Blind carrier frequency estimation for phase shift keying signals in low signal to noise ratio

In order to solve the problem of carrier frequency blind estimation of PSK signals in electronic reconnaissance, a new estimation method was proposed. The phase shift keying（PSK） signal was divided into several overlapping intervals which had equal length, and the spectrum concentration measures of every interval were extracted by the FFT. And then, using the grid-density clustering, the spectrum concentration measures were classified into two categories, the narrowband spectrum interval and the wideband spectrum interval. The narrowband spectrum interval was regarded as the characteristic class. The spectrums of the characteristic class were accumulated to estimate the carrier frequency of PSK signal. The proposed method had avoided the non linear operation in the traditional PSK signal carrier frequency estimation algorithm. Thus, the signal to noise ratio （SNR） threshold was remarkably decreased. Moreover, the proposed method did not need the prior knowledge of the signal, which was suitable to the electronic reconnaissance occasion. Experimental results had verified the validity of the proposed estimation method in low SNR.

An innovative detection method of high frequency BPSK signal with low signal-to-noise ratio

Based on chaotic oscillator system, this paper proposes a novel method on high frequency low signal- to-noise ratio BPSK（ Binary Phase Shift Keying） signal detection. Chaotic oscillator system is a typical non-lin- ear system which is sensitive to periodic signals and immune to noise at the same time. Those properties make it possible to detect low signal-to-noise ratio signals. The BPSK signal is a common signal type which is widely used in modern communication. Starting from the analysis of advantages of chaotic, os~.illator system and signal features of the BPSK signal, we put forward a unique method that can detect low signar-to-noise ratio BPSK sig- nals with high frequency. The simulation results show that the novel method can dclct.t low signal-to-noise ratio BPSK signals with frequency in an order of magnitude of l0s Hz, and the input Signal-to-Noise Ratio threshold can be -20 dB.

Blind parameter estimation of pseudo-random binary code-linear frequency modulation signal based on Duffing oscillator at low SNR

Conventional parameter estimation methods for pseudo-random binary code-linear frequency modulation(PRBC-LFM)signals require prior knowledge,are computationally complex,and exhibit poor performance at low signal-to-noise ratios(SNRs).To overcome these problems,a blind parameter estimation method based on a Duffing oscillator array is proposed.A new relationship formula among the state of the Duffing oscillator,the pseudo-random sequence of the PRBC-LFM signal,and the frequency difference between the PRBC-LFM signal and the periodic driving force signal of the Duffing oscillator is derived,providing the theoretical basis for blind parameter estimation.Methods based on amplitude method,short-time Fourier transform method,and power spectrum entropy method are used to binarize the output of the Duffing oscillator array,and their performance is compared.The pseudo-random sequence is estimated using Duffing oscillator array synchronization,and the carrier frequency parameters are obtained by the relational expressions and characteristics of the difference frequency.Simulation results show that this blind estimation method overcomes limitations in prior knowledge and maintains good parameter estimation performance up to an SNR of-35 dB.

Research on the Seismic Wave Characteristics of Low Frequency Signals before the Alxa Left Banner MS5.8 Earthquake in Inner Mongolia,China

In order to search for the seismic wave characteristics of low frequency signals in the Alxa Left Banner region,Inner Mongolia,the low frequency signals of seismic wave data are extracted from the earthquakes of MS5. 8 in 2015 and MS5. 0 in 2016 in this area. The results show that:① Before the MS5. 8 earthquake,the seismic stations located near the epicenter in Wuhai,Dongshengmiao,and Shizuishan recorded seismic waves that showed the phenomenon of spectrum shift from high to low frequency.② The low frequency signals recorded by different stations have obvious difference.③ According to the data recorded by the station closest to the epicenter,low-frequency signals were recorded about120 hours before the earthquake and had obvious anomalies. This may reflect slow slip before the earthquake.

Study on System of Low Frequency Signal′s Measurement and Extraction of its Characteristic Quantity

The essay mainly studies a system of low frequency signal′s measurement and the extraction of its characteristic quantity,introduces how matlab analyzes and extracts low frequency signal.MATLAB,a visual programming software,designs testing program for mechanic vibration signal,analyzes and identifies vibration signal.The collected data shows the effectiveness of the system.Based on XM121 modules,the testing system collects and analyzes single channel signal.

Target detection for low angle radar based on multi-frequency order-statistics

For radar targets flying at low altitude, multiple pathways produce fade or enhancement relative to the level that would be expected in a free-space environment. In this paper, a new detec- tion method based on a wide-ranging multi-frequency radar for low angle targets is proposed. Sequential transmitting multiple pulses with different frequencies are first applied to decorrelate the cohe- rence of the direct and reflected echoes. After receiving all echoes, the multi-frequency samples are arranged in a sort descending ac- cording to the amplitude. Some high amplitude echoes in the same range cell are accumulated to improve the signal-to-noise ratio and the optimal number of high amplitude echoes is analyzed and given by experiments. Finally, simulation results are presented to verify the effectiveness of the method.

