Rate-Splitting Multiple Access Over Cell-Free Massive MIMO With Low-Resolution ADCs

In this paper, we explore the potential of rate-splitting multiple access (RSMA) in a cell-free (CF) massive multiple-input multiple-output (MIMO) system integrated with low-resolution analog-to-digital converters (ADCs). Both the access points (APs) and the user equipment (UEs) deploy low-resolution ADCs. Thus, quantization noise (QN) limits the system’s performance. The limited coherence interval and the large number of users result in pilot contamination. This arises since multiple users may share the same pilot sequence in the uplink training. We investigate the spectral/energy efficiency (SE/EE) of an RSMA-enabled CF massive MIMO downlink transmission due to the robustness of RSMA against interference. We derive the closed-form SE expressions for the common and private streams which are shown to be exact when compared to Monte Carlo simulations. We optimize the combining weights of the common precoder to maximize the minimum user SE due to the common stream. It is shown that substantial SE gains and higher EE are achieved using the proposed algorithm in the presence of QN and imperfect channel state information. Our results emphasize that RSMA can be adopted to enhance the SE and EE while reducing the quantization bits.