In recent years, the spread spectrum communication systems are more and more important in digital cellular and personal communication networks. In particular, direct sequence code division multiple access (DS/CDMA) has been adopted in third generation cellular standard.
Improvement of downlink capacity is one of the main challenges for 3G evolution. Transmit diversity provides an attractive avenue for increasing the downlink capacity problem. In order to reduce the interference of downlink transmission to send the better signal to the base station, the transmit diversity technique uses a plurality of orthogonal antennas to send different modulated signals of one message.
The transmit diversity technique can be generally classified as open loop transmit diversity and closed loop transmit diversity. The open loop transmit diversity employs the space time block coding and the full diversity of a plurality of antennas. The distances between antennas should be large enough so that the signal fading received by different antennas can be viewed as independent and the antenna power can be increased. The closed loop transmit diversity mode has tow sub-modes, which use downlink channel measurements and feedback signaling in controlling the phases and/or gain transmit weights in the two transmit antennas.
However, the closed loop transmit diversity is typically applied to a low moving speed environment. In a high moving speed environment (above 40 or 50 km/hour), the variation of the signal is too fast. The fast variation causes signal fading and inaccuracy of channel estimating which decays the system efficiency.