Is Downlink Training Necessary for User-Centric Cell-Free RSMA Systems With Mobile Users?

TL;DR

This paper investigates whether downlink pilots are essential for RSMA in user-centric cell-free mMIMO systems with mobile users, demonstrating their importance for spectral efficiency under realistic channel conditions.

Contribution

It reveals that downlink pilots are necessary for RSMA to outperform SDMA in practical, mobile user scenarios with channel aging and spatial correlation.

Findings

Downlink pilots improve RSMA spectral efficiency in mobile scenarios.

Higher user velocity and longer resource blocks reduce RSMA performance.

Increasing the number of clusters can mitigate spectral efficiency loss.

Abstract

We study the spectral efficiency (SE) of a ratesplitting multiple access (RSMA) enabled multi-clustered cell-free (CF) massive multiple-input multiple-output (mMIMO) system. The access points (APs) in each cluster serve mobile user equipments (UEs) by employing RSMA. The UEs employ successive interference cancellation to decode their data. This work emphasizes the role of downlink (DL) pilots in realizing RSMA benefits in practical CF systems with spatially-correlated Rician channels which observe random phase shifts, pilot contamination, and channel aging due to UE mobility. We numerically show that DL pilots are required for RSMA in user-centric CF mMIMO systems with channel aging to outperform spatial division multiple access. We show that the degraded channel quality due to higher UE velocity and longer resource block lengths significantly reduces the RSMA SE. Increasing the number of clusters can compensate for the SE loss.