2x2 VS 4x4 APs

     I'm regularly asked to recommend an access point for an installation or refresh. The information that typically comes up when discussing the key differences between two different AP types are the radios. I go into the differences between WiFi 6 and 6E, making sure the customer is aware of the benefits of 6GHz and how it can help their wireless when implemented well, it is NOT a silver bullet to fix bad designs. After these discussions, the key difference between many different AP models tends to be the radio count. Whether the AP is 4x4:4 or 2x2:2 in most cases. What makes one better than the other?

    It's difficult to talk about radio chains without talking about MU-MIMO. I have posted about this in an earlier discussion. Basically, it is rare to see the advantages of MU-MIMO. So, if that's not a large benefit, what is the driving factor for choosing a 4x4 AP over a 2x2?

4x4:4 Clients

    The most obvious benefit would be if you're using clients that are 3x3 or greater. On a 4x4 AP a 3x3 or 4x4 client would be able to use its higher number of spatial streams to achieve higher throughput. This would be fantastic IF there were many 3x3 or 4x4 clients. Typically, due to the power draw of multiple wireless antennas, you do not see more than 2x2 on most wireless clients. Many lower end clients might only be 1x1! I've seen some high end laptops have a 3x3 antenna. With this being said, the majority of your client base will most likely be 2x2 or lower. So this isn't necessarily a benefit everyone would experience, unless you have control over the devices your organization purchases and can fork over the extra money for more capable clients. Remember, you don't design for the few 4x4 clients you have on your network, you design for the Least Capable, Most Important Device.

Maximal Ratio Combining (MRC)

    MRC allows a wireless device to listen to multiple received signals and combine them to increase their quality. Effectively making it easier to listen to the signal. With a 4x4 AP, the additional receive radios allow that AP to "hear" the signal more clearly. Think of having 4 ears to listen with instead of 2.

Space Time Block Coding (STBC)

    With STBC, a 4x4 AP transmitting to a 2x2 client can send two copies of the same data over one spatial stream. Meaning radio 1 and 2 transmit Spatial Stream 1 and radio 3 and 4 transmit Spatial Stream 2. With this, the receiving client will be able to receive multiple copies of the same transmission. This reduces the chances of data corruption over the air and allows the client to receive signals at a lower SNR than would be required without STBC.

Conclusion

    With all of these technologies in play, there are still some advantages to 4x4 APs over 2x2. Even without the use of MU-MIMO, 4x4 could improve your wireless experience. However, 4x4 is not a magic bullet to fix a bad design, if you have 2x2 .11ac APs in your hallways and 1:1 swap them with 4x4 6E APs, it will not fix the fundamental design flaw.

Sources:

David Coleman / David Westcott CWNA-108 Guide Sybex Chapter 10