MULTICARRIER code division multiple access (MCCDMA) is one of the promising techniques for fourth generation wireless mobile communication systems. MCCDMA has several advantages, such as efficient spectrum utilization and immunity to multipath impairment. One major drawback associated with MC-CDMA is that multiuser detection is often necessary to overcome the multiple access interference (MAI) problem in uplink systems. Many multiuser detection approaches have been proposed in the literature .Among them, multistage parallel interference cancellation (PIC) has received great attention because of its low complexity and low processing latency, but it fails to guarantee performance improvement with more interference cancellation (IC) stages in moderate to high system load environments. To alleviate this problem, Divsalar et al. proposed the partial PIC (PPIC) detector, which first estimates MAI and then partially cancels it from the received signal on a stage-by-stage basis. Moshavi also indicated in that linearly combining soft tentative decisions of each IC stage can reduce the variance of the signal estimate, which will generate more reliable MAI estimates. As revealed in, a bit estimator that linearly combines the soft decisions from previous stages at a given stage for interference cancellation is inherently used by the PPIC approach. Therefore, the PPIC approach outperforms the PIC approach. Because the PPIC detector introduces weighting factors (WFs) to each IC stage, the performance depends highly on the choice of them. For good performance, two optimal WF (OWF) selection schemes based on analyzing the bit-error-rate (BER) were proposed for CDMA systems. However, the complexity of BER analysis increases greatly with the number of IC stages, and these approaches generally are not suitable for applications with more than two IC stages.
In this project, an improved version of PPIC, called variance-reduced PPIC (VRPPIC),for C-CDMA uplink systems. First,the main PPIC operations, where (the number of active users) multiplications of the original PPIC scheme are replaced with additions for each IC stage.Then we derive VRPPIC based on this simplified PPIC, where a new bit estimator is used for linearly combining soft tentative decisions from PPIC to reduce the conditional variance of the final signal estimate. In addition, an OWF selection algorithm for VRPPIC based on minimizing a conditional variance function of the final signal estimate. With the monotonically increasing property of the conditional variance function, it is shown that the OWFs for all IC stages are equal and can be approximately expressed as a linear function of the number of active users. Computer simulation results show that the proposed VRPPIC using the derived OWFs significantly improves the system performance.
In this project, an improved version of PPIC, called variance-reduced PPIC (VRPPIC),for C-CDMA uplink systems. First,the main PPIC operations, where (the number of active users) multiplications of the original PPIC scheme are replaced with additions for each IC stage.Then we derive VRPPIC based on this simplified PPIC, where a new bit estimator is used for linearly combining soft tentative decisions from PPIC to reduce the conditional variance of the final signal estimate. In addition, an OWF selection algorithm for VRPPIC based on minimizing a conditional variance function of the final signal estimate. With the monotonically increasing property of the conditional variance function, it is shown that the OWFs for all IC stages are equal and can be approximately expressed as a linear function of the number of active users. Computer simulation results show that the proposed VRPPIC using the derived OWFs significantly improves the system performance.
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