Multi-carrier Cdma Frequency Offset Estimation In Joint Detection Multiple Access Interference

For wireless communications, direct sequence code division multiple access (DS-CDMA) techniques have received a great deal of attention due to the expectation of capacity increase, interference rejection capability, security, and so on. In a high data rate application such as multimedia services, however, the capacity of DS-CDMA systems is limited by both the inter-chip, or inter-symbol interference. A demand for very high bit rates brought OFDM systems into focus. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream.MC-CDMA system is a combination of direct-sequence CDMA and multicarrier modulation systems. Much attention has been paid to it, because the system can achieve better performance than conventional CDMA in high data applications and it is potentially robust to channel frequency selectivity. But MC-CDMA system still has some problems. Some of the most important sources for performance degradation of MC-CDMA systems are carrier frequency offset and the multiple access interference (MAI). In OFDM systems similar to MC-CDMA systems, many schemes have been proposed to estimate the frequency offset. These schemes, however, cannot be directly applied to MC-CDMA systems due to the MAI and the full correlation between the different subcarrier data. In DS-CDMA systems, many detection schemes have been proposed to eliminate the MAI. However, using these schemes for MC-CDMA systems, their performances get considerably worse even in the presence of the small frequency offset. Therefore, the frequency offset and the MAI should be jointly considered. Our research is focused on these two problems in MC-CDMA system. We hope to clarify the types and originations of interference and consider the main interference synthetically in MC-CDMA system. Our target is to find out a solution to estimate the frequency and eliminate the MAI jointly and also by this solution we can improve the performance and capacity in MC-CDMA system.The work in this thesis is part of the International Cooperate Project "Research on OFCDMA for beyond 3G mobile communications" and Nature Science Research Project of Anhui Education Department, "Research on the blind adaptive multiuser detection in CDMA systems".In this thesis we firstly analyze the MAI and frequency offset in multicarrier CDMA system respectively. Then we evaluate the effect of frequency offset in multicarrier CDMA systems by computer simulation and theoretical analysis. The performance analysis of some conventional multiuser detectors in multicarrier CDMA with frequency offset is performed. Based on the analysis of frequency offset and multiuser detectors in multicarrierCDMA system, we propose a method, named ML-MPIC method, to estimate the frequency offset for the desired user and to eliminate the MAI jointly .The performance analysis of the scheme is given, simulation results with different user number show its better performance than that of systems which deal with these two problems separately.