MIMO and MU-MIMO

Read about and configure IEEE 802.11n MIMO (multi input, multi output) and 802.11ac Wave 2 MU-MIMO (multiuser multi input, multi output) technologies in this topic.

MIMO (Multiple In, Multiple Out) is a major WLAN advancement introduced in the IEEE 802.11n standard in which multiple RF links are formed on the same channel between the transmitter and receiver simultaneously. To accomplish this, the transmitter separates a single data stream into multiple spatial streams, one for each RF chain (an antenna + various digital signal processing modules linked to the antenna). The transmit antennas at the end of each RF chain then transmit their spatial streams. The recipient’s receive antennas obtain streams from all the transmit antennas. In fact, due to multipath, they receive multiple streams from each transmit antenna. The receive antennas pass the spatial streams to the digital signal processors in their RF chains, which take the best data from all the spatial streams and reassemble them into a single data stream once again.

MU-MIMO stands for multiuser multiple input, multiple output, and was introduced with 802.11ac Wave 2. MU-MIMO provides concurrent downstream communications to multiple client devices for more efficient use of the spectrum. Wave 2 MU-MIMO support is required on both the access point and client device to work. Extreme Networks AP250 access points support MU-MIMO.

In previous 802.11 standards, access points and clients each employed a single set of components, or RF chain, for transmitting or receiving. Although two antennas are often used for diversity, only the one with the best signal-to-noise ratio is used at any given moment, and that antenna makes use of the single RF chain while the other antenna remains inactive. A significant improvement that MIMO introduces is to permit each antenna to have its own RF chain and for all antennas to function simultaneously. For the AP340, for example, you can connect up to three antennas per radio and configure the radio to use two or three transmit chains and two or three receive chains. Using two or three transmit and receive chains simultaneously increases the amount of data that can flow across the WLAN and accelerates the processing of that data at each end of the wireless link.

Another major aspect of MIMO is how it turns multipath signals from a curse to a boon. As a radio signal moves through space, some objects reflect it, others interfere with it, and still others absorb it. The receiver can end up receiving multiple copies of the original signal, all kind of muddled together. However, the digital signal processors in the multiple receive chains are able to combine their processing efforts to sort through all the received data and reconstruct the original message.

Furthermore, because the transmitter makes use of multiple RF chains, there is an even richer supply of signals for the receive chains to use in their processing. To set the transmit and receive RF chains for a radio profile, enter the following commands:

radio profile <name> transmit-chain { 1 | 3 }

radio profile <name> receive-chain { 1 | 3 }

When deciding how many antennas to use, consider the types of wireless clients - 802.11n only, 802.11 g/n, 802.11b/g/n, 802.11a/n, or 802.11 ac (5 GHz only) - the area needing coverage, and the RF environment.

 


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