# Energy-Efficient Power Control in Cell-Free and User-Centric Massive MIMO at Millimeter Wave

## TL;DR

This paper explores energy-efficient power control in cell-free and user-centric massive MIMO systems at millimeter wave frequencies, introducing novel channel models, low-complexity beamforming, and power allocation strategies that enhance performance without requiring channel estimation at mobile stations.

## Contribution

It introduces a multiuser clustered millimeter wave channel model and proposes channel-independent beamforming and power control schemes for energy efficiency in cell-free and user-centric massive MIMO.

## Key findings

- Power control significantly improves energy efficiency.
- Hybrid beamforming incurs manageable performance loss.
- Proposed schemes do not require channel estimation at mobile stations.

## Abstract

In a cell-free massive MIMO architecture a very large number of distributed access points simultaneously and jointly serves a much smaller number of mobile stations; a variant of the cell-free technique is the user-centric approach, wherein each access point just serves a reduced set of mobile stations. This paper introduces and analyzes the cell-free and user-centric architectures at millimeter wave frequencies, considering a training-based channel estimation phase, and the downlink and uplink data transmission phases. First of all, a multiuser clustered millimeter wave channel model is introduced in order to account for the correlation among the channels of nearby users; second, an uplink multiuser channel estimation scheme is described along with low-complexity hybrid analog/digital beamforming architectures. Third, the non-convex problem of power allocation for downlink global energy efficiency maximization is addressed. Interestingly, in the proposed schemes no channel estimation is needed at the mobile stations, and the beamforming schemes used at the mobile stations are channel-independent and have a very simple structure. Numerical results show the benefits granted by the power control procedure, that the considered architectures are effective, and permit assessing the loss incurred by the use of the hybrid beamformers and by the channel estimation errors.