SU-MIMO & MU-MIMO
With Single User MIMO (SU-MIMO), the AP can use multiple spatial streams to send a large amount of data to compatible clients. Devices such as laptops can sometimes support two or three streams, allowing for high speed connections. SU-MIMO is the technology traditionally used in 802.11n and 802.1ac Wave 1 networks.
With Multi User MIMO (MU-MIMO), the AP can use the individual spatial streams to send separate transmissions to distinct clients simultaneously. This increases the total network performance and improves the end user experience, especially when large numbers of devices are connected.
MU-MIMO complements Single User MIMO (SU-MIMO). An AP can choose the best way to transmit: Simultaneously to multiple devices as efficiently as possible, or consecutively to individual devices as fast as possible. MU-MIMO requires hardware support from the latest 802.11ac Wave 2 APs and compatible clients.
Beamforming occurs when an AP and a specific client focus RF signals directly at each other to improve throughput to that client. Explicit beamforming requires information to be sent from the client device to the access point in order to work. The additional information makes the beamforming more accurate, but the tradeoff is that it places more data in the airwaves.
In a departure from 802.11n, 802.11ac defines only one beamforming sounding method: Explicit sounding. Although this method is optional, it allows an 802.11ac AP-to-client pair to perform beamforming. If the access point implements the beamformer side and the client implements the beamformee side (which may be typical), unidirectional beamforming is achieved. If the access point and client implement both beamformer and beamformee functionality, bidirectional beamforming is achieved.
Cisco expects that 802.11ac access points and clients are more likely to perform 802.11ac sounding and beamforming.
Beamforming and MU-MIMO/SU-MIMO
SU-MIMO uses beamforming to improve the signal strength and achieve higher bitrates to a single client. MU-MIMO uses beamforming to direct energy to one client and steer that energy away from other clients addressed by the MU-MIMO transmission. MU-MIMO will also do the same energy direction for any subsequent clients that connect.
MU-MIMO in 802.11ac Wave 2 sends to multiple receivers:
- An access point with four antennas sends one stream each to three smartphones, all at the same time.
- The access point must beamform one space-time stream to each receiver and simultaneously null-steer that space-time stream to the two other receivers.
Both SU-MIMO and MU-MIMO are enabled on Meraki APs by default without any additional configuration by the network administrator. Please refer to the MR Datasheet to verify which technologies are supported by each MR.
As described in the sections below, MU-MIMO is negotiated by the client and the AP which will ensure that capabilities are matched for each client connecting to the Meraki MR. This means that the MR AP may serve MU-MIMO enabled clients without sacrificing backwards compatibility with legacy, non-MU-MIMO clients.
Below are some 802.11 frames that detail how MU-MIMO is negotiated between the AP and the client. A similar technique is used to negotiate SU-MIMO as well.
The MU Beam-former flag will be set by the AP in these following frames:
- Probe Response
The MU Beam-formee flag will be set by the client in these following frames:
- Directed Probe Request
- Association Response