  Massive machine-type communications protocols have typically been designed
under the assumption that coordination between users requires significant
communication overhead and is thus impractical. Recent progress in efficient
activity detection and collision-free scheduling, however, indicates that the
cost of coordination can be much less than the naive scheme for scheduling.
This work considers a scenario in which a massive number of devices with
sporadic traffic seek to access a massive multiple-input multiple-output (MIMO)
base-station (BS) and explores an approach in which device activity detection
is followed by a single common feedback broadcast message, which is used both
to schedule the active users to different transmission slots and to assign
orthogonal pilots to the users for channel estimation. The proposed coordinated
communication scheme is compared to two prevalent contention-based schemes:
coded pilot access, which is based on the principle of coded slotted ALOHA, and
an approximate message passing scheme for joint user activity detection and
channel estimation. Numerical results indicate that scheduled massive access
provides significant gains in the number of successful transmissions per slot
and in sum rate, due to the reduced interference, at only a small cost of
feedback.
