The Generation Of The Improved Chaotic Spread Spectrum Sequence And The Research On Its Application

Chaos is the non-linear dynamic system with characteristics of certainty and similar randomness. It is an unperiod and an irregular course. It is not convergence but dose have a bound, while it is sensitive to its initial value. For all these special performance, chaos has an excellent outlook in the application of secure communication and spread spectrum communication.In the3G-based mobile communication technology, the CDMA-based spread spectrum has become the key technology due to its advantages of anti-jam and anti multi-access. For a spread spectrum communication, the performance of system will be directly decided by the spread spectrum code of the generator. Thus for the signal source, how to improve the performance of spread spectrum code becomes an important issue for a spread spectrum communication system. The present spread spectrum communication system uses the m-code and Gold-system generated from the maximal length linear shift register, yet they still have the drawbacks of length and quantity limitation. The discovery of the chaotic dynamics provides the new ideas for the use and research of the spread spectrum sequence. The application of chaotic sequence in spread spectrum communication becomes the hot research topic.In this paper, three typical chaotic mappings are presented, including Logistic, modified Logistic, and Chebyshev. The balance and correlation of these mappings are analyzed in detail, which supplies the basic theory for the improved chaotic sequence. Based on the Shannon Theory, the bases of spread spectrum are introduced, including classification, system consistent, mathematic model, characteristic, and performance index. And the direct sequence (DS) spread spectrum is also introduced.By the research of the original Logistic model, the improved Logistic and the Chebyshev chaotic map, a new method to generate a chaotic spread spectrum sequence based on the combined chaotic model, is proposed. The basic contents of this method are as follows:(1) put original given data into the mixed model to iterate;(2) preprocess the result, a sequence, generated by (1);(3) make a bit-L-selection from the sequence generated by (2) to obtain a new sequence. Justifying this new method is to analyze the balance property, security property and correlation of this new sequence. By the analysis and the comparison with the properties of the current sequence, it is proved that this new method is effective on the grounds that the sequence generated by it has a similar correlation, better balance and security property.The simulation model of the modified chaotic spread spectrum sequence is constructed by SIMULINK. A single user chaotic spread spectrum communication system is constructed based on a modified chaotic sequence. The theory and SIMULINK simulation model of this system are given. The waves of all signals in the system are simulated. The effect of the modified chaotic spread spectrum sequence is emphasized based on the analysis of the frequency spectrum. The bit error rate (BER) is analyzed, comparing its effect on the system between the modified chaotic sequence and the traditional chaotic sequence. The results show that the system has very good ability against noise and multiple access interference, and the performance of the modified chaotic sequence is much better than that of the traditional chaotic sequence. Moreover, the comparison of different modulation in the system is given.Finally, the multi-user user chaotic spread spectrum communication system and DS-SS are constructed based on the modified chaotic spread spectrum sequence. The diagram and simulation model are given in SIMULINK. From the theory basis, the mathematic express of BER is presented. And the effect of the BER depending on the amount of users is obtained. The results show that the system has very good ability against noise and multiple access interference, and the performance of the modified chaotic sequence is much better than that of the Chebyshev chaotic sequence. The BER of DS-SS system, based on modified chaotic spread spectrum sequence, is lower than the traditional chaotic spread spectrum. The BER of DS-SS system changes slow when the amount increases. Thus, the system has an advantage of big content.