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4X4 VS 4X2 and MU-MIMO

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  • 1.  4X4 VS 4X2 and MU-MIMO

    Posted Sep 16, 2022 06:19 PM
    * There is a mistake in the Subject, I wanted to say 2x2 and not 4X2

    Hello guys,

    I'd like to know your opinion about the performance of 4x4 APs vs 2x2, does it worth to go to a 4x4 AP at these days? If I'm not wrong, to be able of taking advantage of the benefits of 4X4 APs you need 4X4 compatible client devices. I can see that most of the new 6XX series APs are 2x2.... so I'm confused about this. In addition, if you have a 4x4 AP which can provide up tu 4.8Gbps but you have an infrastructure with 1Gbps ports, the access point connected to these ports are not going to be able of providing these data rates...

    And another question about MU-MIMO, again I can see that most of the new 6XX APs are not MU-MIMO. I think that deploying APs with these feature, will improve the most critical communications such as voice and video, Any feedback about this? Is there any reason because these new APs ar not MU-MIMO?



  • 2.  RE: 4X4 VS 4X2 and MU-MIMO
    Best Answer

    Posted Sep 16, 2022 06:47 PM
    The main value of a 4x4 MIMO radio in an AP is not to serve 4x4 client devices (because those do not exist ;-)  but you're right that those would be needed to consistently achieve the highest datarates. AFAIK there are no 4x4 (4SS) client devices, and even the few 11ac 3x3 ones do not appear to have successors in 11ax.
    Of course there's MU-MIMO, and in theory you could combine two 2x2 clients with a 4x4 AP radio and take advantage of the 4.8Gbps, shared between the two clients. Unfortunately there are lots of limitations (many devices either do not support MU-MIMO at all, or only in the downlink direction), and due to overhead and other considerations, the efficiency tends to be pretty bad. Best case gain (in the lab) is about 1.5x rather than the 2x gain that people hope/look for, and in the real world, with a mix of clients and traffic (up/down) it will be far worse than that. Especially when the number of simultaneous clients grows, at some point the value of MU-MIMO may end up being negative and you could be better off using SU-MIMO.
    This is even more pronounced when trying to take advantage of MU-MIMO with a 2x2 radio on the AP side (and a mix of 2x2 and 1x1 clients), which is the main reason that we do not recommend using it with 2x2 AP radios, and even if we do support it on some 2x2 APs, we do not make any claims around that.
    You may wonder if there's any value in a 4x4 AP radio at all, and fortunately there is. When connecting 2x2 clients to a 4x4 AP radio (SU-MIMO), the connection is significantly more robust, and you'll find that the 2x2 rates are maintained further (range) and in far more challenging conditions (noise, reflections, attenuation, etc.). That's a little harder to quantify, but the performance impact is quite significant, with client devices maintaining higher MCS rates and/or continue to operate on 2SS much longer, before dropping to 1SS.
    AP-655 has three 4x4 radios that all support MU-MIMO in both directions, whereas AP-635 has 2x2 radios and no MU-MIMO support. The AP -655 also supports a much higher number of OFDMA clients, but that's a different story altogether..
    Finally, keep in mind that the 4.8Gbps number is the peak "over-the-air" datarate. Also, Wi-Fi traffic is half-duplex. The 5Gbps speed limit of the wired Ethernet connection is application level TCP/UDP throughput, and is full-duplex.
    Even when maxing out an AP-655 on the wireless side, the wired side will not become a bottleneck. Even in the lab that is hard to achieve. However, you will be limiting these APs (depending on how they're configured) when using 1Gbps switch ports. You could aggregate two of them to at least double the capacity, upgrade your switches, or accept the limitations. For example, if the WLAN network is deployed with 40MHz channel bandwidth in 5GHz and 6GHz, you'll be fine with a single 1Gbps wired connection for the APs..

  • 3.  RE: 4X4 VS 4X2 and MU-MIMO

    Posted Sep 23, 2022 10:11 AM
    Thank you onno for this detailed explanation, this has been very useful for me.


  • 4.  RE: 4X4 VS 4X2 and MU-MIMO

    Posted Sep 29, 2022 11:13 AM
    This is a fantastic answer, I was about to post a similar question.  I guess I could use some additional details if you have them, and it may end up simply being a judgment call.  The question revolves around the choice of  615 vs 515.. with the 600 series you get the addition of the 6Ghz radio space, but all the radios are 2x2.  With the current lack of 6ghz clients the additional radio spectrum does not have current advantages, but would be helpful likely in the future.  AX clients would be able to take advantage of  OFDMA with  2x2 WAPs but not with the same client density as a 4x4 (but ok for non-HD spaces??)   The big question i suppose is that  it sounds like you are indicating that I would see a drop in coverage compared to the 4x4 radio as devices on the outskirts will not get the robustness of connection that a 4x4 WAP would provide.  Moving or adding WAPs is an expensive proposition(much higher cost than the WAPs themselves) so if there would be a need to add 2x2 WAPs for coverage this may be a deal breaker.  Am I understanding this correctly?


  • 5.  RE: 4X4 VS 4X2 and MU-MIMO

    Posted Sep 30, 2022 01:45 AM
    Yes, I would agree, but keep in mind that in most enterprise deployments there will (should) be plenty of coverage, since those deployments are typically capacity drives rather than coverage driven. So it would be more likely that when using 2x2 AP radios versus 4x4 ones, clients may see a somewhat lower MCS rate, or drop to single SS rates sooner, but they would typically still connect just fine to the network.
    So yes, I think your 5GHz clent devices will have a better experience with an AP-515 than they will with an AP-635, but that should not imply a significant impact or connectivity issues (unless the design was providing borderline coverage to begin with).
    With more and more client devices taking advantage of the 6GHz spectrum, that will also reduce the load on the 5GHz network which will eventually help there as well (less contention, interference, etc.) which may offset some of the impact from going to 2x2.
    Another great option of course is to use the AP-655, but that one does come at a price premium..