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My company is replacing an old wireless bridging system with new 802.11ac gear. Layout is a central building with 4 remote buildings within 180 degrees line of sight. Bridging only.

I think that I want to use less expensive 2x2 radios with directional antenna on the remote buildings and a 4x4 radio with omni directional at the head end.

My question is: does it make sense to use a 4x4 radio with omni antenna at the headend, i.e., does 4x4 mean that it can simultaneously talk to 4 remote AP's at once? The alternative would be to deploy multiple gateway radios with directional antenna at the main building...

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does it make sense to use a 4x4 radio with omni antenna at the headend, i.e., does 4x4 mean that it can simultaneously talk to 4 remote AP's at once?

No. I forget if MU-MIMO beam forming requires a single extra radio chain or an extra radio chain per client. But you certainly won't be able to send traffic from a 4x4 headend to 4 clients simultaneously.

I also wouldn't use an omni in this case as you only require 180 degrees of coverage (not 360). Look for a patch/panel/sector antenna that provides you a more ideal coverage pattern.

The alternative would be to deploy multiple gateway radios with directional antenna at the main building...

This would be my preference for this situation. This provides a dedicated connection to each downstream location. It also means a failure at the head end disrupts the network to only one location and not all four.

Additionally, you said:

2x2 radios with directional antenna on the remote buildings

I would be concerned about this setup as you can easily run into performance issues. In a point-to-multipoint setup like this, the clients would not be aware of each other, so you would likely run into the hidden node issue and possbly need to implement RTS/CTS. Without proper configuration, you could have worse performance than you have now with your current equipment.

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WiFi is half duplex, so only one station can talk at a time. To simultaneously talk to multiple station, you will need multiple radios on separate channels.

  • MIMO enables multiple stations to talk at the same time, where talk means that one speaks while the other listens (half duplex), right? So, 4x4 at the headend would enable each remote 2x2 radio to talk to the head end concurrently, right? – Ron Royston May 25 at 19:43
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    @RonRoyston, MU-MIMO in 802.11ac allows a base station (i.e. an AP) to transmit to multiple clients at the same time under certain conditions. Client side MU-MIMO probably won’t be available in practical terms until 802.11ax wave 2 chipsets, if not later. – YLearn May 25 at 20:58
  • @YLearn MU-MIMO is also known as 802.11ac Wave 2. So, in the case of point-to-point backhaul (aka wireless bridging), Client-side MU-MIMO is relevant only insofar as there are multiple headend/gateway radios, right? In other words, there would only be a single transmitter to listen for. – Ron Royston May 26 at 3:44
  • 802.11ac MU-MIMO is only infrastructure side. So only the "AP" can transmit to multiple clients. Multiple clients cannot transmit to the AP at the same time. You will need to way for 802.11ax wave 2 for client side MU-MIMO (multiple clients speaking to infrastructure at the same time). – YLearn May 26 at 3:53

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