  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.
