I. Introduction
The third generation (3G) of mobile communications is intended to increase system performance, which is required to meet the rising demand for Internet and wireless services. In particular, the Third Generation Partnership Project (3GPP) [2] and 3GPP Two [3] have developed the wideband code-division multiple access (WCDMA) [4] and CDMA2000, respectively. Multiple antenna techniques, which are used in 3G systems, are known to enhance the capacity and quality of wireless communication [5] by exploiting the available spatial diversity. On the other hand, the improvement of the downlink capacity is one of the main challenges of the 3G systems because many of the proposed services are expected to be downlink intensive. Therefore, transmit antenna diversity has been considered, which is typically suitable for downlink applications [6].