺点.该方法结合正交频分复用调制方式,无需导频即可完成相位盲估计,且计算量较少.本文基于112 Gb/s 1 040 km的相干光正交频分复用传输系统进行了算法验证,并与传统的4次方纠偏方法进行了比较,实验证明该算法适用于正交频分复用和16-QAM调制平台,相位纠偏结果显著优于基于导频的4次方算法.     

Blind frequency offset estimation based on cyclic prefix and virtual subcarriers in CO-OFDM system

A new blind frequency offset estimation method based on cyclic prefix and virtual subcarriers in coherent optical orthogonal frequency division multiplexing （CO-OFDM） system is presented. It is able to estimate the fractional part and integral part of frequency offset at the same time. Its estimation range is about [-3.5 GHz, 3.5 GHz]. The influence of the integral frequency offset is comprehensively analyzed in COOFDM system. Its performances in the additive white Gaussian noise （AWGN） channel and the dispersive channel are investigated, respectively. Simulation results indicate that even in the dispersive channel, when the optical signal-to-noise ratio （OSNR） is low, it can still work very well.     

Noise reduction of speech signals using time-varying and multi-band adaptive gain control for smart digital hearing protectors

In this paper, a single-channel speech enhancement algorithm based on non-linear and multi-band Adaptive Gain Control (AGC) is proposed. The algorithm requires neither Signal-to-Noise Ratio (SNR) nor noise parameters estimation. It reduces the background noise in the temporal domain rather than the spectral domain using a non-linear and automatically adjustable gain function for multi-band AGC. The gain function varies in time and is deduced from the temporal envelope of each frequency band to highly compress the frequency regions where noise is present and lightly compress the frequency regions where speech is present. Objective evaluation using the PESQ (Perceptual Evaluation of Speech Quality) metric shows that the proposed algorithm performs better than three benchmarks, namely: the spectral subtraction, the Wiener filter based on a priori SNR estimation and a band-pass modulation filtering algorithm. In addition, blind subjective tests show that the proposed algorithm introduces less musical noise compared to the benchmark algorithms and was preferred 78.8% of the time in terms of signal quality. The proposed algorithm is implemented in a miniature low power digital signal processor to validate its feasibility and complexity for smart hearing protection in noisy environments.     

Coherent optical OFDM scheme with inter-carrier interference self-cancellation and common phase error compensation

Schemes integrating inter-carrier interference （ICI） self-cancellation and common phase error （CPE） com- pensation for coherent optical orthogonal frequency division multiplexing （CO-OFDM） systems are investi- gated. The purpose of our research is to counteract the impacts of laser phase noise and fiber nonlinearity. We propose two ICI self-cancellation-based CO-OFDM schemes, and adopt a pilot-aided decision feedback （DFB） loop for CPE compensation. The proposed schemes are compared with conventional CO-OFDM schemes at the same spectral efficiency. Simulations show that our schemes can not only enhance laser linewidth tolerance of the CO-OFDM system, but also present strong robustness against fiber nonlinearity.     

Multiple trellis-coded modulation,per-survivor processing and Reed–Solomon coding in the presence of phase noise

The aim of this paper is to consider trellis coded modulation (TCM) in the presence of phase noise. The starting point is a two-dimensional 32 point cross (32-CR) signal set that has served as a benchmark for other power and bandwidth efficient modulation schemes. Per-survivor processing (PSP) is used for carrier phase estimation, and both the phase acquisition (transient state) and phase tracking (steady state) performances are considered in an additive white Gaussian noise (AWGN) channel, in which phase offset or time varying phase noise is introduced. Based on the theoretical and simulation results it can be said that the 32-CR constellation is itself more sensitive to a phase error rather than the PSP algorithm. Due to the phase error the TCM scheme becomes a catastrophic code. Possible solutions include stringent phase noise requirements for oscillators, multiple trellis-coded modulation (MTCM) and concatenated TCM and Reed–Solomon (RS) coding. Our simulation results show that a simple combined MTCM, PSP and RS scheme has around 3 dB advantage compared to the pure TCM and PSP in the presence of phase noise. A semianalytical approach is used for performance evaluation, and the method is a particularly convenient tool to determine the rate of RS coding in detail.     

Two-stage blind deconvolution scheme using useful priors

Hand shake blurry image is a common phenomenon in our daily life. In this paper, a novel blind deconvolution scheme is proposed to recover a single hand shake blurry image. The algorithm is subdivided into two main stages, kernel estimation stage and non-blind deconvolution stage. In the kernel estimation stage, we propose a cost function taking a selected map into consideration. In the non-blind decovolution stage, another cost function is designed using image derivatives prior. We also present an adaptive kernel size selection method instead of traditional manual selection. Extensive experiments on real world blurry images are conducted to demonstrate the performance of our algorithm.     

