Multi-user MIMO (MU-MIMO) is a wireless communication technology that uses multiple antennas to improve communication by creating multiple connections to the same device at the same time. MIMO is an acronym which stands for multiple-input and multiple-output. MU-MIMO is commonly used in devices such as routers and works with mobile devices such as smartphones or laptops. The technology is primarily used to support environments where multiple users access the same wireless network at one time.
Generally, when multiple users connect to the same router, congestion will begin to build up as the router services the first device's requests, while the other devices wait to be serviced. The amount of time each device waits is generally not long but can build up with enough devices. MU-MIMO helps relieve this potential congestion by creating multiple connections to a device at the same time, increasing network efficiency.
MU-MIMO takes advantage of multi-path, which is when a radio signal gets reflected and bounces around surrounding objects to be picked up by a receiver in a user’s device at slightly different times and angles. MU-MIMO will typically have multiple antennas at the transmit end and one at the receiving end of the signal. MU-MIMO devices will separate bandwidth into individual streams sharing an equal connection. Typically these streams are divided as 2x2, 3x3, 4x4 or 8x8 (referring to the number of streams). The data streams are directed to one device, meaning that an MU-MIMO router can only send and receive data from one device at a time. Typically an algorithm is used to allow fair access to multiple devices to an access point.
Initially, only routers supported MU-MIMO, but as time went on, other endpoint devices started catching up. The Wi-Fi Alliance has a list containing over 550 devices that support MU-MIMO, as an example.
Types of MU-MIMO
There are multiple types of multi-user MIMO, each type with its own use case. Examples of MU-MIMO include:
- MIMO-MAC – Which is used as a multiple access channel in uplink scenarios. In MIMO-MAC, the receiver performs most of the processing and it requires large levels of uplink capacities.
- MIMO-BC – Which is used MIMO broadcast channels, and enables more throughput.
- Cross-layer MIMO – Which improves the performance of MIMO links. This is done by solving problems that may happen when employing MIMO configurations.
- Dirty Paper Coding – Which would be used in telecommunications to make the transmission of data through a channel with known interference more efficient.
- Cooperative MIMO (CO-MIMO) – Which uses distributed antennas belonging to other users instead of ones only belonging to a logical terminal.
Multi-user MIMO benefits from its construction and operations in multiple ways including:
- MU-MIMO's use of beamforming to direct signals toward a wireless device.
- A general decrease in the time each device has to wait for signals.
- A general increase in the capacity and efficiency of a user’s router.
- MU-MIMOs are affected less by antenna correlation and channel rank loss.
- Access points do most of the involved processing.
- A user’s devices do not have to use multiple antennas.
- Devices that do not support the technology will still run faster on a MIMO network, as all devices connected to an MU-MIMO network do not have to wait as long for signals from the Wi-Fi
- MU-MIMO can improve video playback streams and eliminate some buffering or lower video quality.