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This paper considers the problem of beam alignment in a cell-free massiveMIMO 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, aprotocol is developed that permits estimating, for each UE, the strongestpropagation path from each of the surrounding APs, and to perform user-centricassociation between the UEs and the APs. Estimation of the strongest paths fromnearby APs is realized at the UE in a one-phase procedure, during which all theAPs simultaneously transmit on pseudo-randomly selected channels withpseudo-random transmit beamformers. An algorithm for orthogonal channelsassignment to the APs is also proposed, with the aim of minimizing the mutualinterference between APs that transmit on the same channels. The performance ofthe proposed strategy is evaluated both in terms of probability of correctdetection of the directions of arrival and of departure associated to thestrongest beam from nearby APs, and in terms of downlink and uplinksignal-to-interference-plus-noise ratio. Numerical results show that theproposed approach is effective and capable of efficiently realizing beamalignment in a multi-UE multi-AP wireless scenario.

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