The Study Of Mismatched Sequence And Its Application In DS-CDMA System

Spread spectrum communication modulates the information data with spreading sequences,and then transforms the modulated signal with a much wider bandwidth,at the receiver, after demodulation and correlation processing with the same spreading sequences,the transmitted information data can be getted back exactly. On the basis of this,code division multiple access(CDMA) communication system takes advantage of orthogonal spreading sequences to differentiate users. The more the number of spreading sequences satisfying this character,the more the number of users,viz. the capacity of system is larger. Therefore it is important for improving the performance of direct sequence spqead spectrum CDMA(DS-CDMA) system to apply the sequences with good correlation, here good indicates big main peak of auto-correlation function, small sidelobe of auto-correlation function and small cross-correlation function.At present,the widely-used spreading sequences include m sequence,Gold sequence,etc.But the main shortage of these spreading sequences is that they exist only at some given length and the number of existing sequences is limited,which restrict their applications in practice.However,the mismatched sequence studied in this paper exist at much more lengths,which provide more choice for practical applications. So mismatched sequence can make up the shortage of the actual spreading sequences as an effective supplement.Furthermore,applying mismatched sequence to the CDMA communication system puts forward a new concept and idea, viz. from mismatched point of view, to study optimal signal,which is more important.In this paper,firstly,the optimal mismatched sequence sets satisfying certain given conditions, including zero sidelobe and low correlation zone, are generated; Secondly,analyze their properties of auto-correlation and cross-correlation and some results with better correlation function than Gold sequence are found ;Then mismatched sequence is applied to DS-CDMA communication system,the BER performance of which is studied by experimental simulation and afterwards is compared with Gold sequence DS-CDMA communication system,the results prove that mismatched sequence can be applied to spread spectrum communication system and the BER performance of mismatched sequence DS-CDMA communication system is better than Gold sequence DS-CDMA communication system;Finally,the existed code lengths and number of sequences of mismatched sequence are stated through computer search and afterwards are compared with that of Gold sequence, the results show that the mismatched sequence is much richer.