Detection Of Direct Sequence Spread Spectrum Signal Based On Fractal Theory

Direct sequence spread spectrum (DSSS) signal obtains widespread developments and applications in the modern military and the field of communications because of its characteristics such as low density and concealment, anti-interference ability, low intercept and low probability of detection, etc. Therefore, it is an urgent task to do the research of DS communication countermeasure technique and find effective methods to antagonize it. The blind detection, parameters estimation and recovery of spreading sequences are the main contents of DSSS communication antagonism. Generally speaking, the signal detection in this situation is noncooperative detection, and it lacks of some prior information, so it's a type of blind detection or approximate blind detection.With the development of nonlinear science in recent years, wavelet, chaos, fractal theory have become more and more important in various fields. And for fractal theory's unique point of view of analyzing problems, it has great development in the fields of radar target detection, digital image processing and weak signal detection. This makes it possible for the theory to be applied to the detection of the direct sequence spread spectrum (referred to as DS) under the condition of low signal-to-noise ratio. Moreover, the application of fractal theory to the blind detection of DS signals is not only an innovation in the field of fractal theory, but also a new idea for the blind detection of DS signal, and it has great potential in improving the performance of detection. In this thesis, the DS-BPSK modulated signal has been analyzed. The methods of blind detection through fractal theory are explored and investigated, and the key work and innovations of the thesis mainly include:1. Studied the development history of the fractal theory, the fundamental nature of fractal and the basic calculation methods of fractal dimension, and deeply analyzed the concept of multifractal.2. Proved DS signal has multifractal characteristics through verifying DS signal's non-Gaussian and long-range correlation, and provided a theoretical basis for applying fractal theory to the detection of DS signal.3. According to the variance fractal dimension (VFD) theory, the algorithm of Variance Fractal Dimension Track (VFDT) is creatively used to blind detection of DSSS/BPSK signal in very low signal-to-noise ratio (SNR) below -25dB.4. Studied the typical ways of blind detection and parameter estimation in the DSSS/BPSK signal, builded on the union processing system including the DSSS/BPSK signal detection, parameter estimation and code sequences restoration, and a lot of simulation and analysis are done for this system.