Suppressing Pilot Contamination for Massive Access in User-centric Cell-free Massive MIMO Systems

Abstract

In this paper, an effective pilot assignment algorithm is proposed to counter the pilot contamination (PC) issue caused by limited pilot sequences during massive access in a user-centric cell-free massive multiple-input multiple-output system. The problem of pilot assignment is formulated as max-min spectral efficiency (SE) optimization problem which is NP-hard. Hence, the paper proposes a heuristic-based algorithm to assign pilots to users (UEs) in such a way that the repetitive assignment among the associated UEs for all the access points can be avoided. Numerical results demonstrate that the proposed algorithm achieves up to 37% average SE gain while reducing the interference power by up to 52% relative to the random and two state-of-the-art pilot assignment schemes. Moreover, the proposed pilot assignment scheme exhibits the best balance in the trade-off between the performance criteria and the computational complexity relative to its comparatives.