  Millimeter wave (mmWave) massive multiple-input multiple-output (MIMO)
communication system is expected to achieve enormous transmission rate,
provided that the transmit and receive beams are properly aligned with the MIMO
channel. However, existing beam alignment techniques rely on either channel
estimation or beam sweeping, which incur prohibitively high training overhead,
especially for future wireless systems with further increased antenna
dimensions and more stringent requirement on cost-effective hardware
architectures. In this paper, we propose a new beam alignment technique, which
is environment-aware and training-free, by utilizing the emerging concept of
channel knowledge map (CKM), together with the user location information that
is readily available in contemporary wireless systems. CKM is a site-specific
database, tagged with the transmitter/receiver locations, which contains useful
channel information to facilitate or even obviate real-time channel state
information (CSI) acquistion. Two instances of CKM are proposed for beam
alignment in mmWave massive MIMO systems, namely channel path map (CPM) and
beam index map (BIM). It is shown that compared with existing training-based
beam alignment schemes, the proposed CKM-enabled environment-aware beam
alignment is able to drastically improve the effective communication rate, even
with moderate user location errors, thanks to its significant saving of the
prohibitive training overhead.
