A Fast Pilot Assignment for Cell-Free Massive MIMO: Using Anchor-Based Clustering Scheme

Abstract

In this letter, we study the pilot assignment (PA) in cell-free (CF) massive multiple-input multiple-output (MIMO). PA is a key issue to facilitate signal processing, e.g., beamforming, by providing reliable channel estimation at access points (APs). In the context of non-orthogonal pilots, we propose an anchor-based (AB) clustering scheme to reduce the pilot contamination (PC) among the user equipments (UEs). The proposed approach aims at partitioning the potential high-PC UEs into different clusters. More specifically, we first construct an interference graph to model the PC. Then a binary space partitioning (BSP) algorithm is leveraged for generating clustering anchors. To obtain the target clustering of UEs, we perform a spectral analysis on the UE-anchor clustering matrix. Comprehensive numerical results on several different configuration settings, show that the proposed approach achieves near-optimal performance in terms of the normalized mean square error (NMSE) of channel estimation.