  Consider the problem of a Multiple-Input Multiple-Output (MIMO)
Multiple-Access Channel (MAC) at the limit of large number of users. Clearly,
in practical scenarios, only a small subset of the users can be scheduled to
utilize the channel simultaneously. Thus, a problem of user selection arises.
However, since solutions which collect Channel State Information (CSI) from all
users and decide on the best subset to transmit in each slot do not scale when
the number of users is large, distributed algorithms for user selection are
advantageous.
  In this paper, we analyse a distributed user selection algorithm, which
selects a group of users to transmit without coordinating between users and
without all users sending CSI to the base station. This threshold-based
algorithm is analysed for both Zero-Forcing (ZF) and Minimum Mean Square Error
(MMSE) receivers, and its expected sum-rate in the limit of large number of
users is investigated. It is shown that for large number of users it achieves
the same scaling laws as the optimal centralized scheme.
