With 802.11n, a device
can transmit multiple spatial streams at once, but only directed to a single
address. For individually addressed frames, only a single device (or user) gets
data at a time. This is referred to as Single-User MIMO (SU-MIMO). With the
advent of 802.11ac Wave-2, a new technology is defined, called Multi-User MIMO
(MU-MIMO). Using this technology, an AP can use its antennas and radio systems
to transmit to different clients, all at the same time over the same frequency
Figure 47. Example of
Figure 48. Example of
MU-MIMO with the AP
1850 is limited to a maximum of 3 spatial streams simultaneously.
To send data to user
1, the AP forms a strong beam towards user 1, shown as the top right lobe of
the blue curve (Figure 3).
At the same time the AP minimizes the energy for user 1 in the direction of
user 2 and user 3. This is called “null steering” and is shown as the blue
notches. In addition, when the AP sends data to user 2, it forms a beam towards
user 2, and forms notches (nulls) towards users 1 and 3, as shown by the red
curve. The yellow curve shows a similar beam towards user 3 and nulls towards
users 1 and 2. In this way, each of users 1, 2, and 3 receives a strong copy of
the desired data that is only slightly degraded by interference from data for
the other users.
Figure 49. MU- MIMO Using a
Combination of Beamforming and Null Steering to Multiple Clients in
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. Because the channel changes
over time, the AP has to keep measuring the channel, which adds overhead.
Meanwhile, the client
receives 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 transmits to two or three clients, the effective
speed increase varies from a factor of unity (no speed increase) up to a factor
of two or three times, according to wireless channel conditions.
You cannot configure
to use MU-MIMO, the clients will advertise this capability and the AP will put
them into groups.
is similar to traditional 802.11n MIMO, except instead of one client at a time,
it supports up to several clients that can all receive different data
pre-coding for all the clients; basically aggregating clients that advertise
MU-MIMO capability into groups that participate in MU-MIMO process.
It is an
elaborate process. In MU pre-coding, when the AP is beamforming the signal, for
example at 1SS or 2SS rate to a particular client, the space-time streams have
to be constructed so that it also simultaneously null-steers to those
space-time streams being sent to the other clients.
All users’ MPDUs
are padded to the same number of OFDM symbols. The AP seeks out opportunities
to send similar size packets out to the MU-MIMO clients to optimize on air time
increasing throughput by sending data out to 2 – 4 clients per transmission.
The number of clients depends on the type of stream, for example one client can
receive 1SS while another is receiving 2SS.
certification for Wave-2 includes MU-MIMO.
MU-MIMO is designed to maximize downlink capacity by overlapping clients in
space (multiplexing). Best performance occurs with high client counts and high
(CSI) channel state information to maintain deep nulls so that each MU-MIMO
client can properly decode its data without too much interference from the
pre-coding group data adds overhead as does their acknowledgments and so on.
is slow – Wave-2 clients to be integrated into new laptops, tablets, and phones
but this moves slowly.
MU-MIMO is a new
technology and needs to develop and mature. So, issues such as lower end
clients may be sensitive to MU grouping overhead and client driver version
Figure 50. Example of a
MU-MIMO protocol advertisement