Channel covariance is emerging as a critical ingredient of the acquisition ofinstantaneous channel state information (CSI) in multi-user Massive MIMOsystems operating in frequency division duplex (FDD) mode. In this context,channel reciprocity does not hold, and it is generally expected that covarianceinformation about the downlink channel must be estimated and fed back by theuser equipment (UE). As an alternative CSI acquisition technique, we propose toinfer the downlink covariance based on the observed uplink covariance. Thisinference process relies on a dictionary of uplink/downlink covariancematrices, and on interpolation in the corresponding Riemannian space; once thedictionary is known, the estimation does not rely on any form of feedback fromthe UE. In this article, we present several variants of the interpolationmethod, and benchmark them through simulations.
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