Graph Coloring-Based Pilot Assignment for User-Cluster-Centric Cell-Free Massive MIMO Systems

A user-cluster-centric (UCC)-based cell-free massive multiple-input multiple-output (CF-mMIMO) system, which realizes multi-user multiplexing in each user-cluster formed by neighboring users and served by surrounding antennas, guarantees system scalability and improves the user capacity. However, accurate channel estimation is necessary for multi-user multiplexing. In this paper, we propose a graph coloring (GC)-based pilot assignment method to efficiently reuse the limited number of orthogonal pilots. It is verified by simulation that our proposed GC-based pilot assignment can effectively avoid pilot contamination.