Cisco Aironet 2702i AIR-CAP2702I-E-K9 Folheto

 

 

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Figure 8.    MU-MIMO Using a Combination of Beamforming and Null Steering to Multiple Clients in Parallel 

 

For all this to work properly, especially the deep nulls, the AP has to know the wireless channel from itself to all of 

the users very accurately. And since the channel changes over time, the AP has to keep measuring the channel, 

which adds overhead. Some APs might use the higher-overhead 802.11ac sounding protocol only, but the greatest 

benefit of MU-MIMO comes if the AP can minimize the number of explicit sounding exchanges, such as with the 

ClientLink mechanisms. 

Meanwhile, the client is receiving its desired signal distorted by some interference from the signals intended for 

other users. This interference makes the highest constellations such as 256QAM infeasible within an MU-MIMO 

transmission. 

In summary, MU-MIMO allows an AP to deliver appreciably more data to its associated clients, especially for 

small-form-factor clients (often BYOD clients) that are limited to a single antenna. If the AP is transmitting to two or 

three clients, the effective speed increase varies from a factor of unity

3

 (no speed increase) up to a factor of two or 

three times, according to wireless channel conditions. 

The 802.11ac project authorization request (PAR) that kicked off 802.11ac includes some throughput numbers: 

500-Mbps single-user throughput and 1-Gbps multiuser throughput. These numbers are requirements in the 

802.11ac amendment (that is, the document), not in individual products. The amendment defines that the 

minimum product allowed to call itself 802.11ac can operate at 290 Mbps for a single user and not support 

multiuser at all. 

                                                 

3

 If the speed-up factor drops below unity, the AP uses SU-MIMO instead.