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基于FRFT的复杂电磁环境下未知信号辨识
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摘要
信息化战争中,由于作战双方在相对有限的战场空间内大量使用各种电子信息武器装备、电子干扰设备以及探测系统,加上空间内已有的众多电磁辐射体,导致在该范围内形成了一个信号特征繁杂、信号密集度极大的电磁环境——复杂电磁环境。随着信息化进程的推进,复杂电磁环境将成为战场环境的重要表现形态,并将对现有系统中的电子信息设备的效能发挥带来多方面影响。在此背景下,为了提高电子信息武器系统的抗干扰能力,同时又能够实施有针对性地电子干扰,就要求对各种侦察及干扰信号能够实现精、准、快的检测、分选与识别。然而,由于复杂电磁环境中的信号具有种类多、频段宽、体制新、分布广等特点,信号本身多呈现非平稳特性,信号之间存在严重的时频耦合。因此,要从如此复杂多变的环境中分选出未知的干扰信号极其困难,传统的信号处理方法已无法胜任。
     分数阶Fourier变换(FRFT, Fractional Fourier Transform)是一种新兴的非平稳信号处理方法,其具有较高的时频分辨率,是一种全时域的线性变换,不存在交叉项干扰问题,因此分数阶Fourier变换对分析和处理多分量时频信号具有优良的特性。本文将分数阶Fourier变换应用到复杂电磁环境下未知信号的处理中,对典型时频交叠信号的快速检测与特征参数估计进行了深入研究,主要体现在:
     1、对分数阶Fourier变换的基本原理及性质进行学习与研究,重点讨论了基于分数阶Fourier变换的二维搜索检测法以及基于分数阶频谱四阶原点矩的一维搜索检测法的信号检测原理,并通过仿真实验对两种方法的检测性能进行了比较分析。
     2、针对现有基于分数阶Fourier变换的Chirp信号参数估计算法具有一定的应用局限性的问题,论文通过对实际观测信号形式进行分析,推导得出现有估计算法的估计误差,在此基础上提出了一种通用的参数估计方法,并且进一步将新方法推广应用到了时变幅度Chirp信号的参数估计中。
     3、研究并提出了两种最佳分数阶Fourier变换旋转角度的快速确定方法,即基于FRFT的Chirp信号快速检测方法。方法一,在研究了基于FRFT的Chirp信号欠采样快速检测算法的基础上,从离散分数阶Fourier变换的角度出发,证明了将欠采样技术推广应用于基于分数阶频谱四阶原点矩的Chirp信号检测方法的可行性,进而从欠采样信号处理的角度,降低该Chirp信号一维搜索检测方法的运算量。第二种方法则是利用Chirp信号分数阶频谱四阶原点矩的对称特性,推导得出最佳FRFT旋转角度理论值与粗搜索检测估计结果的差值,进而在粗搜索的基础上快速计算得到更为准确的最佳FRFT旋转角度估计值。
     4、针对当前基于FRFT的多分量时频信号检测中,一轮检测迭代只检测一个最强分量的情况,在研究了多分量Chirp信号间的遮蔽关系的基础上,提出了一种基于分数阶频谱四阶原点矩的多分量Chirp信号快速检测的阈值设置方法,利用该阈值设置方法可实现在一轮检测迭代中同时检测出未发生遮蔽的多个信号分量,进而提高检测效率。
     5、在对复杂电磁环境的定义及构建准则进行研究的基础上,在一定范围内构造了由常规通信信号、雷达信号、以及干扰信号等组成的复杂电磁环境仿真系统,并利用该仿真系统产生的复杂电磁环境信号验证了论文所提出的基于分数阶Fourier变换的复杂电磁环境下未知信号的识别分选系统架构的有效性。
In the future information warfare, numerous electronic weapons and equipments willbe widely used in a relatively limited space in the battlefield. With the addition of exsitingelectromagnetic radiation, a special electromagnetic environment will be formed which iscalled complex electromagnetic environment. There are many kinds of intense signals inthe complex electromagnetic environment. Thus, in order to improve the anti-interferenceability of the electronic weapons and equipments, the jamming signals need to be detectedand identified precisely and quickly. However, the signals in the complex electromagneticenvironment are usually non-stationary and there are serious time frequency couplingbetween different signals. As a result, the traditional signal processing methods can not bequalified for detecting and recognizing signals in the complex electromagneticenvironment.
     As a newly emerged signal processing method, fractional Fourier transform (FRFT) issuitable for dealing with non-stationary signals. What’s more, FRFT is a lineartransformation and there is no cross-term interference. So fractional Fourier transform canachieve good performance when processing multi-component time-frequency signals. Thispaper applies FRFT to the recognition of unknown signal in the complex electromagneticenvironment. The fast detection methods and parameter estimation of typicaltime-frequency signals based on FRFT are studied in depth. The main contributions of thisthesis are summarized as follows.
     1. The fundamentals and properties of FRFT is studied and analized. On that basis, thesignal detection method based on fractional Fourier transform is researched, especially themethod based on the2-D peak searching and the one based on the fourth order originmoment of fractional spectrum (OMFrS). The performance of these methods is comparedthrough simulations.
     2. On the basis of analyzing the practical signal form, the estimation error of theexisting parameter estimation method is derived and then a novel and universal parameterestimation method is proposed. Furthermore, the proposed algorithm is developed whichallows estimation of the practical observed Gaussian windowed chirp signal.
     3. The fast detection method of the optimal transform angle of FRFT is studied andtwo new algorithms are proposed. The first method is realized based on sub-Nyquistsampling. The relationship between the optimal transform angle of the undersampledsignal and that of the original signal is studied. It is proved that the chirp-rate of noiselessChirp signal can be estimated correctly even though the signal is undersampled. Thesecond method is based on the symmetric property of the fourth order OMFrS of chirpsignal. By deducing the difference between the theoretical optimal transform angle and thedetected result, a more accurate estimation can be obtained. Compared to the existingmethods, the proposed algorithm achieves better accuracy with high detection speed.
     4. The existing detection methods based on FRFT is usually only detects one Chirpcomponent that has the largest energy accumulation in each iteration. Thus, it will needseveral iterations to detect multicomponent Chirp signals. In order to improve the detectionefficiency, the shading relations between the fourth order OMFrS of different Chirpcomponent is studied. On this basis, a new threshold setting method for rapid detection ofmulticomponent Chirp signals based on the fourth order OMFrS is proposed. Byemploying the proposed method, several LFM components whose fourth order OMFrS attheir own optimal rotation angles are not shadowed by strong components can be detectedat the same time in one iteration.
     5. The connotative meaning of complex electromagnetic environment and itsconstruction principle is studied. On this basis, a simulation system of complexelectromagnetic environment is realized by adopting the computer simulation. Using thecomplex signals produced by the simulation system, the recognition method of unknownsignal in the complex electromagnetic environment using FRFT which is proposed basedon the works of this thesis is verified.
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