Performance of direct sequence spread spectrum in a fading dispersive channel with jamming

The performance of a binary phase-shift-keyed direct-sequence spread-spectrum communication system in a fading dispersive channel with jamming is examined. The channel is modeled as a wide-sense-stationary uncorrelated scattering channel, dispersive only in time. The channel output consists of a strong stable component of the signal plus a continuum of dispersive components and is referred to as a frequency-selective Rician fading channel. To obtain specific results, the multipath intensity profile is modeled by triangular function. Both the uncoded and coded performances are analyzed. The multipath spread is chosen to limit intersymbol interference to K adjacent coded symbols in the coded case and to only adjacent data bits in the uncoded case. No specific interleaver design is considered. A closed-form expression for the probability of error is obtained, assuming random spreading sequences. Numerical results are presented for values of the system and channel parameters. It is shown that a jammer can optimize its strategy if the probability of error is not too high.<>     

Time-Frequency Characterization and Receiver Waveform Design for Shallow Water Environments

We investigate a frequency-domain characterization of shallow water environments based on normal-mode models of acoustic mediums. The shallow water environment can be considered as a time-dispersive system whose time-varying impulse response can be expressed as a superposition of time-frequency components with dispersive characteristics. After studying the dispersive characteristics, a blind time-frequency processing technique is employed to separate the normal-mode components without knowledge of the environment parameters. This technique is based on first approximating the time-frequency structure of the received signal and then designing time-frequency separation filters based on warping techniques. Following this method, we develop two types of receivers to exploit the diversity inherent in the shallow water environment model and to improve underwater communication performance. Numerical results demonstrate the dispersive system characterization and the improved processing performance of the receiver structures.     

Improving GPR Image Resolution in Lossy Ground Using Dispersive Migration

As a compact wave packet travels through a dispersive medium, it becomes dilated and distorted. As a result, ground-penetrating radar (GPR) surveys over conductive and/or lossy soils often result in poor image resolution. A dispersive migration method is presented that combines an inverse dispersion filter with frequency-domain migration. The method requires a fully characterized GPR system including the antenna response, which is a function of the local soil properties for ground-coupled antennas. The GPR system response spectrum is used to stabilize the inverse dispersion filter. Dispersive migration restores attenuated spectral components when the signal-to-noise ratio is adequate. Applying the algorithm to simulated data shows that the improved spatial resolution is significant when data are acquired with a GPR system having 120 dB or more of dynamic range, and when the medium has a loss tangent of 0.3 or more. Results also show that dispersive migration provides no significant advantage over conventional migration when the loss tangent is less than 0.3, or when using a GPR system with a small dynamic range.     

Bi-end dispersion compensation for ultralong optical communicationsystem

Bi-end dispersion compensation (DC) for ultralong nonreturn-to-zero (NRZ) optical transmission system is studied. Both the loss and dispersion of the transmission fiber are periodically compensated. Two dispersive elements are placed at the input and output ends of a compensation period, respectively, to compensate for fiber dispersion. The pulse compression owing to self-phase modulation (SPM) can be adjusted by the compensation ratios of the dispersive elements at the two ends of a compensation period. Therefore, the pulse compression can be optimized and the system performance can be improved to compare with the system with either pre- or postdispersion compensation. The rules to design the system are considered. The transmission system of 10-Gb/s bit rate, 9000-km transmission distance, and 100-km compensation period is taken as an example. The second-order fiber dispersion is assumed to be completely compensated. Wave equation is numerically solved to study the system performance which is represented by Q factor. The relations of several system parameters and Q factor are studied. The system parameters include the compensation ratios of the dispersive elements at the two ends of a compensation period, dispersion of transmission fiber, signal power, and the compensation ratios of third-order fiber dispersion. If the third-order fiber dispersion cannot be completely compensated, it is found that one can use a higher signal power to improve the system performance     

An Analysis of Interchannel Interference of OFDM Systems in Time-Varying Channel

1　IntroductionRecentlyOFDMtechniquehasregainedlotsofinterestforitspromisingtosupportbroadbandwire lessaccesssystems[1～ 4] .Comparedwithsingle car riersystems,OFDMsystemshavemanyobviousad vantages,suchasrobustnessagainstmultipathdelayspread,feasibilityinhardwareimplementation ,flexibilityinsubcarrierallocationandadaptabilityinsubcarriermodulation[5～6] .However,thepriorityofOFDMsystemscomesfromtheorthogonaldivi sionofbandwidth .Themoredivision ,thelongerthesymbolperiodisandthemorerobustt…     

SAW dispersive delay line utilising apodised IDT with periodic electrodes

In designing SAW dispersive delay lines (DDLs), one usually applies dispersive interdigital transducers with electrode spacing varying according to the frequency-delay time relation of the delay line impulse response. A new method is proposed in the letter, in which a DDL is treated as a bandpass filter with a nonlinear phase response. Such a filter can be synthesised with an IDT having periodic electrodes with apodisation. The experimental result is ?40 dB of sidelobe level obtained in a system of two DDLs, one of which is Hamming-weighted.     

