摘要
扩频通信由于其具有工作信噪比低、强抗干扰性、低截获率、能够抑制多径干扰等优点,在移动通信、雷达、导航等多领域中都有着广泛的应用。随着电子战与信息战技术的发展,如何在低信噪比和先验知识缺乏的条件下,对直扩信号进行检测以及参数估计已经成为一个重要的研究课题。
信号检测与参数估计有很多方法,这些方法大多是针对常规信号进行检测与参数估计。由于直扩信号是宽带微弱信号,不符合常规信号处理方法,在低信噪比下,这些方法的检测性能急剧恶化。从目前研究情况来看,直扩信号的检测与参数估计方法还不够完善。目前,直扩信号的检测与参数估计已有一些方法,但大多方法各自对某一参数有很好的估计效果,不能对所有参数系统性地进行估计。
本文主要研究了高阶累积量检测法、高阶循环累积量检测法、延时相乘法、自相关等方法,在归纳了前人理论的基础上,提出了一些改进方法,主要研究成果如下:
1.在对高阶累积量的研究的基础上,针对直扩信号载频检测问题,利用四阶累积量理论上可以有效地抑制高斯白噪声的特性与自适应干扰对消器可以有效消除噪声的特性相结合,提出了一种低信噪比条件下直扩信号的载频的估计方法。实验仿真验证了此方法的有效性。
2.在对高阶循环累积量的研究的基础上,针对直扩信号载频与码片速率检测问题,利用循环统计量的循环特性体现循环平稳信号的频率参数的特点,以及高阶循环累积量可以有效地抑制任何平稳噪声与非平稳高斯有色噪声的特性,提出了一种可以同时对直扩信号的载频与码片速率进行盲估计的方法。实验仿真验证了此方法的有效性。
3.在对延时相乘法与自相关研究的基础上,针对长伪码直扩信号伪码周期、码片速率、载频参数检测问题,利用延时相乘法可以检测出直扩信号的码片速率与载频参数以及自相关运算可以有效地检测出直扩信号的周期性等特性,提出了一种延时相乘多重自相关的综合性检测方法。此方法不但可以检测出长伪码直扩信号的载频、码片速率,而且可以检测出长伪码直扩信号的周期。实验仿真验证了此方法的有效性。
上述三种方法在无需任何先验知识的情况下,完成了直扩信号参数的检测,对直扩信号实现盲解扩具有一定的意义。实验表明,在低信噪比下,可获得较好的估计效果。
The spread spectrum communication has the advantages of low signal noise ratio (SNR), good anti-jamming, low probability of interception and the capability of anti-multipath interference. It is widely used in mobile communications, radar, navigation and orientation. With the development of the electron and information technology, the detection of the Direct Sequence Spread Spectrum (DSSS) signal and its parameters estimation have become an important research direction on condition of little prior knowledge and low SNR.
A lot of DSSS signal estimation methods have been developed. These methods are mostly based upon the theory of conventional signal detection and estimation. However, DSSS signals are wide-band weak signals, and they are impracticable to the theory of conventional signal processing. Therefore, when the SNR of the received signals becomes lower, the performance of the methods is worsened. At Present, the methods of detecting DSSS signals and estimating its parameters are far from mature. Although there have been some methods for the detection, most of them can only estimate a different parameter of the signal with good performance but can not make a systematic detection for all parameters. In this paper, some methods for the high-order cumulant, the high-order cyclic cumulant, the delay-multiply and autocorrelation are discussed. The improved methods based on predecessors are presented.
1. The fourth-order cumulant theoretically can suppress the Gaussian White Noise and the adaptive noise cancellation can also eliminate noises effectively. Based on the fourth-order cumulant, this paper proposes a method of estimating the carrier frequency of DSSS signals on the condition of the low SNR. This method can solve the difficult problem of the carrier frequency detection. The validity of this method is shown by the computer simulation.
2. The cyclic-character of cyclic-cumulant embodies the frequency of cyclic stable signals, and the high-order cyclic-cumulant can suppress the stationary and non-stationary Gaussian coloured noise. Based on the high-order cyclic cumulant, this paper proposes a method of estimating the carrier frequency and the chip rate of DSSS signals. This method can solve the difficult problem of detection for the carrier frequency and the chip rate. The validity of this method is shown by the computer simulation.
3. The delay-multiply can estimate the chip rate and the carrier frequency of DSSS signals. And the autocorrelation processing can detect the period of DSSS signals. Based on delay-multiply and autocorrelation, this paper proposes a method of delay-multiply multi-autocorrelation. This method not only can estimate the carrier frequency, the chip rate, but also detect the period detection of the long Pseudo Noise (PN) code. The validity of this method is shown by the computer simulation.
The above mentioned three methods can estimate parameters for DSSS signals effectively with little prior knowledge, which have significance for the blind estimation. The simulations show that they can achieve good precision on the condition of low SNR.
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