Improved Approach for the Maximum Entropy Deconvolution Problem

The probability density function (pdf) valid for the Gaussian case is often applied for describing the convolutional noise pdf in the blind adaptive deconvolution problem, although it is known that it can be applied only at the latter stages of the deconvolution process, where the convolutional noise pdf tends to be approximately Gaussian. Recently, the deconvolutional noise pdf was approximated with the Edgeworth Expansion and with the Maximum Entropy density function for the 16 Quadrature Amplitude Modulation (QAM) input but no equalization performance improvement was seen for the hard channel case with the equalization algorithm based on the Maximum Entropy density function approach for the convolutional noise pdf compared with the original Maximum Entropy algorithm, while for the Edgeworth Expansion approximation technique, additional predefined parameters were needed in the algorithm. In this paper, the Generalized Gaussian density (GGD) function and the Edgeworth Expansion are applied for approximating the convolutional noise pdf for the 16 QAM input case, with no need for additional predefined parameters in the obtained equalization method. Simulation results indicate that improved equalization performance is obtained from the convergence time point of view of approximately 15,000 symbols for the hard channel case with our new proposed equalization method based on the new model for the convolutional noise pdf compared to the original Maximum Entropy algorithm. By convergence time, we mean the number of symbols required to reach a residual inter-symbol-interference (ISI) for which reliable decisions can be made on the equalized output sequence.     

Effect of multipath and antenna diversity in MIMO-OFDM systems with imperfect channel estimation and phase noise compensation

The effect of phase noise in multiple-input–multiple-output systems employing orthogonal frequency division multiplexing is analyzed in a realistic scenario where channel estimation is not perfect, and the phase noise effects are only partially compensated. In particular, the degradation in terms of SNR is derived and the effects of the receiver and channel parameters are considered, showing that the penalty is different for different receiver schemes. Moreover it depends on the channel characteristics and on the channel estimation error. An analytical expression is used to evaluate the residual inter-channel interference variance and therefore the degradation. The effects of multipath and antenna diversity are shown to be different for the two types of linear receivers considered, the zero-forcing scheme and the minimum mean squared error receiver.     

基于小波变换的长距离光正交频分复用系统中四波混频引起的相位噪声分析

四波混频（FWM）效应是相干光正交频分复用（CO-OFDM）系统中最主要的非线性作用。分析基于小波变换的相干光正交频分（WT-OOFDM）系统原理,研究小波变换对FWM效应和光放大器自发辐射（ASE）引起的相位噪声的改善效果。数值结果表明,在采用常规G. 652光纤,100 Gbit/s的WT-OOFDM传输1500 km时,非线性相位噪声降低20%,总相位噪声降低15%。     

Phase-Measuring Algorithms to Suppress Spatially Nonuniform Phase Modulation in a Two-Beam Interferometer

Phase modulation of presently used phase-shifting interferometers is assumed to be spatially uniform across the observing aperture. However, calibration errors or the configuration of an interferometer can cause a spatial nonuniformity in the phase modulation. Spatial nonuniformity causes a significant error in the measured phase when the phase modulator has nonlinear sensitivity. An even-order nonlinearity in the phase modulation in particular contributes to the errors. Lowest-order errors can be suppressed by adding a new symmetry to the sampling functions of the phase-shifting algorithm, however the algorithm suffers from large random noise. The random noise is shown to be decreased substantially by applying one more sampled frame to the algorithm. We derive new seven-sample and eight-sample algorithms that can compensate for a nonuniform phase shift and has much less random noise than the previous algorithm we proposed.     

天文图像多帧盲反卷积收敛性的增强方法

天文图像多帧盲反卷积的收敛性受到初始目标、约束条件和光子噪声等因素的影响.提出了用实际光学成像系统参数确定频率带宽有限约束的方法.用Knox-Thompson 方法重构初始目标相位形成盲反卷积算法的初始目标函数.研究了一种新颖的有效减小光子噪声、边缘效应和振铃现象的方法.根据最大似然估计理论，用期望最大化的优化方法建立了改进的严格约束多帧盲反卷积算法.模拟图像和实际天文图像的复原结果表明，所建立的多帧盲反卷积，可以有效克服大气湍流和减小光子噪声，改善天文观察图像的分辨率，并部分消除光学系统衍射效应对恢复图像的影响. 关键词： 大气光学 天文观测 图像处理和恢复     

基于相位调制-解调的光纤激光相位噪声检测方法研究

在主动相位控制相干合成中,常用的光纤激光主动相位控制方法主要有外差法、多抖动法和随机并行梯度下降算法等. 基于多抖动法和外差法的原理,提出了一种新型的基于相位调制-解调的相位检测与控制方法. 该方法利用周期信号对参考光进行相位调制,将调制后的参考光与待检测信号光进行相干检测,通过对相干光强信号和相位调制信号的融合处理,实现相位噪声的检测和控制. 对该方法进行了数值模拟和实验研究. 实验结果表明,对于频率为2 kHz,噪声范围为[0,2π)的正弦噪声,相位检测精度优于λ/50,控制精度可达 关键词： 光纤激光 相干合成 相位噪声检测 调制-解调     

