  The problem of finding the optimal number of data streams to transmit in a
multi-user MIMO scenario, where both the transmitters and receivers are
equipped with multiple antennas is considered. Without channel state
information at any transmitter, with a zero-forcing receiver each user is shown
to transmit a single data stream to maximize its own outage capacity in the
presence of sufficient number of users. Transmitting a single data stream is
also shown to be optimal in terms of maximizing the sum of the outage
capacities in the presence of sufficient number of users.
