  A critical hindrance to realize frequency division duplex (FDD) massive
multi-input multi-output (MIMO) systems is overhead associated with downlink
channel state information at the transmitter (CSIT) acquisition. To address
this challenge, we propose a novel framework that achieves robust performances
while completely eliminating downlink CSIT training and feedback. Specifically,
by exploiting partial frequency invariance of channel parameters between the
uplink (UL) and downlink (DL), we adopt the 2D-Newtonized orthogonal matching
pursuit (2D-NOMP) algorithm to reconstruct DL CSIT from UL training. Due to
inherent discrepancies arising from a carrier frequency difference between two
disjoint bands, however, the multi-user interference is inevitable. To overcome
this, we propose a precoding method that employs rate-splitting multiple access
(RSMA) and also develop an error covariance matrix (ECM) estimation method by
using the observed Fisher information matrix (O-FIM). We find that this ECM
estimation is crucial for our precoding design in maximizing the sum spectral
efficiency (SE). Simulation results show that our method significantly improves
the sum SE compared to other state-of-the-art approaches, underscoring the
importance of our ECM estimation.
