iPhone XS and iPhone XS Max contain 4 × 4 MIMO for cellular data connections.  MIMO is an integral part of modern wireless communication technology, whether you're talking about 802.11ac Wi-Fi or 4G LTE cell data.
Traditionally, a device had only one antenna inside it. This would be called a 1 × 1 MIMO device because it has an antenna and can support a 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 to receive data and to send data. The more antennas your device has, the more data it can transfer at once – and this means faster wireless download and upload speed.
Think of it as lane on a highway. If you have a 4-lane highway, more traffic can flow simultaneously than on a two-way or one-lane road.
Going from 1 × 1 MIMO to 4 × 4 MIMO means that the theoretical maximum data transfer rate is doubled. This is because each antenna supports a separate data stream up to the maximum theoretical limit. The exact limit varies depending on the wireless networking standard they are using.
These faster speeds require you to be connected to a mobile network that supports 4 × 4 MIMO. It will not work everywhere on all operators, but mobile operators have gradually rolled out this feature on their networks across the United States for a few years.
More MIMO means a better signal, for
Only iPhone XR supports 2 × 2 MIMO.
New tests have shown going from 2 × 2 MIMO to 4 × 4 MIMO can also give you improved wireless signal strength. PC Magazine had Cellular Insights run some tests that compared the iPhone XR with iPhone XS. The iPhone XR and iPhone XS have the same wireless modem, so the biggest difference should only be the fewer antennas on the iPhone XR compared to the iPhone XS-2 × 2 MIMO on XR vs. 4 × 4 on XS.
When both phones were both connected to a 4 × 4 MIMO LTE network, 4 × 4 iPhone XS picked up a download speed of barely 400 Mbps. The 2 × 2 MIMO iPhone XR peaked at just under 200 Mbps with the same signal strength.
It is expected to show the benefits of 4 × 4 MIMO versus 2 × 2 MIMO-it can transfer data twice as fast.
But 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 iPhone XR even when it was connected to a mobile network that only supported 2 × 2 MIMO.
It does not matter if you have a solid connection and your iPhone XRs download speeds are good enough for you. But when you have a weak cell signal, it seems that the additional 4 x 4 MIMO antennas can result in an improved wireless signal. 4 × 4 MIMO is not only about speed, it also seems to improve your all-round signal strength.
Cellular vs Wi-Fi
MIMO technology is used for both cellular and Wi-Fi connections. But cell and Wi-Fi have separate antennas.
4 × 4 MIMO is now common on advanced phones like Apple's iPhone XS and iPhone XS Max. Samsung Galaxy S9 and S9 + also support 4 × 4 MIMO, like Google's Pixel 3 and Pixel 3 XL phones. They can all support four separate data streams at once when connected to a mobile network that offers them.
However, it refers only to the cellular connection. For example, the iPhone XS and Pixel 3 have both 4 × 4 MIMO LTE (mobile), but 2 × 2 MIMO Wi-Fi. Even though you are 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 also support MU-MIMO. This refers to "multi-user multiple input, multiple output." A 4×4 MU-MIMO router has four antennas that it can communicate at once. If you had multiple 4 × 4 MIMOs connected to that router, they would all have a connection of four data streams at the same time.
Or if you have a laptop with 3 × 3 MIMO as Apple's newer MacBook Pros, you can connect to a 4 × 4 MIMO access point with three data streams at the same time.
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 does not support MIMO at all, it will only get a single data stream. If you connect a 3 × 3 MIMO device to a 2 × 2 MIMO router, it only uses two data streams.
RELATED: What is MU-MIMO and I need my router Do I need 4 × 4 MIMO?
Samsung Galaxy S9 and S9 + support 4 × 4 MIMO, like many other flagship Android phones.
The more MIMO, the better. Everything else 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, but then you pay often for that. It's just more hardware. Modern flagship phones generally have 4 × 4 MIMOs. The IPhone XR is a bit unusual in its price range with only 2 × 2 MIMO. Hopefully Apple will include 4 × 4 MIMO in the sequel to iPhone XR next year.
The optional wireless hardware will use some extra power, so 4 × 4 MIMO can reduce battery life a little bit compared to 2 × 2 MIMO. But we doubt that it's a big factor compared to anything else that drains power on a mobile device.
Overall, the faster wireless speed and improved signal strength are always good to have. You may only need to pay extra for devices with this feature.
Image Credit: GobyOneKenobi / Shutterstock.com, Apple, Apple, Samsung