Multi-User MIMO is a form of MIMO (Multiple Input Multiple Output) whereby a multi-antenna transmitter simultaneously communicates with multiple receivers, each having one or multiple antennas.
To distinguish the intended datastream at the receiver from the others, there is a need for precoding at the transmitter. The transmitter will need information on the transmission channel between the transmitter and the receiver to perform this precoding. Precoding requires a great deal of processing power. The processing power will rise exponentially with the number of users to be served simultaneously.
Multi-User MIMO is used in e.g. IEEE 802.11ac and LTE.