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Understanding 802.11n MIMO
and Multiple Spatial Streams

Multiple-Input / Multiple-Output (MIMO) is the often misunderstood 802.11n capability that allows more than one antenna to transmit a separate data stream at the same time and on the same channel thereby multiplying the throughput accordingly

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The Path to Understanding 802.11n Multiple-Input / Multiple-Output (MIMO)

Signal Reflection and Multipath Distortion

Key terms explained in this section that you need to understand to fully appreciate 802.11n MIMO:
out-of-phase, amplitude, constructive interference, destructive interference, multipath distortion


When the two reflected signals reach the 'X' the signal traveling across the yellow path
will have traveled a longer distance than the one on the red path. The two signals will
arrive slightly out of phase with each other.
Above we see a single RF signal being transmitted from an antenna and propagating outwards in all directions. The light areas represent the peaks of the signal envelope and the dark areas are the valleys between them.
Above, the propagated wave has hit an obstruction and we see two rays of resulting reflected signal bouncing back. The dark areas are the result of destructive interference and the light areas are the result of constructive interference.

Overcoming Multipath Distortion Using Antenna Diversity

Key terms explained in this section that you need to understand to fully appreciate 802.11n MIMO:
antenna diversity, signal wavelength, reflected rays

The access point selects the antenna experiencing
the least amount of multipath distortion

Moving From Diversity to MIMO

Key terms explained in this section that you need to understand to fully appreciate 802.11n MIMO:
cluster, spatial stream, multiple input, multiple output, MIMO

In the example given by the diagram above, imagine what would happen if the client device moved to the right by 1/2 wavelength. It could be imagined that there would be a position that would result in the right-hand antenna now being the better choice. This is the situation depicted in the diagram below. Now the black and orange ("good") rays are converging at the right-hand antenna.

If the client device moves to the right it may be possible
for the right-hand access point antenna to be the better receiver
It was from this concept that the engineering community devised a remarkably clever scheme for doubling the capacity of a single wireless channel. If two antennas are available for simultaneous transmission and two antennas are available for reception then there may be some locations where the left-hand transmission antenna is the "good" signal for the left-hand receiver antenna and, at the same time, the right-hand transmission antenna is the "good" antenna for the right-hand receiver antenna. Remember that there is no guarantee that all locations for all client devices will actually be in a place where this left/right relationship will be possible - but some locations will have this dual relationship. The locations where two simultaneous transmissions on the same channel can be differentiated by the two receiver antennas is called a "cluster" location. The two transmissions are referred to as two "spatial streams" and the result is that multiple transmissions are made into the air ("multiple input") and two spatial streams are recovered from the air ("multiple output") - and this is 802.11n MIMO. The situation is depicted below.
With two antennas the client device can transmit two spatial streams in a MIMO system