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 home broadband routers.

How MIMO Works

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 aggressively transmitting and receiving data across a Wi-Fi link. It organizes the network traffic that flows 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 streams into single messages.

MIMO Technology in Wi-Fi Networks

Wi-Fi incorporated MIMO technology as a standard beginning with 802.11n. 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.

SU-MIMO and MU-MIMO

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 the router 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 requires routers to manage 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 that are 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 in one direction: from the router to the client.
• It supports a limited number of simultaneous client connections (usually between two and four), depending on the router antenna configuration.

MIMO in Cellular Networks

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

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