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MU-MIMO
As far as I recall, MU-MIMO was first introduced for Downlink only in 802.11ac and evolved to support both downlink and uplink in 802.11ax.
MU-MIMO (Multi User MIMO) is a type of MIMO technique that enable the transmitter to send signals to multiple users at the same time (simultaneously). We have two keywords here. Multiple User and Simultaneous are the keywords. The basic principle of this technology is to do a specific type of beamforming that are directing to multiple directions.
MU-MIMO is often illustrated as follows. As you see, a special antenna radiation pattern is used. The radiation pattern form a specific pattern which forms multiple boresight (main direction).
The illustration above shows only the spatial aspect of MU-MIMO and does not indicated anything about temporal aspect. So I would like to represent MU-MIMO as follows to show both spatial and temporal aspect of the operation.
MU-MIMO Process for Downlink only
Let's assume a case where MU-MIMO is supported only for downlink and is not supported for Uplink. In this case, DL packet can be transmitted simultaneously as shown in the first diagram, but ACK/NACK from each STA cannot be received simultaneously.
MU-MIMO Process for Downlink and Uplink
The overall protocol sequence for DL+UL MU-MIMO is very similar to OFDMA DL protocol sequence. The main difference between MU-MIMO and OFDAM protocol would be at step (3). In MU-MIMO, DL-PPDU for every user occupies the same spectrum and is just transmitted through different beam, but in OFDMA each of DL-PPDU is transmitted different RF spectrum (NOTE : this difference that I mentioned is based on the assumption is that the system support OFDMA only or MU-MIMO only, not both acting at the same time).
References :
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