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

Multiple in, multiple out — pronounced "my-mo" and abbreviated as MIMO — is a method for the coordinated use of several radio antennas in wireless network communications. The standard is common in modern home broadband routers.

MIMO-based Wi-Fi routers use the same network protocols that conventional (single-antenna, non-MIMO) routers do. A MIMO router achieves higher performance by more aggressively transmitting and receiving data across a Wi-Fi link. 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) them into single messages.

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 deployed in a MIMO Wi-Fi router varies. 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 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 that was introduced with 802.11n supported single-user MIMO (SU-MIMO). Compared to basic MIMO, in which 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 allocated separately to individual client devices.

Multi-user MIMO technology (MU-MIMO) works 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 manage connections with several 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.

MIMO technology is used in other kinds of wireless networks, too — for example, in cell networks (4G and 5G technology) — in several forms:

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