What is Mu-Mimo?
Multiple-input multiple-output, or MIMO, is a range of technologies used to multiply the capacity of a wireless connection without requiring more spectrum. Many modern technologies, from Wi-Fi to LTE, use MIMO techniques to achieve more capacity without more spectrum.
Although traditional MIMO techniques are focused on increasing the bandwidth available between two wireless nodes, multi-user MIMO (MU-MIMO) applies these technologies to increase overall wireless network capacity by allowing an access point to communicate wirelessly with more than one wireless node at once.
An MU-MIMO access point features an array of antennas. When multiple wireless nodes wish to communicate, the array is divided such that some antennas are used to communicate with one node, whilst other antennas are used to communicate with another node – simultaneously.
This is contrast to a traditional wireless system, where two wireless nodes cannot communicate on the same channel to the same access point at the same time, without causing significant self-interference and degrading the overall wireless network performance.
An MU-MIMO access point estimates and measures what a transmission from each wireless node ‘sounds like’, by applying knowledge of the wireless path characteristics between the access point and node. Known as channel estimation, this process is of vital importance; without it, the access point cannot distinguish properly between wireless nodes, affecting performance.
Channel estimation is achieved by the access point sending a specific signal to a wireless node, which the node then reflects back. By measuring how the signal was received back from the node, the access point can estimate the wireless conditions between itself and the node, and know to expect these same conditions to be applied to other communications from that node. This is known as channel sounding.
Channel estimation and sounding must be regularly repeated to ensure wireless network performance remains high; the speed at which a system is able to accurately estimate the channel has a large impact on performance.
Once channel estimation is completed for a wireless node, the MU-MIMO access point can electrically tune each antenna to provide the highest performance for that node. The access point uses beamforming to create a radio beam to that node which is tuned for optimum performance and avoids beams directed to other nodes, reducing interference and helping to improve overall wireless network capacity.
An MU-MIMO access point can communicate to multiple wireless nodes simultaneously using this process. As the majority of nodes are unable to make full use of the whole access point capacity at once, communicating with several nodes simultaneously can greatly improve the overall capacity achieved in the wireless network.
As with single-user MIMO, if a system has greater than 8×8 antennas it is considered a Massive MIMO system; if the system also supports MU-MIMO, it is considered a Massive MU-MIMO system.
As the complexity of implementing both Massive MIMO and MU-MIMO are very high and increase with each antenna added, Massive MU-MIMO systems are not yet commercially available.