Channel Training for Analog FDD Repeaters: Optimal Estimators and Cram\'er-Rao Bounds

For frequency division duplex channels, a simple pilot loop-back procedure has been proposed that allows the estimation of the uplink (UL) and downlink (DL) channel subspaces between an antenna array and a fully analog repeater. For this scheme, we derive the maximum likelihood (ML) estimators for the UL and DL subspaces, formulate the corresponding Cramér-Rao bounds, and show the asymptotic effic...