Research On Code Acquisition Of Chaotic Spread Spectrum Sequence

Chaos refers to the noise-like and deterministic behavior of non-linear dynamic systems that are highly sensitive to initial condition, thus a large number of almost uncorrelated signals can be generated and reproduced. Quite a lot of chaos-based techniques have been developed in telecommunication. Chaotic sequences are used instead of traditional PN code in spread spectrum systems. The code acquisition method of chaotic sequences is discussed in the paper.Firstly, chaos theory is introduced, including the definition and characteristic of several chaos mapping, as well as the production method of digital chaotic sequence. Then we studied some performance of chaotic sequences by simulation. On the basis of analysis, an algorithm is proposed in order to pick out sequences with good property. A new sequence using M code embedded by chaotic sequence is presented to make better balance performance. It can be concluded that with the increasing of iteration times, the length of chaotic sequence increases, the balance performance and correlation performance gets better. But while being applied in real spread spectrum system, the length of sequence is limited, it's necessary to search for a compromise between the sequence length and the performance. The goal can be achieved by optimization or generating embedded code.For code acquisition of shortened chaotic sequence with a fixed period, a method based on sequential detection is discussed, which is also named large step fast acquisition method. Circular flow-graph is used for system modeling and analyzing the principle. The mean and variance of acquisition time is discussed, under different threshold, some important varieties such as detection probability, false alarm probability and penalty time are analyzed. The code acquisition method for non-periodic chaotic sequence is also discussed, the method is based on the principle that the correlation of time domain signals can be transferred to frequency domain. According to the simulation result, acquisition method based on FFT/IFFT is effective.