直扩系统中线性调频干扰抑制技术研究
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摘要
扩频通信技术因具有发射功率谱密度低、抗干扰能力强、截获概率低、多址能力、较好地抗衰落等优点,在通信领域中得到广泛应用。其中直接序列扩频(Direct Sequence spread Spectrum,DSSS)技术的应用最为普遍。然而在直扩系统中,当外部干扰的强度超过了系统的干扰容限时,会导致系统的性能急剧下降,因此要采取相应的措施来抑制干扰对系统性能的影响。目前,对于干扰抑制的研究主要集中于窄带干扰,对于宽带非平稳干扰抑制技术的研究相对较少,而在宽带非平稳干扰中,最为典型的就是线性调频干扰。由于线性调频(chirp, LFM)干扰的瞬时频率是时间的线性函数,而传统的傅里叶变换是一个全局性的变换,因此无法描述干扰的时频特性。
基于以上理论,本文首先采用一种基于分数傅里叶变换的干扰抑制方法。利用线性调频干扰能够在某个分数傅里叶变换域产生能量聚集的特点,通过对混有干扰的直扩信号进行相应阶数的分数傅里叶变换探测出线性调频干扰的位置,并采用窄带带阻滤波器来实现对干扰的抑制,从而改善线性调频干扰对系统性能的影响。然后,针对信号截断频谱泄露的问题,对信号进行加窗处理。通过对分数域线性调频干扰加窗特性的分析表明,干扰能量相对更集中。最后,考虑到加窗会使部分有用信号的能量损失,本文采用分段重叠加窗的方法来实现对干扰的抑制。结果表明,分段重叠加窗的方法比带阻滤波器更易于实现,并且能更有效的提高系统干扰抑制性能。
Spread spectrum communication technology has been widely applied in communications for its advantage of low launched power spectral density, strong anti-jamming capability, low intercept possibility, multiple access capability, better anti-fading capability, and so on. Specially, DSSS technology is the most widely used. But in the system of DSSS, when the external interference exceeds the system's interference tolerance, it will deteriorate system performance rapidly. So it's necessary to take corresponding measures to suppress the external interference. At present, the researches of interfering suppression focus on narrowband interference, but less on broadband non-stationary interference suppression; and in Broadband non-stationary interference, one of the most typical kinds is linear frequency interference. Instantaneous frequency of LFM is a linear function of time. But Fourier transform is used for global transformation, so it can't describe the time-frequency characteristics of LFM.
Based on the above theory, first of all, this paper introduces an interference suppression method based on fractional Fourier transform. Using the characteristics that linear frequency modulation interference can generate energy aggregation in a fractional Fourier transform domain, the location of LFM interference is detected through corresponding fractional Fourier transform of DS signals that mixed with interference, and the effect of LFM interference on system performance can be reduced by using narrow-band band-stop filter device to achieve suppression of the interference. Then, in the light of the leakage of the spectrum due to cutting off the signal, it processes the signal for adding windows. The analysis of adding windows characteristics of linear frequency modulation interference in the domain of fractional Fourier transform shows that the energy of interference is relatively more concentrated. Finally, considering that there will be a sectional energy loss of the signal because of adding windows, this paper uses overlapping sub-windowed approach to achieve interference suppression. The result shows that overlapping sub-windowed approach is easier to implement than the band stop filter and it can improve system performance of interfering suppression more effectively.
基于以上理论,本文首先采用一种基于分数傅里叶变换的干扰抑制方法。利用线性调频干扰能够在某个分数傅里叶变换域产生能量聚集的特点,通过对混有干扰的直扩信号进行相应阶数的分数傅里叶变换探测出线性调频干扰的位置,并采用窄带带阻滤波器来实现对干扰的抑制,从而改善线性调频干扰对系统性能的影响。然后,针对信号截断频谱泄露的问题,对信号进行加窗处理。通过对分数域线性调频干扰加窗特性的分析表明,干扰能量相对更集中。最后,考虑到加窗会使部分有用信号的能量损失,本文采用分段重叠加窗的方法来实现对干扰的抑制。结果表明,分段重叠加窗的方法比带阻滤波器更易于实现,并且能更有效的提高系统干扰抑制性能。
Spread spectrum communication technology has been widely applied in communications for its advantage of low launched power spectral density, strong anti-jamming capability, low intercept possibility, multiple access capability, better anti-fading capability, and so on. Specially, DSSS technology is the most widely used. But in the system of DSSS, when the external interference exceeds the system's interference tolerance, it will deteriorate system performance rapidly. So it's necessary to take corresponding measures to suppress the external interference. At present, the researches of interfering suppression focus on narrowband interference, but less on broadband non-stationary interference suppression; and in Broadband non-stationary interference, one of the most typical kinds is linear frequency interference. Instantaneous frequency of LFM is a linear function of time. But Fourier transform is used for global transformation, so it can't describe the time-frequency characteristics of LFM.
Based on the above theory, first of all, this paper introduces an interference suppression method based on fractional Fourier transform. Using the characteristics that linear frequency modulation interference can generate energy aggregation in a fractional Fourier transform domain, the location of LFM interference is detected through corresponding fractional Fourier transform of DS signals that mixed with interference, and the effect of LFM interference on system performance can be reduced by using narrow-band band-stop filter device to achieve suppression of the interference. Then, in the light of the leakage of the spectrum due to cutting off the signal, it processes the signal for adding windows. The analysis of adding windows characteristics of linear frequency modulation interference in the domain of fractional Fourier transform shows that the energy of interference is relatively more concentrated. Finally, considering that there will be a sectional energy loss of the signal because of adding windows, this paper uses overlapping sub-windowed approach to achieve interference suppression. The result shows that overlapping sub-windowed approach is easier to implement than the band stop filter and it can improve system performance of interfering suppression more effectively.
