Robust Precoding and Rate-Splitting Multiple Access for FDD Massive MIMO Without CSI Feedback

The overhead of acquiring downlink (DL) channel state information at the transmitter (CSIT) poses a significant challenge for frequency division duplex (FDD) massive multipleinput multiple-output (MIMO) systems. To address this, we propose a framework that reconstructs DL CSIT solely from uplink (UL) pilots, leveraging partial frequency invariance of channel parameters. However, since perfect reconstruction is infeasible due to inherent discrepancies between the UL and DL bands, multi-user interference (MUI) arises. To address this, we formulate the ergodic sum spectral efficiency (SE) by incorporating the estimated channel and error covariance matrix (ECM). Additionally, we propose an ECM estimation technique using observed Fisher information and enhance system performance with a rate-splitting multiple access (RSMA) precoder optimization design. The simulation results validate the effectiveness of the proposed framework, demonstrating the critical role of ECM estimation in mitigating MUI and providing robust SE gains in practical FDD-based massive MIMO scenarios.