What Is Multiple-In Multiple-Out (MIMO) Technology?

MIMO (Multiple In, Multiple Out) - pronounced "my-mo" - is a method for the coordinated utilization of multiple radio antennas in wireless network communications, common in modern home broadband routers.

MIMO-based Wi-Fi routers utilize the same network protocols that traditional (single antenna, non-MIMO) routers do. A MIMO router achieves higher performance by more aggressively transmitting and receiving data across a Wi-Fi link Specifically, it organizes the network traffic flowing between Wi-Fi clients and the router into individual streams, transmits the streams in parallel, and enables the receiving device to re-assemble (reconstitute) the back into single messages.

MIMO signaling technology can increase network bandwidth, range, and reliability at an increased risk of interfering with other wireless equipment.

Wi-Fi incorporated MIMO technology as a standard beginning with 802.11n. Using MIMO enhances the performance and reach of Wi-Fi network connections compared to those with single-antenna routers.

The specific number of antennas utilized in a MIMO Wi-Fi router can vary. Typical MIMO routers contain three or four antennas instead of the single antenna that was standard in older wireless routers.

Both a Wi-Fi client device and the Wi-Fi router must support MIMO in order for a connection between them to take advantage of this technology and realize the benefits.  Manufacturer documentation for router models and client devices specify whether they are MIMO capable. Beyond that, there is no straightforward way to check whether your network connection is using it.

The first generation of MIMO technology introduced with 802.11n supported Single User MIMO (SU-MIMO). Compared to traditional MIMO where all of a router's antennas must be coordinated to communicate with one client device, SU-MIMO enables each antenna of a Wi-Fi router to be separately allocated to individual client devices.

Multi-User MIMO (MU-MIMO) technology has been created for use on 5 GHz 802.11ac Wi-Fi networks. Whereas SU-MIMO still requires routers to manage their client connections serially (one client at a time), MU-MIMO antennas can manage connections with multiple clients in parallel. MU-MIMO improves the performance of connections able to take advantage of it. Even when an 802.11ac router has the necessary hardware support (not all models do), other limitations of MU-MIMO also apply::

• it supports traffic only in one direction - from the router to the client
• it supports only a limited number of simultaneous client connections (usually between two and four) depending on the router's antenna configuration

Multiple-In Multiple-Out technology can be found in other kinds of wireless networks beside-Fi. It is also increasingly found in cell networks (4G and future 5G technology) in several forms:

• Network MIMO a.k.a. Cooperative MIMO - coordinated signaling among multiple base stations
• Massive MIMO - utilizing large numbers (hundreds) of antennas at a base station
• Millimeter Wave (mmWave) - utilizing very high-frequency bands where spectrum availability is larger than on bands licensed for use on 3G/4G