Combining Radon-ambiguity transform with second-order difference to improve detection probability of LFM signals in low SNR

The Radon-ambiguity transform （RAT）, although efficient for detecting the linear frequency modulated signals （LFMs）, is troubled by the energy accumulation of noise in low signal-to-noise ratio （SNR）. A secondorder difference （SOD） method is proposed to treat with this problem. In the SOD method, the optimal search step and difference step are derived from the LFM rate resolution formula. The sharpness of the peaks of RAT is measured by curvature, and the sharpness, but not the magnitude of the peaks, is used to detect the LFMs. The SOD method removes the noise energy accumulation and reserves the drastically changing components integrally; thus, it improves the detection probability of LFMs in low SNR. The expected performance of the new method is verified by 100 Monte Carlo simulations.

Timedelay based all-frequency correction method for discrete spectrum

Many spectrum correction methods have been developed, but their performance degrades significantly when they are applied to the correction of low frequency component （ LFC ）. It owns to that the model underlying the conventional approaches neglects the interference of the negative frequency in the real signal. A new approach for the correction of the LFC is proposed, which suits all kinds of symmetrical windows. It divides a signal into three sections and makes use of the first spectrum line of each section. Then this approach is modified so that it is also applicable to the correction of the high frequency component. Thus a timedelay-based all-frequency correction method is proposed. The simulation results show that this method is simple and feasible. By this method, the accurate frequency, amplitude and phase of the spectral line can be obtained whether it is close to or far from OHz.

Tracking algorithm of BPSK signal in low bit SNR and high dynamic scenarios

A new tracking algorithm is proposed aiming at the tracking problem in low bit signal-to- noise ratio （i. e. , Eb/N0 ） scenarios, in which the bit clock regenerated by bit synchronization loop decides loop update moment. The double frequency processing and non-coherent accumulation tech- nologies are applied to eliminate the impact of data polarity inversion, and then long time accumula- tion improves the input signal-to-noise ratio of discriminator. The frequency locked loop and phase locked loop constitute a carrier loop in parallel, which can meet the high dynamic demands. The ef- fectiveness of this algorithm has been corroborated by theoretical analysis, simulation and measure- ments, and the new tracking algorithm has been used in an aerospace engineering project successfully.

高速低载频比传感信号下的逆变器有源阻尼自适应控制

逆变器工作在高频运行和控制载波比低的情况下,存在谐振尖峰抑制,提出了高速低载频比传感信号下的逆变器有源阻尼自适应控制技术。在高速低载频比传感信号下,构建逆变器数学模型。输出电流到电容电压与定子电流之间的传递函数,将逆变器的有源阻尼等效成一个控制电压环。调整有源阻尼自适应控制回路的参数,制定逆变器有源阻尼自适应控制策略。实验结果表明:文中技术的稳定裕度由2.46 dB下降至1.18 dB,可以保证逆变器的稳定运行;在不同采样频率下,可以控制逆变器输出与电网电压相同相位和频率的并网电流,其中在采样频率为5.9f时,输出并网电流和电压均表现为正弦状态,波动范围分别为[35 A,58 A]和[185 V,225 V],具有较好的控制效果。

基于超前滞后环节附加前馈阻尼的VSG控制策略

虚拟同步发电机(virtual synchronous generator,VSG)在引入同步机二阶转子运动方程,增大电力系统等效惯量的同时,也引入了同步发电机的振荡特性,有功低频振荡等动态稳定性问题也随之而来。引入调速侧电力系统稳定器(governor power system stabilizer,GPSS)能有效抑制VSG的功率低频振荡,但其在超调量及调节时间方面的控制效果仍有待提高。通过建立VSG的小信号模型从极点配置角度分析其稳定性,揭示基于GPSS的VSG控制策略在功率动态响应上存在较高超调和较长调节时间的原因。基于此,参考GPSS控制思想,提出了一种基于超前滞后环节附加前馈阻尼补偿的虚拟同步发电机控制策略。并从理论上分析验证了所提控制策略在不影响系统稳态特性的前提下,能够提供调整自由度更高的正阻尼,在有效地抑制功率超调的同时提高了系统的调节速度,从而更好地抑制了有功功率的低频振荡。最后通过MATLAB/Simulink进行对比仿真,仿真结果与理论分析结果一致,证明了所提控制策略的正确性和有效性。

