MIMO

Multiple-input and multiple-output (MIMO) is a method of multiplying the throughput of wireless connection through a Wi-Fi or cellular signal.

Adoption by Apple
Apple first adopted MIMO in June 2013 for its AirPort Extreme 802.11ac, which allowed a 3x3 antenna configuration to triple Wi-Fi bandwidth over preceding 802.11n protocols. All Apple devices have since adopted 802.11ac.

Apple first adopted 4x4 4G antennae in the iPhone X S and X S Max, doubling the maximum cellular bandwidth over that of the 2x2 antennae in the iPhone X and X R.