基于光波映射展宽技术的稀疏图像实时获取

针对传统成像技术的成像速度慢且灵敏度低,本 文采用光波映射展宽技术搭建一个一 维稀疏图像实时获取系统,以实现高速图像的实时获取。基于空间光栅的空-频映射以 及色散光纤的 频-时映射,充分利用傅里叶变换的色散效果,使得系统简化并能够实现实时处理的性能 。从光栅公式入手,对系统的量程和空间分辨率进行了初步分析,并通过系统实验将 上述理论结果 进行验证。实验表明,本文系统能够将一维的条形码以20.9MHz的帧 率在示波器上实时成像,比目前的常用成像技术高4个量级。     

Photonic Generation of Microwave Signals Based on Pulse Shaping

A novel approach to generating microwave signals based on optical pulse shaping is proposed and experimentally demonstrated. The proposed system consists of a femtosecond pulse laser source, a Sagnac-loop filter (SLF), a dispersive element, and a photodetector. The spectrum of the femtosecond pulse is shaped by the SLF that has a sinusoidal spectral response. Thanks to the frequency-to-time conversion in the dispersive element, time-domain pulse exhibiting the shape of the optical power spectrum is obtained. Depending on the free-spectral range of the SLF and the total dispersion of the dispersive element, signals with frequencies up to terahertz can be generated. A model to describe the signal generation is developed. Experimental results agree well with the theoretical analysis     

HF信道的色散带宽及其对脉冲波传播的影响

电离层是时变色散媒质，色散带宽是描述HF信道色散程序的物理量，它安全由电离层的特性决定。文中采用Lin等人给出的色散带宽的定义，将电离层电子浓度进一步推广到球面分层，并给出计算球面分层、各向同性的电离层的色散带宽的普遍关系式。以电离层电子浓度剖面为准抛物层为例求解了色散带宽及其同收发间距和发射频率的关系。计算结果表明当收发间距D＞0且发射频率f＜MUF时，射线分为高角射线和低角射线会发生重合，色散带宽相等。高角射线穿入电离层较深，其色散带宽小于低角射线的色散带宽。提出并通过模拟计算证实了从信号恢复的角度看存在一个最佳的色散带宽，即高、低角射线重合时所对应的色散带宽。     

含吸收型色散介质的光子晶体的腔极化激元研究

对色散介质层采用洛仑兹振子模型,通过考虑色散介质层两边的边界条件,可以严格得到表征色散介质层的转移矩阵.通过改变介质层的不同参数,详细研究了含吸收型色散介质的对称光子晶体的腔极化激元.研究发现吸收型色散微腔和增益型色散微腔在简正模耦合,无定型展宽效应和失谐效应方面都可以表现出大致相似的性质,但其在透射强度和耦合模频率飘移方面都有着明显的不同.最后对同时含有吸收型色散介质层和Kerr非线性层的一维对称光子晶体的光双稳行为进行了研究,发现其形成双稳态所需的阈值入射光强比仅含Kerr非线性层的阈值入射光强有所降低.     

圆柱型涂层/基底系统中的激光超声表面波

为了分析圆柱型涂层/基底系统中声表面波的特点及其传播特性,以热弹激发机理为基础建立了脉冲激光在圆柱型涂层/基底系统中激发超声表面波的有限元程序。在此基础上,计算了激光在铝(涂层)/镍(基底)和镍(涂层)/铝(基底)系统中激发的超声表面波波形,它们分别对应了硬涂层系统和软涂层系统。结果表明,圆柱型涂层/基底系统中的超声表面波是色散的,并且其色散特性由弯曲柱面引起的几何色散和涂层/基底系统引起的色散共同决定,这使得圆柱型涂层系统中色散特性远比板状涂层系统杂,不存在简单的正常和反常色散规律。     

Nonlinear control in optical bistability

We study the influence of a small periodic modulation of the input field amplitude in dispersive optical bistability. When the system is initially near one of the two limit points in a stable state, the addition of a small periodic modulation may either stabilize or destabilize the system. We prove that destabilization occurs as a result of critical slowing down when the modulation frequency is decreased.     

Ultrafast and Precise Interrogation of Fiber Bragg Grating Sensor Based on Wavelength-to-Time Mapping Incorporating Higher Order Dispersion

An interrogation scheme based on wavelength-to-time mapping to achieve ultrafast, high-precision, and large dynamic range interrogation of fiber Bragg grating (FBG) sensors is proposed and experimentally demonstrated. The wavelength-to-time mapping, also called temporal self-imaging effect, is realized in the optical domain, using a dispersive element that has a large group velocity dispersion. For a practical dispersive element, higher order dispersions exist, which makes the wavelength-to-time mapping nonlinear. Thus, an interrogation system based on wavelength-to-time mapping without considering the high-order dispersion would reduce the interrogation accuracy. In this paper, for the first time to the best of our knowledge, a mathematical model that incorporates higher order dispersion to achieve an accurate wavelength-to-time mapping is developed, which is then verified by a numerical simulation. An FBG-based strain sensor interrogated based on the developed wavelength-to-time mapping scheme is experimentally investigated. The system has a sampling speed of 48.6 MHz, a dynamic range as large as 20 nm, and a sensing accuracy as high as 0.87 $mu varepsilon$ for a single-shot measurement.      

