The evolution of MIMO
Single input, Single Output
It all began with SISO, Single Input Single Output. One antenna is used for all communication at both the source and destination transceivers. This all changed with the introduction of 802.11n in 2009. (although work began on 802.11n in 2002).
Multiple input, Multiple Output
|What||MIMO is the transmission of multiple streams of data at once across the same channel|
|When||802.11n officially published in 2009.|
|Where||Where multipath exists|
|How||Using spatial streams, MIMO takes advantage of multipath. Because there is more than 1 path, you can double the throughput by transmitting different bits on different paths.|
MIMO relies on the access point having multiple radio chains, and individual antennas. The amount of transmitters and receivers is defined in technical documentation using ZxY – Z=Transmitters, Y=Receivers (eg. 2×3 MIMO is 2 transmitters, but 3 receivers)
OK, but what’s a radio chain?
A radio chain is comprised of a single radio and all associated components.
The Oscillator creates the frequency for the transmission, it has to be stable, accurate, known. (there can be 2 or more oscillators because of different circuits in the tranceiver). The Oscillator is usually linked to a frequency synthesieser to give you a range of frequencies. The signal is amplified, cleaned with a filter, and mixed with the frequency created by the oscillator. Then it’s cleaned, amplified again, and finally transmitted via an antenna.
MIMO uses Spatial Multiplexing to achieve greatness
Spatial Multiplexing is the act of transmitting different sets of data across multiple spatial streams. Obviously, both transmitter and receiver need to support the same amount of spatial streams.
MIMO & MU-MIMO Notation
When referring to datasheets, you will come across a specific notation which represents transceiver MIMO capability.
T x R : S
T = Transmit Radio Chains
R = Receiver Radio Chains
S = Spatial Streams
T and R must be greater than or equal to S.
OK, what about MU-MIMO?
|What||The transmission of data to multiple users during the same timeslot on the same channel. MU-MIMO is downstream to the client only|
|Where||Rarely, in-fact, never really used. Needs client support and perfect conditions to be used.|
|How||Using spatial streams, MU-MIMO takes advantage of multipath. Because there is more than 1 path, you can double the throughput by transmitting different bits on different paths.|
MU-MIMO is MIMO, except instead of using 3 spatial streams for one client, it uses it for multiple clients. This was to be the greatest new feature in 802.11 that never came to be.