基于可靠性的相位去包裹算法

提出了一种新的基于可靠性的相位去包裹算法。采用了队列算法及去包裹可靠性的判别标准。与传统方法相比,能够自动避开噪声区域,解决了相位去包裹中的阴影遮挡以及空洞等问题,克服了传统去包裹方法中的误差传播效应,能准确快速地对全场进行相位去包裹。设计了一套基于此算法的三维形面测量系统。     

数字水印的鲁棒性评测模型

数字水印的鲁棒性是水印技术实用化的一个重要指标.与通过StirMark测试和各种仿真测试不同,定义互信息作为代价函数,建立水印系统鲁棒性描述和度量的一般模型.以扩频水印和量化水印为范例,推导出评测鲁棒性的互信息度量计算模型,并仿真分析了高斯噪声和JPEG压缩条件下互信息函数对鲁棒性的评估结果.实验以统计误比特率的方法计算图像DCT域中低频系数为载体的扩频水印误码率和一段音频数据三级小波细节系数为载体的量化水印误码率.互信息函数和误码率之间的匹配关系验证了互信息度量模型的有效性,互信息函数可以作为代价函数预测误码率的变化趋势. 关键词： 数字水印 鲁棒性 互信息 误码率     

语音通信降噪研究

语音通信系统中，语音通过信道传输将不可避免地引入码间串扰和信号畸变，同时受到噪声污染。本文在分析自适应盲均衡算法CMA(constant modulus algorithm）和改进盲均衡算法的基础上，考虑到自适应盲均衡技术在语音噪声控制方面能力有限，将自适应盲均衡技术与小波包掩蔽阈值降噪算法联合使用，形成一种基带语音增强新方法。仿真试验结果显示自适应盲均衡技术可以使星座图变得清晰而紧凑，有效减小误码率。研究证实该方法在语音信号ISI和畸变严重情况下，在白噪及有色噪声不同的噪声环境中都具有稳定的降噪能力，消噪同时可获得汉语普通话良好的听觉效果。     

最大似然参数估计水声通信帧同步技术研究

水声通信中传统帧同步方法仅仅依赖于匹配滤波输出的相关系数,在复杂信道条件下,容易出现误触发和漏同步。针对这一问题,分析了水声信道中噪声、多普勒和多径对帧同步的影响,提出了一种基于参数估计的帧同步方法.在同步过程中引入参数估计,将信号参数作为检测判决依据,以降低误触发率和漏检率;同时对最大似然估计算法进行适当简化,以降低实现复杂度。计算机仿真实验验证了新的帧同步方法相对于传统方法在定时精度和相关系数方面的优势。实际水声通信实验结果表明,无论是静止平台还是移动平台之间的通信,新的帧同步方法均能有效提高匹配滤波器输出的相关系数,提升接收机检测能力,从而避免出现误触发和漏同步。      

A novel adaptive synchronization algorithm for intermediate frequency architecture CO-OFDM system

An adaptive algorithm of fine synchronization for intermediate frequency architecture coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission system is proposed. The whole synchronization procedure is separated into two steps. In the first step, we use an auto-correlation-style algorithm to achieve the coarse symbol synchronization. In the second step, the proposed algorithm is used to achieve the adaptive fine synchronization. The merit of the proposed algorithm is that it can realize fine synchronization under different environments without adjusting the detect-threshold of timing metric in a dynamically reconfiguration network. And also the algorithm can implement in field-programmable gate array (FPGA) or other digital signal processing (DSP) chips easily. In this paper, the CO-OFDM model is analyzed in theory firstly. Secondly, an analysis on the performance of coarse synchronization algorithm used in this paper is made. Finally, the proposed fine synchronization algorithm is introduced in detail. The simulation experiment result demonstrates that the proposed algorithm achieves fine synchronization under different signal noise ratio (SNR) conditions after 6 fiber-spans transmission.     

Reliability-guided phase unwrapping algorithm following noncontinuous path based on color fringe projection

The intrinsic drawbacks of existing reliability-based algorithms, they often fail to handle wrapped phase data contains error sources, such as phase discontinuities, noise and undersampling. These algorithms often calculate the reliability only relying on the recorded pixel intensity or wrapped phase value, but the intensity of pixel always exist many noises. In this paper, we present a new method to define the reliability of pixel and path of phase unwrapping, the pixel's intensity and color are employed in fringe analysis simultaneously. The reliability function is calculated based on modulation, second difference of wrapped phase, and second difference of modulation, then we present the concept of smoothness of path, the reliability of path is calculated based on reliability of pixel and smoothness of path. Experiment results validate our proposed algorithm is more accurate than many previous algorithms. The proposed algorithm can be used to unwrap the complex phase map with discontinuous jump, and can be implemented with high efficiency without any manual interface.