基于直流电压同步的构网型变流器低频振荡分析与阻尼控制

基于直流电压同步的构网型变流器能够在稳定直流侧电压的同时实现变流器自同步,然而直流电压动态与控制环相互作用可能会引起系统低频振荡。为了研究低频振荡产生的机理,建立了计及直流电压同步控制环与交流电压环的构网型变流器“功率-功角”稳定性分析模型,分析发现直流电压同步相比于功率同步会引入更多的相位滞后,同时交流电压环也会在环路中引入相位滞后,导致系统环路增益在具有高幅值的低频范围内存在-180°相位穿越,进而引发低频振荡问题。基于低频振荡机理分析,提出两种对直流电压同步控制进行相位补偿的方法,通过重塑系统环路低频高增益区的相位,实现对低频段的阻尼控制和振荡抑制。最后,通过仿真和实验验证了理论分析的正确性和所提方法的有效性。

风电机组附加有功控制参数对系统低频振荡的影响

附加有功控制环节使风机参与至同步机的机电振荡模式中,这使得系统功率振荡特性机理更加复杂,不利于附加有功控制环节的参数设计。为了研究风机附加有功控制对系统稳定性的影响,首先建立了考虑锁相环特性的风机与同步机动态作用的小信号模型;然后基于该模型利用阻尼转矩法分析了风机附加有功控制对系统振荡稳定性的影响机理,并提出了综合考虑系统等效惯性时间常数和阻尼比的附加有功控制环节系数设计方法;最后通过“10机39节点系统”的仿真结果量化分析了各系数对系统低频振荡稳定性的影响规律。结果表明,合理设置风电机组附加有功功率控制系数、滤波时间系数和锁相环带宽均能提升系统频率稳定性;理论分析的正确性也得到了验证。

基于FRFT的低信噪比LFM信号参数快速估计算法

基于分数阶傅里叶变换(Fractional Fourier Transform,FRFT)对线性调频(Linear Frequency Modulated,LFM)信号参数进行估计,问题关键是确定FRFT最佳阶数,根据误差迭代思想提出新的参数估计算法,该算法利用归一化带宽和旋转角的转化关系,由估计误差推算角度差值,有效降低了运算量,不需要调频斜率正负的先验信息,改进的对数搜索算法可以进一步提高参数估计结果的稳定性和可靠性。仿真结果表明,信噪比在-8 dB以上时该方法在高效率的前提下仍具有良好的参数估计性能,平均估计误差在1%以内,估计结果接近Cramer-Rao下限,满足工程实时处理需求。

基于微纳光纤结型谐振腔的低频水声传感器

通过将谐振腔置入有利于将声压变化有效传导到谐振腔的材料中,可以实现基于光学微纳结型谐振腔的水声传感器。提出了一种基于微纳光纤技术的结型谐振腔水声传感器,其尺寸更小,对于低频信号有出色的探测能力。将直径为3.5μm的光学微纳纤维制备成曲率半径为550μm的结型谐振腔,采用聚二甲基硅氧烷(PDMS)薄膜将其以上、下包裹的形式封装到镂空玻璃基底上,完成水声传感器的制备。PDMS具有比SiO_(2)低的折射率,并表现出良好的传输性能。采用PDMS封装谐振腔提高了传感器的品质因数(1.45×10^(4)),也有利于在水下更好地保护谐振腔。采用滤波放大技术来解调接收到的声信号,测试结果表明,该传感器在12.5~100 Hz内的灵敏度均值为-148.6 dB,信噪比均值为40 dB。

电针联合低频经颅超声刺激对创伤性脑损伤大鼠脑电信号的影响

背景:创伤性脑损伤是由头部受到撞击、打击而导致大脑正常功能被破坏的疾病,目前需要寻找有效治疗方式和客观指标,帮助医生判别损伤状况及恢复患者脑功能。目的:探究电针联合低频经颅超声刺激对创伤性脑损伤大鼠脑电信号的影响。方法:将40只6周龄SPF级雄性SD大鼠随机分为假手术组、模型组、电针组、低频经颅超声刺激组和联合组(n=8),后4组采用Feeney自由落体法造模,假手术组只开骨窗而不打击。各干预组均于造模后1 d开始实施干预,电针组进行电针干预,低频经颅超声刺激组进行低频经颅超声刺激干预,联合组进行两者联合干预,共干预7 d。造模后8 h,用改良神经功能缺损评分评定大鼠神经功能缺损情况;干预7 d后观察大鼠Y迷宫自发轮流行为百分比,而后采集脑电信号,利用快速傅里叶变换分解出α、β、θ和δ波段,计算各频段振荡幅值、能量占比百分比以及Lempel-Ziv复杂度、样本熵。结果与结论:①造模后8 h,模型组、电针组、低频经颅超声刺激组和联合组的改良神经功能缺损评分显著高于假手术组(P<0.05);②造模后第7天,模型组的α波、δ波频带振荡幅值、δ波能量占比百分比显著高于假手术组(P<0.05),自发轮流行为百分比、α波、β波能量占比百分比、Lempel-Ziv复杂度、样本熵显著低于假手术组(P<0.05);③与模型组比较,联合组的α波、δ波频带振荡幅值显著下降(P<0.05),电针组、低频经颅超声刺激组、联合组的α波、β波频带能量占比百分比显著升高(P<0.05),δ波能量占比百分比显著下降(P<0.05);④与电针组和低频经颅超声刺激组相比,联合组的δ波能量占比百分比显著降低(P<0.05),自发轮流行为百分比、α波、β波能量占比百分比、Lempel-Zi复杂度、样本熵显著升高(P<0.05);⑤结果显示,创伤性脑损伤大鼠出现脑电信号异常,而电针联合低频经颅超声刺激干预后可以改善大鼠脑电信号的异常情况,提示脑电频域特征和非线性特征可用来评估创伤性脑损伤情况。

