What is single-user MIMO?
Single-user MIMO (SU-MIMO) is a multi-transmitter and receiver technology that lets a wireless access point send multiple, simultaneous data streams to one compatible endpoint at a time. SU-MIMO is typically compared in contrast with multi-user MIMO (MU-MIMO).
SU-MIMO began as part of the 802.11n wireless standard in 2007. SU-MIMO enables multiple data streams to be transmitted to or received between Wi-Fi devices.
In SU-MIMO, a device such as an access point or router uses multiple antennas to simultaneously send multiple data streams to a single endpoint device, such as a phone, laptop or tablet. Dividing available bandwidth into multiple spatial streams increases the efficiency and reliability of transmissions.
SU-MIMO technologies also make use of MIMO functions such as spatial multiplexing and beamforming to direct and improve signal strengths.
For SU-MIMO to work, both the transmitting and receiving devices -- such as an AP and a laptop -- must support MIMO. That means both devices must have at least two antennas, which may be a burden for smaller devices, such as smartphones.
A potential downside to using SU-MIMO compared to another strategy such as multi-user MIMO is only being able to send multiple data streams to one device at a time.
How MIMO works
MIMO is an acronym that stands for multiple-input and multiple-output. It can be divided into these three groups: spatial multiplexing, precoding and diversity coding.
Multiplexing is a way of sending multiple signals or streams of data over a communications link simultaneously in the form of a single signal. Networks will use multiplexing to have multiple devices connect to each other and to stretch scarce network resources further.
Spatial multiplexing, used in SU-MIMO for example, splits a high rate signal into multiple low rate signals, transmitting them via separate antennas at the same frequency channel. The number of possible data streams in spatial multiplexing is limited to the number of antennas at the transmitter and receivers. Spatial multiplexing can also be used in multi-user MIMO to transmit data to multiple receivers at the same time.
MIMO also uses precoding, which is multi-stream beamforming. Precoding can also be considered as all the spatial processing which occurs in the transmitter. Beamforming is used to increase signal gain and directs signals toward a wireless device.
Diversity coding is a coding technique that sends multiple copies of data through multiple antennas at the transmitter. This is done to improve reliability when there is no knowledge of the channel at the transmitter. When only some channel knowledge is available, diversity coding can be combined with spatial multiplexing.
Single-user vs multi-user MIMO
MU-MIMO is another, more advanced technology used for wireless communication between devices. It debuted in Wave 2 of the 802.11ac standard, also known as Wi-Fi 5.
MU-MIMO is mainly used to support environments where multiple users access the same wireless network at one time. MU-MIMO helps relieve potential congestion caused by multiple devices trying to connect by creating multiple connections to a device at the same time.
Single-user MIMO transmits data streams to a single device at a time, while multi-user MIMO can transmit data to multiple devices at the same time. In SU-MIMO, any multiplexing gain is limited by the number of transmitting and receiving antennas. In MU-MIMO however, multiplexing gain can scale with the number of transmit antennas used.