Multiple-input Multiple-Output (MIMO) is a communication method for wireless transmission where both transmitter and receiver use multiple antennas. The use of multiple antennas at both sides of the link creates a multi-path propagation environment between transmitter and receiver that can be used to significantly improve the system throughput.

The origin of the improvement can be intuitively understood by thinking of the multi-path propagation MIMO environment as the equivalent of several separate paths between transmitter and receiver. Every one of these paths (or “streams”) can be independently used for data transmission with each employing the full bandwidth available to the system. This is referred to as spatial multiplexing. The higher the number of antennas, the higher the multiplexing gain.

In addition, MIMO enables other advanced techniques like beamforming and MU-MIMO. The higher the number of antennas, the more efficient these techniques become.


Multi-User MIMO (MU-MIMO) provides an alternative use of the spatial multiplexing that is provided by MIMO. MIMO enables the transmission of multiple independent streams between a transmitter and a receiver with multiple antennas. However, without MU-MIMO, the spatial multiplexing gain is limited by the number of antennas at the client devices (typically one or two), leaving a lot of spatial diversity unused. MU-MIMO extends the MIMO concept to the case with one transmitter and multiple receivers (Downlink MU-MIMO) or to the case with one receiver and multiple transmitters (Uplink MU-MIMO), allowing the full use of the spatial diversity provided by the channel. Instead of being able to transmit to a single client, an MU-MIMO capable system can transmit to several clients simultaneously, using the full bandwidth of each link.

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