Overloaded Pilot Assignment with Pilot Decontamination for Cell-Free Systems

TL;DR

This paper proposes overloaded pilot assignment methods combined with pilot decontamination to enhance spectral efficiency in cell-free massive MIMO systems, especially under high user density conditions.

Contribution

It introduces overloaded pilot assignment strategies tailored for pilot decontamination, improving spectral efficiency in cell-free systems beyond traditional non-overloaded methods.

Findings

Overloaded PA methods outperform non-overloaded PA at high user densities.

Numerical simulations confirm increased spectral efficiency with overloaded PA.

Decontamination via subspace projection effectively separates co-pilot interference.

Abstract

The pilot contamination in cell-free massive multiple-input-multiple-output (CF-mMIMO) must be addressed for accommodating a large number of users. In previous works, we have investigated a decontamination method called subspace projection (SP). The SP separates interference from co-pilot users by using the orthogonality of the principal components of the users' channel subspaces. Non-overloaded pilot assignment (PA), where each radio unit (RU) does not assign the same pilot to different users, limits the spectral efficiency (SE) of the system, since SP channel estimation is able to deal with co-pilot users that have nearly orthogonal subspaces. Motivated by this limitation, this paper introduces overloaded PA methods adjusted for the decontamination in order to improve the sum SE of CF systems. Numerical simulations show that the overloaded PA methods give higher SE than that of non-overloaded PA at a high user density scenario.