Robust transmit eigen-beamforming based on imperfect channel state information

Transmit beamforming is a powerful approach to enhance the performance and increase the throughput of wireless communication systems that employ multiple antennas at the transmitter. A major drawback of existing transmit beamforming techniques is that most of them require a nearly perfect knowledge of the channel at the transmitter, which is not available in practice. Transmitter designs that address the imperfect channel state information (CSI) problem commonly use statistical models for the channel and/or mismatch between the presumed and actual transmitter CSI. Since these approaches are model-based, they can suffer from mismodeling. We propose a more robust framework in which we make no statistical assumptions about the CSI mismatch and the channel. Our robust transmit beamformer is designed to have the best performance under the worst-case CSI mismatch. It turns out to be an eigen-beamformer along the eigenvectors of the spatial channel correlation matrix. Power loading across the beams is achieved by spatial water-filling-type strategy in which the water level is determined in a closed form.