single-user MIMO

Single-user MIMO (SU-MIMO) is a multi-transmitter and receiver technology for wireless communication that allocates the bandwidth of a wireless access point to a single device. SU-MIMO is typically compared in contrast with multi-user MIMO (MU-MIMO).

SU-MIMO began being implemented with the 802.11n wireless standard in 2007. SU-MIMO enables multiple data streams to be transmitted to or received between Wi-Fi devices.    

A device such as a router that uses SU-MIMO will transmit multiple data streams using multiple antennas on the device to send data to a single endpoint device such as a phone, laptop or tablet. Utilizing the antennas of a device (commonly referred to as the DAS or distributed antenna system) is vital in SU-MIMO systems to improve resistance, interference, reliability and capacity. SU-MIMO technologies will also make use of MIMO functions such as spatial multiplexing or beamforming to direct and improve signal strengths.  

A potential downside to using SU-MIMO compared to another strategy such as multi-user MIMO is only being able to send data to one device at a time. SU-MIMO also requires both transmitting and receiving technologies to support MIMO to operate. Additionally, both devices need to use multiple antennas, which may be a burden for smaller devices.

Functions of MIMO

MIMO is an acronym which stands for multiple-input and multiple-output and 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 which 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 technology used for wireless communication between devices. 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 transmit and receiving antennas. In MU-MIMO however, multiplexing gain will scale with the number of transmit antennas used.

Even though it is possible for MU-MIMO to transmit multiple data streams at the same time, administrators normally limit this since MU-MI is most optimal when serving one device at a time. Generally, administrators will use an algorithm to split the time a MU-MIMO router, will complete requests for each device.

This was last updated in June 2019

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