引文
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[2]郭林.直扩信号码序列恢复算法及系统性能仿真研究[D].电子科技大学硕士学位论文.2009,1-3页.
[3]朱春华.直接序列扩频系统的线性调频干扰抑制技术研究[D].郑州大学硕士学位论文.2004,5-11页
[4]韦惠民.扩频通信技术及应用[M].西安电子科技大学出版社.2007,12-13页.
[5]乔海龙.直扩系统抗窄带干扰技术的研究与实现[D].西安电子科技大学硕士学位论文.2009,1-2页.
[6]边海龙.非平稳信号联合时频分析方法的若干问题研究与应用[D].电子科技大学博士学位论文.2008,1-2页.
[7]任孝民,绍高平,钱学锋.直扩通信中时变幅度LFM干扰的抑制[J].信号处理.2009,25(10)：1560页.
[8]朱全银,邓建平.基于分数阶傅里叶变换的线性调频干扰抑制[J].探测与控制学报.2009,31(1)：10-11页.
[9]Xuemei Ouyang,M.G.Amin.Short-Time Fourier Transform Receiver for Nonstationary Interference Excision in Direct Sequence Spread Spectrum Communications [J].IEEE Transactions on Signal Processing.2001,49(4): 851-863P.
[10]Ouyang X, Amin M. Performanace analysis of the DSSS communications receiver implementing a short time Fourier transform interference excision system [A]. IEEE International Symposium on Time Fre-quency and Time Scale Analysis [C]. Pittsburgh:IEEE,1998.393-396P.
[11]Oyang X, Amin M. Time frequency concentration measures for nonstationary interference excision in DSSS communications [A]. Proceedings of IEEE Sarnoff Symposium on Advanced in Wired and Wireless Communications[C]. Trenton, NJ:College of New Jersey Press, 1998.246-249P.
[12]陶然,耿鹏飞.基于Radon_STFT的DSSS系统线性调频干扰抑制[J].北京理工大学学报.2005,25(10)：900-904页
[13]S.R.Lach,M.G.Amin,A.R.Lindsey.Broadband nonstationary interference excision for spread spectrum communications using time-frequency synthesis[C].Proceedings of the IEEE International Conference on Acoustics,Speech and Signal Processing,Sealtle,1998,6:3257-3260P.
[14]Barbarossa S, Analysis of multicomponent LFM signals by a combined Wigner-Hough transform [J].IEEE Trans Signal Processing.1996,3(4):112-115P.
[15]S. Barbarossa and A. Scaglione. Adaptive time-varying cancellation of wideband interferences in spread-spectrum communications based on time-frequency distributions [J]. IEEE Trans on SP,1999,47(4):957-965P.
[16]冉鑫,马世伟,曹家麟.利用时频方法去除扩展频谱信号中的线性调频干扰[J].微计算机信息.2001,17(3)页
[17]M. Amin and G. Mandapati.Nonstationary interference excision in spread spectrum communications using projection filtering methods. Proceedings of the ASILOMAR Conference on Signals,systems and Computers. Pacific Grove,CA:1998,827-831P.
[18]潘文玲.宽带非平稳干扰信号的时频分析及抑制技术研究[D].电子科技大学硕士学位论文.2007,1-8,11-12页.
[19]文亮.分数傅里叶变换及其应用[D].重庆大学硕士学位论文.2008,1-2页.
[20]冉启文,谭立英.小波分析与分数傅里叶变换及应用[M].国防工业出版社.2002,103页.
[21]邹欲越.采用分数傅立叶变换的光学图像处理方法研究[D].哈尔滨工业大学工学硕士学位论文.2006,1-3页,11页.
[22]陈旭.分数傅里叶变换在地震资料处理中的应用初探[D].吉林大学硕士学位论文.2009,1-4页
[23]牛虹.基于分数阶Fourier变换的时变幅度线性调频信号的检测[D].郑州大学硕士学位论文.2005,10-12页,22-27页
[24]陶然,齐林,王越.分数阶Fourier变换的原理与应用[M].清华大学出版社.2004,1-5页,23-68页.
[25]E.U.Condon.Immersion of Fourier transform in a continuous group of functional transformations[J].Proc.National Academy of Sciences.1937, 23:158-164P
[26]H.Kober.On fractional integrals and derivatives[J].Quart.J.Math.Oxford Ser. 11.1940,193-211P
[27]田瑞卿.基于分数阶傅里叶变换的图像数字水印[D].北京化工大学硕士研究生学位论文.2006,11-16页
[28]刘婉莹.基于分数阶傅氏变换的地震信号分析方法研究[D].中国海洋大学硕士学位论文.2009,6页
[29]杨文涛.分数傅里叶变换在数字图像处理中的应用研究[D].华中科技大学博士学位论文.2007,1-6,92页
[30]Ozakta H.M,Aytur O.Fractional Fourier Domains.IEEE.Signal Processing. 1995,46:119-124P
[31]李宏飞.直扩信号的同步与跟踪技术研究[D].西北工业大学硕士学位论文.2005,1-5页
[32]丁兴文.直接序列扩频通信系统中干扰抑制技术研究[D].南京理工大学 学位论文.2007,1-2页
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