Splitting Messages in the Dark- Rate-Splitting Multiple Access for FDD Massive MIMO Without CSI Feedback

TL;DR

This paper introduces a CSI feedback-free framework for FDD massive MIMO systems using uplink pilots and rate-splitting multiple access, effectively mitigating multi-user interference and improving spectral efficiency.

Contribution

The paper proposes a novel method to reconstruct downlink CSI without feedback and optimizes precoding with RSMA, addressing key challenges in FDD massive MIMO.

Findings

Reconstructed DL CSI using uplink pilots and 2D-NOMP algorithm.

ECM estimation improves interference mitigation.

Simulation confirms enhanced spectral efficiency with the proposed method.

Abstract

A critical hindrance in realizing frequency division duplex (FDD) massive multi-input multi-output (MIMO) systems is the overhead associated with the downlink (DL) channel state information at the transmitter (CSIT) acquisition. To address this, we propose a novel framework that eliminates the need for CSI feedback, while achieving robust sum spectral efficiency (SE). Specifically, by leveraging partial frequency invariance of channel parameters, we reconstruct the DL CSIT using uplink (UL) pilots with the 2D-Newtonized orthogonal matching pursuit (2D-NOMP) algorithm. Due to discrepancies between the two disjoint bands, however, perfect DL CSIT acquisition is infeasible; resulting in multi-user interference (MUI). To account for this, we reformulate the sum SE maximization problem using the reconstructed channel and its error covariance matrix (ECM). Then, we propose an ECM estimation method based on the observed Fisher information matrix and introduce a precoder optimization technique with rate-splitting multiple access (RSMA). Our simulation results verify the validity of the proposed framework in the practical FDD massive MIMO scenarios, highlighting the essential role of ECM estimation in mitigating MUI to attain RSMA gains.