Research On Data Detection Techniques For Downlink And Uplink MC-CDMA System

MC-CDMA is a spread-spectrum transmission technique introduced in 1993. It combines the advantages of OFDM and CDMA techniques, introduces parallel transmission theory of OFDM into CDMA, and avoids the side effects of intersymbol interference (ISI) and multipath interference as well as performing spread spectrum and has such good characteristics as high spectral efficiency and fast implementation by the fast Fourier transform. MC-CDMA has been considered to be a promising candidate for the air interface of the fourth generation (4G) wireless communication systems.Current CDMA-based wireless communications systems have to cope with a large mount of multiuser interference (that is also called multiple access interference (MAI)), due to many users that need to share the spectrum to achieve high-rate multimedia communication. Therefore, the capacity of MC-CDMA system is also essentially limited by MAI. Multiuser detection (MUD) has been extensively researched and used to combat MAI and increase system capacity as one of advanced signal processing technologies. In this thesis multiuser detection techniques are focused on for both downlink and uplink MC-CDMA system. The base station receivers have much higher complexity allowance than mobile terminals. For this reason, downlink receivers are supposed to exploit relatively simple data detection schemes to guarantee reasonable trade-off between performance and system complexity, while uplink receivers at the base station can employ relatively complex suboptimal multiuser detection schemes.The single-user detection and multi-user detection schemes in MC-CDMA system are analyzed in detail. Owing to simple structure and easy implementation, single-user detection is more suitable for downlink. The following single-user detection strategies are analyzed and simulated, such as MRC, EGC, MMSEC and ORC. MMSEC has superior performance in downlink. As for uplink, MMSE multiuser detection and PIC are given more investigation. Afterwards, a multi-user detection scheme with better performance is proposed in uplink for MC-CDMA system: MMSE-PIC. Based on this, MMSE-PIC is made an improvement. Multistage partial Parallel Interference Cancellation (PPIC) detection scheme is presented. That is MMSE-PPIC. The main merits of this scheme are employing MMSE as the initial stage to guarantee more accurate initial data estimate and eliminating a fraction of the reconstructed MAI for each user to further reduce the error propagation inherited from PIC multiuser detector. The simulation results show that MMSE-PPIC can further improve the performance and system capacity for MC-CDMA system without channel estimation error. However, compared with MMSE-PIC, MMSE-PPIC has no significant performance improvement when the number of active users is small. In addition, further simulation manifests that different choice of partial cancellation factor in each stage has an impact on MMSE-PPIC system performance. It follows that the selection of appropriate partial cancellation factor is in favor of system performance improvement.