Rate-Splitting for SIM-Enhanced Cell-Free Massive MIMO in High Speed Railway Scenarios

Abstract

To address the interference issues caused by channel aging in high-speed railway (HSR) communications and further reduce the signal processing latency to meet the ultra-low latency requirements of accelerating HSR speed, considering the native electromagnetic signal processing of stacked intelligent metasurfaces (SIMs) and the interference cancellation capability of rate-splitting multiple access (RSMA), we propose a novel RSMA-aided SIM-enhanced cell-free massive MIMO architecture for HSR train-ground wireless communications. Based on the minimum mean square error channel estimates, closed-form expressions for the spectral efficiency (SE) are derived for both perfectly known and unavailable initial line-of-sight (LoS) phase conditions to delineate the performance bounds of the system. Additionally, to address the challenge of communication quality degradation caused by high-speed movement, a heuristic algorithm is developed to jointly optimize the SIM beamforming matrix and the rate-splitting factor, thereby maximizing the sum SE. Moreover, an access points (APs) selection based scheme on normalized channel estimation is proposed to improve the scalability of HSR communication system. The simulation results demonstrate the effectiveness of the proposed joint optimization algorithm. It is also shown that RSMA effectively mitigates channel aging-induced interference, especially under a perfectly known LoS initial phase, and that the proposed SIM-enhanced cell-free massive MIMO system, enabled by the beamforming algorithm, outperforms the conventional system when the initial phase of LoS path is available at APs.