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MIMO stands for "multiple input, multiple output." A 4x4 MIMO device has four antennas for four simultaneous data streams, while a 2x2 MIMO has two. The iPhone XR is 2x2 MIMO, while the iPhone XS and XS Max are 4x4 MIMO.
What Is MIMO?
MIMO is an integral part of modern wireless communications technologies, whether you're talking about 802.11ac Wi-Fi or 4G LTE cellular data.
Traditionally, a device had just a single antenna inside it. This would be called a 1x1 MIMO device because it has one antenna and can support one data stream at once.
However, there are also devices with more antennas. A 2x2 MIMO device has two antennas for two simultaneous data streams, a 3x3 MIMO device has three antennas for three data streams, and a 4x4 MIMO device has four antennas for four data streams.
More MIMO, More Speed
Each antenna on a device is used both for receiving data and sending data. The more antennas your device has, the more data it can transfer at once---and that means faster wireless download and upload speeds.
Think of it like lanes on a highway. If you have a four line highway, more traffic can flow through at the same time than on a two or one lane highway.
Going from 1x1 MIMO to 4x4 MIMO means quadrupling the theoretical maximum data transfer speed. That's because each antenna supports a separate data stream up to a maximum theoretical limit. The precise limit varies depending on the wireless networking standing they're using.
These faster speeds to require you're connected to a cellular network that supports 4x4 MIMO. It won't work everywhere on every carrier, but cellular carriers have been gradually rolling out this feature on their networks across the USA for a few years now.
More MIMO Means a Better Signal, Too
Recent tests have demonstrated that going from 2x2 MIMO to 4x4 MIMO can give you improved wireless signal strength, too. PC Magazine had Cellular Insights run some tests comparing the iPhone XR to the iPhone XS. The iPhone XR and iPhone XS have the same wireless modem, so the main difference should just be the fewer antennas on the iPhone XR compared to the iPhone XS---2x2 MIMO on the XR versus 4x4 on the XS.
When both phones were both connected to a 4x4 MIMO LTE network, the 4x4 iPhone XS topped out at a download speed of just under 400 Mbps. The 2x2 MIMO iPhone XR topped out at right under 200 Mbps at the same signal strength.
That's to be expected, and demonstrates the advantages of 4x4 MIMO compared to 2x2 MIMO---it can transfer data twice as fast.
However, the tests also found that the iPhone XS had better signal strength than the iPhone XR on the 4x4 MIMO network. More surprisingly, the iPhone XS had better signal strength than the iPhone XR even when it was connected to a cellular network that only supported 2x2 MIMO.
This doesn't matter if you have a solid connection and your iPhone XR's download speeds are good enough for you. But, when you have a weak cellular signal, it looks like the extra antennas in 4x4 MIMO can result in an improved wireless signal. 4x4 MIMO isn't just about speed---it seems to improve your all-around signal strength, too.
Cellular vs. Wi-Fi
MIMO technology is used for both cellular and Wi-Fi connections. But cellular and WI-Fi have separate antennas.
4x4 MIMO is now common on high-end phones like Apple's iPhone XS and iPhone XS Max. Samsung's Galaxy S9 and S9+ also support 4x4 MIMO, as do Google's Pixel 3 and Pixel 3 XL phones. They can all support four separate data streams at once when connected to a cellular network that offers them.
However, that only refers to the cellular connection. For example, the iPhone XS and Pixel 3 both have 4x4 MIMO LTE (cellular), but 2x2 MIMO Wi-Fi. Even if you're connected to a 4x4 MIMO router, you only get 2x2 MIMO WI-Fi speeds. The cellular and Wi-Fi antennas are separate.
What is 4x4 MU-MIMO?
Some newer wireless routers support MU-MIMO, too. This refers to "multi-user multiple input, multiple output." A router with 4x4 MU-MIMO has four antennas it can communicate on at once. If you had several 4x4 MIMO devices connected to that router, they would all maintain a connection of four data streams at the same time.
Or, if you have a laptop with 3x3 MIMO like Apple's newer MacBook Pros, they can connect to a 4x4 MIMO access point with three data streams at once.
However, if you have a phone with 2x2 MIMO Wi-Fi or a laptop with 3x3 MIMO and you connect it to an older router that doesn't support MIMO at all, it will only receive a single data stream. If you connect a 3x3 MIMO device to a 2x2 MIMO router, it will only use two data streams.
Do I Need 4x4 MIMO?
The more MIMO, the better. All other things being equal, you should prefer 4x4 MIMO to 2x2 MIMO, and 2x2 MIMO to no MIMO (or 1x1 MIMO, in other words.)
Devices with more antennas are generally more expensive, though, so you'll often pay for it. It's just more hardware. Modern flagship phones generally have 4x4 MIMO. The iPhone XR is a little unusual in its price range with only 2x2 MIMO. Hopefully, Apple will include 4x4 MIMO in the successor to the iPhone XR next year.
That extra wireless hardware will use a bit of extra power, so 4x4 MIMO might reduce battery life a tiny bit compared to 2x2 MIMO. But we doubt that's a huge factor compared to everything else that drains power on a mobile device.
Overall, the faster wireless speed and improved signal strength are always good to have. You may just have to pay extra for devices with this feature.
Image Credit: GobyOneKenobi/Shutterstock.com, Apple, Apple, Samsung
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