Comparison of transmit array algorithms for code division multiple access downlinks operating in the frequency division duplexing mode

It is expected that in the forthcoming 3^rd generation (3G) mobile radio systems the downlinks will have to support much higher data rates than the uplinks. A promising measure to enable high downlink data rates consists in improving the carrier-to-interference ratios at the mobile receivers by the utilization or adaptive mufti-element transmit antennas. However, for the efficient operation of such antennas information about the downlink radio channels is required at the base stations. In the case of the 3G partial standard time division(TD)-CDMA designated for the time division duplexing (TDD) radio frequency bands, such information-thanks to the reciprocity theorem-is readily available from the uplink channel estimators. However, the 3G partial standard W(wideband)-CDMA designated for the frequency division duplexing (FDD) radio frequency bands suffers from the inherent disadvantage that, due to the frequency difference between uplink and downlink, the results of the uplink channel estimation cannot be directly used as the channel information required for adjusting the multi-element transmit antennas. Nevertheless, as long as the frequency difference between uplink and downlink is not too large, approaches to an at least sub-optimum adjustment of the downlink transmit antennas based on uplink channel estimates exist. In the paper such approaches are presented and evaluated by simulations