  Channel state information at the transmitter (CSIT) is a key ingredient in
realizing the multiplexing gain provided by distributed MIMO systems. For a
downlink multiple-input single output (MISO) broadcast channel, with M antennas
at the transmitters and K single antenna receivers, the maximum multiplexing
gain or the maximum degrees of freedom (DoF) is min(M,K). The optimal DoF of
min(M,K) is achievable if the transmitter has access to perfect, instantaneous
CSIT from all receivers. In this paper, we pose the question that what is
minimum amount of CSIT required per user in order to achieve the maximum DoF of
min(M,K). By minimum amount of CSIT per user, we refer to the minimum fraction
of time that the transmitter has access to perfect and instantaneous CSIT from
a user. Through a novel converse proof and an achievable scheme, it is shown
that the minimum fraction of time, perfect CSIT is required per user in order
to achieve the DoF of min(M,K) is given by min(M,K)/K.
