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In a wireless network, a collision is a situation that occurs when two or more devices attempt to send a signal along the same transmission channel at the same time. To avoid collisions we use CSMA/CA so clients can send data to an AP one at a time. CSMA/CA does this by sending RTS/CTS frames to the network.

But knowing this how MU-MIMU avoids collisions?

I know it uses different antennas for communicating with different devices. Does this mean that each antenna works on a different wireless channel to avoid RF interference and as a result avoiding collisions?

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    "CSMA/CA does this by sending RTS/CTS frames to the network." No, that is incorrect. RTS/CTS is an optional feature for a different problem: the hidden workstation that is too far away to hear.
    – Ron Maupin ♦
    Nov 27, 2019 at 19:46
  • But still, RTC frames are sent by the node that wants to communicate to all other nodes including the AP. Then AP retransmits this frame to other nodes that couldn´t get the RTC frame from the node. I saw this in this video. youtube.com/watch?v=mnjgDhF-4ps
    – mavi
    Nov 27, 2019 at 20:47
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    In infrastructure mode (using a WAP), the workstations only communicate with the WAP, not other workstations. To communicate with a different workstation, the workstation communicates with the WAP, then the WAP communicates with the other workstation. RTS is used when there are multiple workstations that are in the range of thw WAP, but not each other. A workstation can then send RTS to the WAP to see if it is safe to send because it cannot hear a different workstation sending to the WAP. That is not CSMA/CA, which is something else.
    – Ron Maupin ♦
    Nov 27, 2019 at 20:58
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    @mavi, I don't speak Portuguese (which I believe the video is in) so I can't validate any of the content of that video. However as Ron points out, the RTS/CTS (or often CTS-to-self) mechanism is an optional addition to CSMA/CA. It is not part of the CSMA/CA process itself. CSMA/CA is required for operation of an 802.11 wireless network, RTS/CTS support is optional.
    – YLearn
    Nov 27, 2019 at 21:18

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But knowing this how MU-MIMU avoids collisions?

MU-MIMO makes use of multiple spatial streams and beamforming. It is actually a pretty complex operation that is taking place so maybe the best thing I could do is to point you to another resource.

This Cisco Tech Field Day video is a good explanation of how MU-MIMO works. It is explained in fairly simple terms, but for really understanding this video it would help if you would need a good basic grounding in 802.11 operations.

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