RT-AC86U with PCE-AC68 ?? (mixing 3x3 with 4x4)

[cmkelley]

Forgive my ignorance here, I'm replacing my AC3200 which had a 3x3 MU-MIMO with an AC-86U, which is 4x4. My multimedia PC in the living room has a PCE-AC68 3x3 wireless NIC. My understanding of how the MU-MIMO technology works is zero. Does it require both ends be the same? Or will the router act as a 3x3 when "talking" to the PCE-AC68?

I'm hoping this is a really stupid question and the answer is, "duh, of course it works".

 

[RMerlin]

Your PCE-AC68 does not support MU-MIMO, so it's irrelevant to your case.

 

[cmkelley]

Clearly I have larger ignorance problems than I thought. What is the difference between 3x3, 4x4, and MU-MIMO? My google-fu failed me tonight on finding an explanation. It appears to my ignorant eye that router manufacturers deliberately obfuscate that they are different technologies.

 

[RMerlin]

https://www.smallnetbuilder.com/wireless/wireless-features/32898-is-mu-mimo-ready-for-prime-time

MU-MIMO is just a feature, it's unrelated to the number of stream beyond the fact that it allows (in theory) better usage for concurrent devices that have fewer streams than the router supports (and these clients must also specifically support MU-MIMO - not many do).

Basically, ignore MU-MIMO even exists when choosing a router. It's not really effective or useful at this time.

 

[cmkelley]

Thanks, that helped immensely.

 

[doczenith1]

Your PCE-AC68 will connect to your new 86U using three streams just like it did to your 3200. You may see a slight increase in throughput with the newer router. I am basing this on my experience with my 86U vs my 68U which was a 3x3 router.

 

[Internet Man]

This page has a decent explanation of how multi-input, multi-output (MIMO) works: http://www.multicap.be/en/technology/mimo-and-spatial-streams

When a wireless device is described as 3x3, it is usually a simplification of the full form of 3x3:3 where the last number after the colon is the number of spatial streams. The first number describes how many transmit (tx) "chains" are supported while the second number describes the number of receive (rx) "chains". A Wi-Fi device must have at least as many tx or rx chains as the number of spatial streams it supports. When sending data, it is split into the largest number of spatial streams supported by both sides of the connection. These streams are then transmitted across all of the available tx chains. When receiving data, the transmitting side would split the data into the maximum number of spatial streams supported by both sides and the receiving side would re-combine the spatial streams. In order for devices to recognize a difference between two spatial streams that are being transmitted at the same frequency, the signal from each stream must travel a different distance so that it they arrive at different times which the router will recognize as a difference in phase of the signal. The separate tx/rx chains hooked up to different antennas that are separated from each other help to make sure that this happens but an indoor signal may take many paths around a room before reaching its destination resulting in a phase difference.

If one side of the wireless connection supports more spatial streams than another, this can result in a performance improvement even though the connection is limited to the lower of the two numbers. This is because additional tx/rx chains can improve the signal strength of the connection for a better quality link. This is why 4x4:4 routers tend to outperform 3x3:3 routers in SNB tests even though the client is always 2x2:2.

The multi-user MIMO (MU-MIMO) feature found in Wave 2 802.11ac devices seeks to allow a single wireless router/access point with a large number of spatial streams to divide its attention simultaneously across multiple clients. For example, a 4x4:4 access point could split into two simultaneous 2x2:2 connections with two 2x2:2 clients. A 4x4:4 access point without MU-MIMO would have to stop talking to one 2x2:2 client in order to talk to a second 2x2:2 client.

Smarter people please correct me where necessary!

This article looks like it would give a better explanation than I have: https://djw.cs.washington.edu/papers/mimo_for_dummies.pdf

 

[mikmock]

Thank you for the links & explanation Internet Man, very good information.
This community rocks!