Cell-Free and User-Centric Massive MIMO at Millimeter Wave Frequencies

TL;DR

This paper explores cell-free and user-centric massive MIMO architectures at millimeter wave frequencies, introducing a multiuser channel model, a novel channel estimation scheme, and hybrid beamforming, demonstrating promising performance especially in lightly loaded systems.

Contribution

It introduces and analyzes cell-free and user-centric massive MIMO architectures at mmWave frequencies with a new channel model and low-complexity beamforming schemes that do not require channel estimation at the MSs.

Findings

UC approach outperforms CF in lightly loaded systems

Proposed schemes do not require channel estimation at MSs

Architectures achieve good performance with simple beamforming

Abstract

In a cell-free (CF) massive MIMO architecture a very large number of distributed access points (APs) simultaneously and jointly serves a much smaller number of mobile stations (MSs); a variant of the cell-free technique is the user-centric (UC) approach, wherein each AP just decodes a reduced set of MSs, practically the ones that are received best. This paper introduces and analyzes the CF and UC architectures at millimeter wave (mmWave) frequencies. First of all, a multiuser clustered channel model is introduced in order to account for the correlation among the channels of nearby users; then, an uplink multiuser channel estimation scheme is described along with low-complexity hybrid analog/digital beamforming architectures. Interestingly, in the proposed scheme no channel estimation is needed at the MSs, and the beamforming schemes used at the MSs are channel-independent and have a very simple structure. Numerical results show that the considered architectures provide good performance, especially in lightly loaded systems, with the UC approach outperforming the CF one.