多途混响环境中声扩频信号线性与相关性分析

为分析浅海多途信道中声扩频信号的相关性和线性,建立了计入频率衰减与海底损耗的浅海Pekeris波导虚源法计算模型;对浅海中的脉冲信号与扩频信号进行了仿真计算,通过混响室试验初步研究了多途混响环境中扩频信号的线性。仿真结果显示:虚源法和简正波法的频域解一致,得到了与简正波理论相同的低频模式频散现象,验证了虚源法适用于浅海理想波导、Pekeris波导中的低频问题;当发生模式频散,低频扩频信号的相关性能强于同扩频参数的高频信号。试验结果表明:低频脉冲的时域波形发生畸变、展宽时,同频率的扩频信号能准确测出反射信号的几何时延,证明了低频多途效果仍可看作每个“路径”信号的线性叠加。

一种基于小信号模型分析的水电机组参与电网一次调频方法

由于水电机组自身复杂的动态特性,导致高比例水电在参与电网调频过程中易产生超低频振荡,不利于电网安全稳定运行。为此,提出一种基于小信号建模分析的水电机组参与电网调频方法。首先,为体现更贴近实际工作状态的动态特性,基于小信号分析对水电机组参与电网调频进行模型构建;然后,考虑到小信号模型会反映系统诸如超低频振荡等危害的发生特征,进而突出系统的非线性,进行分数阶PID(FOPID)控制器设计调速系统;在此基础上,利用遗传算法以频率偏差最小为目标进行控制器参数优化,以进一步改善控制效果。仿真分析表明,所建水电机组小信号模型能够准确反映水轮机的动态特性,同时性能优越的FOPID控制器能有效地抑制超低频振荡。

NWC人工甚低频台站信号产生“条缕状”准捕获电子能谱的模拟研究

遍布全球的人工甚低频台站发射的信号主要用于对潜通信,在夜间,这些信号可以泄漏进磁层与内辐射带中几十至几百keV电子发生回旋共振,从而导致电子沉降.这一过程是导致内辐射带电子损失的重要原因,也是磁层-电离层耦合过程中能量和物质输运的重要环节.被台站信号散射的电子呈现出能量随L增加而减小的“条缕状”能谱结构,与台站信号和电子的一阶回旋共振能量曲线相符.低轨卫星可以对“条缕状”能谱结构进行清晰地观测,为研究近地空间波粒相互作用提供了重要契机.本文基于Drift-Diffusion-Source模型,复现了DEMETER卫星于2009年3月19日多个轨道测量的NWC台站信号导致的“条缕状”能谱,量化了NWC台站信号对辐射带电子的散射作用,明晰了NWC台站信号的幅度和传播角,揭示了内辐射带电子漂移过程中的动态变化规律,为开发人工影响辐射带技术提供了重要理论参考.

面向新型声波激励超低频天线的2FSK通信系统解调算法

针对声波激励超低频天线发射功率较小,低接收信噪比下传统二进制频移键控(2FSK)非相干解调中窄带滤波器设计难等问题,提出了一种基于Duffing混沌振子的2FSK弱信号解调方法。由于Duffing振子本身对相位敏感,首先设计了“粗同步+细同步”的相位同步方法,其中粗同步通过短时傅里叶变换完成,细同步则通过Duffing混沌振子实现;然后,利用混沌振子对同频周期信号敏感、对噪声信号免疫的特性,将信号并行通过2个Duffing振子,二者的内置信号频率与2个载频一致。在某一时刻必定有一个系统处于大尺度周期态,另一个处于混沌态,二者比较判决来实现信号解调。仿真结果表明,文中提出的基于Duffing混沌振子的解调算法比非相干解调和相干解调算法,分别有约5 dB和4 dB的性能提升。