Beam Alignment for the Cell-Free mmWave Massive MU-MIMO Uplink

TL;DR

This paper introduces an interference-aware beam alignment method for cell-free mmWave massive MU-MIMO uplink systems, improving beamforming efficiency and reducing interference in realistic channel conditions.

Contribution

It proposes a novel interference-aware beam alignment approach tailored for cell-free mmWave MU-MIMO uplink, considering full digital AP reception and analog UE transmission.

Findings

Outperforms omnidirectional transmission in realistic channels

Reduces MU interference effectively

Enhances beamforming accuracy in mmWave systems

Abstract

Millimeter-wave (mmWave) cell-free massive multi-user (MU) multiple-input multiple-output (MIMO) systems combine the large bandwidths available at mmWave frequencies with the improved coverage of cell-free systems. However, to combat the high path loss at mmWave frequencies, user equipments (UEs) must form beams in meaningful directions, i.e., to a nearby access point (AP). At the same time, multiple UEs should avoid transmitting to the same AP to reduce MU interference. We propose an interference-aware method for beam alignment (BA) in the cell-free mmWave massive MU-MIMO uplink. In the considered scenario, the APs perform full digital receive beamforming while the UEs perform analog transmit beamforming. We evaluate our method using realistic mmWave channels from a commercial ray-tracer, showing the superiority of the proposed method over omnidirectional transmission as well as over methods that do not take MU interference into account.