Uplink to Downlink Channel Covariance Transformation in FDD Systems

This paper considers frequency division duplexing massive multiple-input multiple-output systems in which the base station (BS) is equipped with either a uniform linear antenna array (ULA) or a uniform rectangular antenna array (URA). For these systems, we develop novel uplink-to-downlink channel covariance mapping schemes. These schemes can be expressed in the form of easy-to-implement affine transformations which depend only on the uplink and downlink carrier frequencies, and the BS array configurations. We derive upper bounds on the estimation errors, and we use these bounds to show that the accuracy of the proposed schemes increases with the number ULA/URA antennas, the compactness and differentiability class of the periodic extension of a non-linearly transformed version of the angular power spectrum. Performance superiority of the proposed schemes over their existing counterparts is confirmed through numerical simulations.