Scalable Cell-Free Massive MIMO Systems with Graph Coloring based Pilot Assignment

Cell-free massive multiple-inputs multiple-outputs (mMIMO) emerges as a technique to address increasing users in 5G. Implementing cell-free mMIMO in real-world situations comes with challenges, and one notable concern is the ability to scale in scenarios with high access demands. Scalability can be addressed by employing an effective pilot assignment technique during uplink transmission and considering usercentric cell-free mMIMO. Various pilot techniques have been proposed, such as random pilot assignment, interference-based pilot assignment, and user group pilot assignment. However, the practical implementation and complexity of scalable cell-free mMIMO systems pose significant challenges. This paper proposes a pilot assignment technique based on graph coloring to overcome these challenges. The proposed technique improved spectral efficiency (SE) and performance compared to existing pilot assignment techniques by reducing pilot contamination. Numerical findings shows that the proposed pilot assignment scheme, utilizing graph coloring, significantly improves the $\mathbf{9 0 \%}$ likely SE compared to traditional pilot assignment methods.