Pilot Contamination Reduction for Access Point Clustering-based Pilot Assignment

Abstract

Abstract—In this paper, we consider an access point clustering- based pilot assignment scheme for user-centric cell-free massive MIMO systems. In such a system, the complexity is reduced by performing the pilot assignment for each cluster independently. However, users at the edge of the cluster will experience high interference from users in neighboring clusters. In this paper, we propose two schemes to reduce pilot contamination for cluster-edge users. The proposed schemes can perform per-group pilot assignment with zero or limited inter-cluster cooperation. Numerical results verify that, compared with conventional pilot assignment schemes, the proposed pilot assignment schemes achieve similar performance while having lower complexity and allowing for decentralized processing. Index Terms—Cell-free MIMO, clustering, user grouping, pilot assignment, graph coloring, pilot contamination.