HARQ Throughput Performance of Multicode DSCDMA with MMSE Turbo Equalization

Hybrid automatic repeat request (HARQ) is an indispensable technique for packet access. This is employed in high speed downlink packet access (HSDPA) using the orthogonal multicode direct sequence code division multiple access (DS-CDMA). As the data rate increases, the frequency-selectivity of the channel becomes severer and some equalization technique other than rake combining is necessary. The use of MMSE frequency-domain equalization (MMSE-FDE) can significantly improve the throughput performance. However, the residual inter-chip-inference (ICI) after MMSE-FDE produces the orthogonality distortion among the spreading codes and the throughput performance is far from the theoretical lower bound. In this paper, we study an MMSE turbo equalization for HARQ using multicode DS-CDMA. It is shown by computer simulation that MMSE turbo equalization can significantly improve the throughput performance. An E_S/N₀ reduction of as much as 2.5-3 dB from the no turbo equalization case is obtained for a throughput range of 2-2.5 bit/s/Hz.