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