Pilot Assignment in NOMA-Aided Cell-Free mMIMO Systems: A Hypergraph Coloring Scheme

A cell-free massive multiple-input multiple-output (CF-mMIMO) system integrated with non-orthogonal multiple-access (NOMA) is investigated under ultra-dense wireless networks (UDWNs) to meet the growing demands of devices and data traffic demand in the future sixth generation (6G) networks. However, intra-cluster pilot contamination due to NOMA severely impairs system performance. To mitigate this issue, a novel hypergraph coloring (HGC)-based algorithm is proposed for pilot assignment considering the complex cumulative interference among multiple users. In this algorithm, the main access points (APs) selection method is employed to construct their interference hypergraph, and pilots are assigned according to the interference weights within the hypergraph. Numerical results demonstrate that the proposed NOMA-hypergraph scheme can significantly improve spectral efficiency in the system.