| Traditional spread spectrum's PN codes are generated by the linear feedback shift register. So its complexity is low and security is bad. With the development of the nonlinear theory, the chaotic sequence have caught wide attention and been researched a lot because of its good correlation, similarity to White Gaussian noise, infinite non-periodic, the extreme sensitivity to initial values, and the ability that can generate a large number of sequences for multi-access communication.At first the background of chaos and the current researches on chaos both in China and abroad are introduced in this paper. The methods of generate the chaotic sequences are simply described. Then four common chaotic sequences named Logistic sequences, Chebyshev sequences, Tent sequences, Improved Logistic sequences are introduced, include their mapping function, the range of the initial value and statistical characteristics. Their correlation properties are simulated and the statistical properties are comparatively analysed.After a brief overview of the traditional direct spread spectrum system(DSSS), the chaotic DSSS theoretical bit error rate is derivated. At the same time several kinds of common interference, include part of the band jamming, single-frequency interference and impulse noise, are introduced. Under White Gaussian noise channel and different kinds of interference, the optimization selected chaotic sequence's BER is simulated, which is compared with the theoretical values of Gold sequence.Then three synchronization strategies are proposed for the chaotic synchronization problems: Blind estimate the chaotic sequence by using the neural network, which can blind estimate the chaotic sequence directly in low SNR, without searching the synchronization point; Using Extended Kalman Filter or Unscented Kalman Filter to estimate and tracking on the chaotic spread spectrum signal, which can traking an infinite non-periodic chaotic spread spectrum signal real-time. And the design of best receiver based on improved Logistic sequence spread spectrum system is analysed; Using the quasi-chaotic characteristics of digital encoder to design the chaotic synchronization system.Finally, the traditional basic principle of pseudo-code ranging is introduced, including the largest non-fuzzy distance, the coherent and non-coherent pseudo-code ranging based on onboard processing mode, and the pseudo-code synchronization. According to the traditional pseudo-code ranging principle and chaos characteristics, a ranging program was designed based on double-sliding correlation theory. The double-sliding correlation theory ranging program is used for regenerative transponder ranging and the chaotic sequences is nonperiodic. The program is analysed in simulation. |
