  We consider a single-cell scenario involving a single base station (BS) with
a massive array serving multi-antenna terminals in the downlink of a mmWave
channel. We present a class of multiuser user MIMO schemes, which rely on
uplink training from the user terminals, and on uplink/downlink channel
reciprocity. The BS employs virtual sector-based processing according to which,
user-channel estimation and data transmission are performed in parallel over
non-overlapping angular sectors. The uplink training schemes we consider are
non-orthogonal, that is, we allow multiple users to transmit pilots on the same
pilot dimension (thereby potentially interfering with one another). Elementary
processing allows each sector to determine the subset of user channels that can
be resolved on the sector (effectively pilot contamination free) and, thus, the
subset of users that can be served by the sector. This allows resolving
multiple users on the same pilot dimension at different sectors, thereby
increasing the overall multiplexing gains of the system. Our analysis and
simulations reveal that, by using appropriately designed directional training
beams at the user terminals, the sector-based transmission schemes we present
can yield substantial spatial multiplexing and ergodic user-rates improvements
with respect to their orthogonal-training counterparts.