复合型二元光学器件补偿色散和相位畸变研究

复合型二元光学器件作为一种新型色散和相位补偿元件,由于其色散的特殊性,在超强超短脉冲激光的色散和波前补偿中具有特殊的意义。提出并研究了利用复合型二元光学器件同时补偿全钕玻璃啁啾脉冲放大(CPA)系统中的色散和相位畸变,并利用光线追迹的方法对二元光学器件模拟了设计和补偿效果。在实验中,利用二元光学器件将100 fs的光束补偿至30 fs左右。实验结果验证了此方法的可行性。     

基于频域分析的光学相干显微镜中的色散补偿

提出基于频域分析的色散补偿方法,对光学相干显微镜的干涉信号进行快速傅里叶变换,得到频率幅值极大值对应的平均波数,将提取的解包裹后的相位以平均波数为中心做多项式拟合,得到二阶色散系数。实验中,通过在参考臂中插入不同厚度的色散介质来引入两个干涉臂色散介质的光程差,并求得相应的二阶色散系数。通过最小二乘线性拟合,证实了二阶色散系数和色散介质的相对厚度具有很好的线性关系。根据该线性关系,可以在参考臂中插入适当厚度的色散介质来完全补偿干涉系统的二阶色散。     

Propagation-medium matched direct-sequence (PM-MDS) spread-spectrumsignals

The distortion of high-data-rate digital radio signals, such as direct-sequence-spread-spectrum (DS-SS) signals, when transmitted through dispersive media greatly degrades the system performance. The design of propagation-medium matched direct-sequence signals that overcome the performance degradation caused by the medium dispersion is described. The performance of PM-MDS signals in multipath dispersive media is evaluated and compared with the corresponding performance when randomly selected (unmatched) DS signals are used. The results show the advantages of PM-MDS transmission. The proposed PM-MDS signals do not require the use of sophisticated signal processing at the receiving ends, and are thus most suited for secure communication systems using one central transmitting station and numerous receiving units     

Comparative study of turbo equalization schemes usingconvolutional, convolutional turbo, and block-turbo codes

Turbo equalizers have been shown to be successful in mitigating the effects of inter-symbol interference introduced by partial response modems and by dispersive channels for code rates of R⩽ 1/2. We comparatively studied the performance of a range of binary phase-shift keying turbo equalizers employing block-turbo codes, namely Bose-Chaudhuri-Hocquenghen (1960, 1959) turbo codes, convolutional codes, and convolutional turbo codes having high code rates, such as R=3/4 and R=5/6, over a dispersive five-path Gaussian channel and an equally weighted symbol-spaced five-path Rayleigh fading channel. These turbo equalization schemes were combined with an iterative channel estimation scheme in order to characterize a realistic scenario. The simulation results demonstrated that the turbo-equalized system using convolutional turbo codes was the most robust system for all code rates investigated     

An Alternative Algorithm for Both Narrowband and Wideband Lorentzian Dispersive Materials Modeling in the Finite-Difference Time-Domain Method

In this study, an alternative algorithm is proposed for modeling narrowband and wideband Lorentzian dispersive materials using the finite-difference time-domain (FDTD) method. Previous algorithms for modeling narrowband and wideband Lorentzian dispersive materials using the FDTD method have been based on a recursive convolution technique. They present two different and independent algorithms for the modeling of the narrowband and wideband Lorentzian dispersive materials, known as the narrowband and wideband Lorentzian recursive convolution algorithms, respectively. The proposed alternative algorithm may be used as a general algorithm for both narrowband and wideband Lorentzian dispersive materials modeling with the FDTD method. The second-order motion equation for the Lorentzian materials is employed as an auxilary differential equation. The proposed auxiliary differential-equation-based algorithm can also be applied to solve the borderline case dispersive electromagnetic problems in the FDTD method. In contrast, the narrowband and wideband Lorentzian recursive convolution algorithms cannot be used for the borderline case. A rectangular cavity, which is partially filled with narrowband and wideband Lorentzian dispersive materials, is presented as a numerical example. The time response of the electric field z component is used to validate and compare the results     

基于子阵结构的大孔径相控阵天线设计

在卫星通信和飞行器测控中,大孔径相控阵天线结构上采用前端模拟子阵和阵间数字波束形成相结合的方式,可以克服天线面临的时间色散问题。子阵规模通过成本和子阵色散确定,阵间波束形成采用空时二维延时滤波器结构,滤波器系数采用查找表方式获得,便于工程实现。计算机仿真验证了系统结构、子阵划分、延时滤波器设计的正确性。