Combined MMSE-PIC in coded OFDM-CDMA systems

In this paper the performance of linear multi-user detection (MUD) and nonlinear parallel interference cancellation (PIC) is analyzed for a quasi synchronous OFDM-CDMA uplink transmission. Specifically, we investigate two different approaches concerning the combination of linear MUD techniques such as the decorrelator and the MMSE approach with nonlinear parallel interference cancellation (PIC). It is pointed out that OFDM-CDMA systems offer great advantages over single carrier systems due to flat fading conditions on each subcarrier leading to much lower implementation costs of MUD techniques. Assuming perfectly known channel impulse responses for each user and a rough synchronization it turns out that the application of the MMSE-MUD filter leads to an insufficient equalization of the channel. Therefore, we propose an approach where the PIC loop works directly on the channel output and the MMSE filter is only active in the initial iteration loop. With this approach, the combination of linear MMSE-MUD and PIC shows excellent performance even for very high system loads.