On spatial covariance matrices for downlink eigen-beamforming in multi-carrier CDMA

The present paper considers downlink eigen-beamforming for a multi-carrier CDMA mobile radio system. Three different versions of the spatial covariance matrix based on either short- or long-term averaging are investigated and their properties are assessed analytically and numerically. We highlight that, for environments with narrow angle spreads, the long-term spatial covariance matrix should be selected. In wide angle spreads, the short-term spatial covariance matrix, averaged over the whole system bandwidth, is an interesting trade-off between the beamforming gain and the robustness to Doppler variations.