Transmit diversity in 3G CDMA systems

Transmit diversity (TD) is one of the key contributing technologies to defining the ITU endorsed 3G systems W-CDMA and cdma2000. Spatial diversity is introduced into the signal by transmitting through multiple antennas. The antennas are spaced far enough apart that the signals emanating from them can be assumed to undergo independent fading. In addition to diversity gain, antenna gain can also be incorporated through channel state feedback. This leads to the categorization of TD methods into open loop and closed loop methods. Several methods of transmit diversity in the forward link have been either under consideration or adopted for the various 3G standards. This article describes the concept of transmit diversity and explains the features of selected TD techniques.