Multi-UE Multi-AP Beam Alignment in mmWave Cell-Free Massive MIMO Exploiting Channel Sparsity

This paper considers the problem of beam-alignment (BA) in a cell-free massive MIMO system in a scenario where multiple user equipments (UEs) simultaneously estimate the direction of arrival and departure of the strongest propagation paths arriving from the surrounding access points (APs). Building on the single-UE, single-AP procedure recently proposed by Song, Haghighatshoar and Caire in [1], we develop a protocol that permits one-shot joint BA for all the UEs and all the APs in the considered system. The proposed procedure relies on the existence of a reliable control channel at sub-6 GHz frequency, so as to enable exchange of estimated values between the UEs and the network, and assumes that APs can be identified based on the prior knowledge of the set of carriers and transmit beamforming codebooks. In order to manage the simultaneous transmissions from the APs, the orthogonality between the resources used by these transmitters in the BA procedure is obtained exploiting both orthogonal subcarriers and orthogonal pilot sequences. Numerical results show the effectiveness of the proposed detection strategies, with the strategy exploiting the two levels of orthogonality performing better than that relying only on subcarriers.