A generalized QS-CDMA system and the design of new spreading codes

A generalized quasi-synchronous code-division multiple-access (QS-CDMA) system for digital mobile radio communications is proposed in this paper. In a QS-CDMA system, the relative time delay between the signals of different users is random and restricted in a certain time range, that is, the signals are quasi-synchronous. The analysis shows that the multiple-access interference (MAI) of the QS-CDMA system is determined by the crosscorrelation between spreading codes around the origin. To minimize the MAI of the QS-CDMA system, we design a new set of spreading codes. The performance is evaluated according to the criteria of the bit error rate (BER). Analytic results of the BER are obtained by using two methods: Gaussian approximation and characteristic function approaches, which are checked by modified Monte Carlo computer simulations known as "importance sampling." The results indicate that the performance of the QS-CDMA system using the spreading codes we construct is much improved.