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In future wireless systems, increasing the downlink capacity becomes more important due to potential high data rate applications such as wireless internet access. Turbo codes and downlink transmit diversity are two of the most explored techniques to improve performance and capacity. In this paper, we evaluate the performance of turbo coded wideband code-division multiple-access (WCDMA) systems with downlink transmit diversity employing a space-time block code on correlated fading channels. The performance improvement using downlink space-time transmit diversity (STTD) is found to be more significant in the case of slow fading when the temporal correlation is high. By considering the spatial correlation between the transmit antennas at the base station, we quantify the tolerable correlation between transmit antennas by means of simulation for which the turbo coded WCDMA systems with transmit diversity still attain performance gains compared to a single transmit antenna system.