Multi-UE Multi-AP Beam Alignment in User-Centric Cell-Free Massive MIMO Systems Operating at mmWave

TL;DR

This paper introduces a protocol for efficient beam alignment in multi-user, multi-AP cell-free massive MIMO systems operating at mmWave frequencies, leveraging a sub-6 GHz control channel for path estimation and user-centric association.

Contribution

It proposes a novel one-phase estimation protocol and an orthogonal channel assignment algorithm to optimize beam alignment and reduce interference in multi-UE multi-AP mmWave systems.

Findings

High probability of correct detection of beam directions

Improved downlink and uplink SNR performance

Effective interference minimization through channel assignment

Abstract

This paper considers the problem of beam alignment in a cell-free massive MIMO deployment with multiple access points (APs) and multiple user equipments (UEs) simultaneously operating in the same millimeter wave frequency band. Assuming the availability of a control channel at sub-6 GHz frequencies, a protocol is developed that permits estimating, for each UE, the strongest propagation path from each of the surrounding APs, and to perform user-centric association between the UEs and the APs. Estimation of the strongest paths from nearby APs is realized at the UE in a one-phase procedure, during which all the APs simultaneously transmit on pseudo-randomly selected channels with pseudo-random transmit beamformers. An algorithm for orthogonal channels assignment to the APs is also proposed, with the aim of minimizing the mutual interference between APs that transmit on the same channels. The performance of the proposed strategy is evaluated both in terms of probability of correct detection of the directions of arrival and of departure associated to the strongest beam from nearby APs, and in terms of downlink and uplink signal-to-interference-plus-noise ratio. Numerical results show that the proposed approach is effective and capable of efficiently realizing beam alignment in a multi-UE multi-AP wireless scenario.