MIMO stands for “multiple input, multiple output.” A 4×4 MIMO device has four antennas for four simultaneous data streams, while a 2×2 MIMO has two. The iPhone XR is 2×2 MIMO, while the iPhone XS and XS Max are 4×4 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 1×1 MIMO device because it has one antenna and can support one data stream at once.

However, there are also devices with more antennas. A 2×2 MIMO device has two antennas for two simultaneous data streams, a 3×3 MIMO device has three antennas for three data streams, and a 4×4 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 1×1 MIMO to 4×4 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 4×4 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 2×2 MIMO to 4×4 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—2×2 MIMO on the XR versus 4×4 on the XS.

When both phones were both connected to a 4×4 MIMO LTE network, the 4×4 iPhone XS topped out at a download speed of just under 400 Mbps. The 2×2 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 4×4 MIMO compared to 2×2 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 4×4 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 2×2 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 4×4 MIMO can result in an improved wireless signal. 4×4 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.

4×4 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 4×4 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 4×4 MIMO LTE (cellular), but 2×2 MIMO Wi-Fi. Even if you’re connected to a 4×4 MIMO router, you only get 2×2 MIMO WI-Fi speeds. The cellular and Wi-Fi antennas are separate.

What is 4×4 MU-MIMO?

Some newer wireless routers support MU-MIMO, too. This refers to “multi-user multiple input, multiple output.” A router with 4×4 MU-MIMO has four antennas it can communicate on at once. If you had several 4×4 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 3×3 MIMO like Apple’s newer MacBook Pros, they can connect to a 4×4 MIMO access point with three data streams at once.

However, if you have a phone with 2×2 MIMO Wi-Fi or a laptop with 3×3 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 3×3 MIMO device to a 2×2 MIMO router, it will only use two data streams.

RELATED: What Is MU-MIMO, and Do I Need It on My Router?

Do I Need 4×4 MIMO?

The more MIMO, the better. All other things being equal, you should prefer 4×4 MIMO to 2×2 MIMO, and 2×2 MIMO to no MIMO (or 1×1 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 4×4 MIMO. The iPhone XR is a little unusual in its price range with only 2×2 MIMO. Hopefully, Apple will include 4×4 MIMO in the successor to the iPhone XR next year.

That extra wireless hardware will use a bit of extra power, so 4×4 MIMO might reduce battery life a tiny bit compared to 2×2 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.