  Remarkable research activities and major advances have been occurred over the
past decade in multiuser multiple-input multiple-output (MU-MIMO) systems.
Several transmission technologies and precoding techniques have been developed
in order to exploit the spatial dimension so that simultaneous transmission of
independent data streams reuse the same radio resources. The achievable
performance of such techniques heavily depends on the channel characteristics
of the selected users, the amount of channel knowledge, and how efficiently
interference is mitigated. In systems where the total number of receivers is
larger than the number of total transmit antennas, user selection becomes a key
approach to benefit from multiuser diversity and achieve full multiplexing
gain. The overall performance of MU-MIMO systems is a complex joint
multi-objective optimization problem since many variables and parameters have
to be optimized, including the number of users, the number of antennas, spatial
signaling, rate and power allocation, and transmission technique. The objective
of this literature survey is to provide a comprehensive overview of the various
methodologies used to approach the aforementioned joint optimization task in
the downlink of MU-MIMO communication systems.
