MMSE Turbo Equalization for Multicode DS-CDMA

Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can replace the conventional rake combining with significantly improved bit error rate (BER) performance of multicode DS-CDMA in a frequency-selective fading channel. However, the presence of residual inter-chip-interference (ICI) after MMSE-FDE produces the orthogonality distortion among the spreading codes and the BER performance degrades as the number of multiplex order increases. Recently, we have proposed a frequency-domain ICI cancellation to improve the uncoded performance. To further improve the BER performance, in this paper, we propose MMSE turbo equalization. In the proposed scheme, MMSE-FDE and ICI cancellation are incorporated into iterative maximum a posteriori (MAP) decoding, resulting in an MMSE turbo equalization. MMSE weight taking into account the residual ICI is updated in each iteration. The effect of MMSE turbo equalization is confirmed by computer simulation