Open Loop Forward Link Beamforming with Diversity Switching

A novel method for using an antenna array to improve the performance of the forward (base station to mobile) link in cellular mobile telecommunications systems is described. Previous methods usually involve either beamforming (forming narrow radiation patterns in the direction of the mobile) or spatial diversity (exploiting uncorrelated channels realized by placing antennas far apart in a fading environment). However, neither strategy is effective in all channel conditions; for example, beamforming is ineffective in conditions with large angle spread, whereas spatial diversity is ineffective in conditions with low angle spread. In the method proposed here, one switches periodically between two beamformers. The beamformers are selected so as to improve signal power delivered to the mobile to the extent possible, while maintaining low correlation between the channels associated with the two beamformers as perceived by the mobile. Thus, switching between the beamformers provides diversity. It is shown that the proposed technique significantly outperforms both a traditional beamforming scheme and a 2-element diversity scheme for flat fading channels with a wide range of angle spreads. Further, the method can be applied to many existing protocols without modification, and requires no calibration of the antenna array.