Error Free Despreading Of Orthogonal Codes In Quasi-Synchronous Cellular CDMA Communication

Abstract

Code division multiple access (CDMA) schemes have been considered as attractive multiple access schemes in the second-generation (2G), third-generation (3G) and future broadband wireless systems. Orthogonal codes are integral part of any broadband cellular systems. In this paper, we studied the performance of DS-CDMA signals with Walsh-Hadamard orthogonal codes, Orthogonal Gold codes and Quadratic-Residue (QR) orthogonal codes in a quasi-synchronous transmission. The synchronization error considered is smaller than the chip duration. We evaluate the maximum synchronization error that can be tolerated so that complete error-free despreading is guaranteed for all the three orthogonal codes in the absence of additive white Gaussian noise. This provides insight in the synchronization demands for QS-CDMA systems. Also system capacity (the total bit rate which can be detected) subjected to this synchronization error has been calculated for all the three codes. It has been found that QR orthogonal codes provide maximum allowable synchronization error and system capacity as compared to other two orthogonal codes. The BER performance at different values of synchronization error is also studied. Monte-Carlo simulations have been carried out to verify the numerical results.