Downlink Error Performance Analysis of an SCMA Aided Cell-Free System with OTFS Modulation

The upcoming sixth-generation (6G) communications is expected to support reliable communications for dense wireless networks even with high-mobility users. Integrating sparse code multiple access (SCMA) aided cell-free massive MIMO (CF-mMIMO) with orthogonal time frequency space (OTFS) modulation, i.e., SCMA-based CF-mMIMO-OTFS, potentially addresses these demands. Such a system can leverage the high-density network capabilities of SCMA-based CF-mMIMO and the robustness of OTFS in time-varying channels. This paper studies the error performance of a downlink (DL) user in such a system. We first derive the pairwise error probability (PEP) of a DL user and then an upper bound on the PEP. Based on this upper bound, we derive the maximum diversity order of the DL user. Our analysis highlights the need for better multi-user interference mitigation and provides insights into how the error performance is affected by the multiple access scheme and system/channel parameters. Simulation results validate the analytical findings, demonstrating significant error performance improvements of SCMA-assisted schemes over standard CF-mMIMO-OTFS and orthogonal multiple access (OMA)-based CF-mMIMO schemes.