On Chaotic Spreading Sequences

Spread梥pectrum communication was firstly applied in military communications due to its good immunity to interference and security. The rapidly developed techniques on VLSI, computer and network have made the application of spread梥pectrum communication in various fields possible, spread梥pectrum communication has been widely used in satellite communication, mobile communication, personal communication, etc.Spread梥pectrum communication is to modulate the signals with the high梥peed spreading codes so as to spread their spectrum and the receiver uses the same spreading codes to decode the signals. The spre&Ling codes which have the good correlation property and randomness property are important to the qualifications of spread梥pectrum communication system. The presently used spreading codes including m sequences, Gold sequences and R桽 sequences have excellent randomness. But they have the disadvantages such as poor complexity, poor security and insufficient amount of sequence numbers among the family of codes, The chaotic signals have noise條ike, broadband frequency spectrum, they can be used as spreading codes, Furthermore, chaos system is extremely sensitive on initial conditions so that the various initial conditions given by chaos system can provide with large sum of spreading sequences and this can meet the demand of CDMA, This thesis sets out to study on using chaos system to generate spreading sequences, There are two parts is this paper:Firstly, We study the basic theory of spread梥pectrum communication and the characteristics of chaos so as to prove the applicability of chaos system to be used in spread梥pectrumiicommunication system.In the second part, a new method is presented to generating spreading sequences from Chebyshev map. Chebyshev map is a one梔imension chaotic map which has simple iteration equation easy to be realized, and extremely sensitive on initial conditions, In this thesis we use various quantization functions to generate binary sequences for DS桽S and multilevel sequences for FH桽S. And the properties such as correlation function, linear complexity and balance property are analyzed theoretically. The analysis shows that chaotic sequences can be used as spreading sequences due to their good properties. The results of simulations prove the reliability of the theoretical analysis. The computer simulation realized direct sequence spread spectrum system which used chaotic spreading sequences instead of PN codes and proved the applicability of using chaotic sequences as spreading codes.