MIMO and Implementation of Multiple Spatial Streams
802.11n and 802.11ac use multiple, simultaneous “spatial streams” to increase data throughput (this is “MIMO” Multiple-Input/Multiple-Output). “Multiple” (in “Multiple-Input/Multiple Output” MIMO) refers to multiple spatial streams all transmitted on the same frequency at the same time. Each spatial stream carries some maximum throughput based on modulation type and channel width and all streams carry the same. Two spatial streams provide twice the throughput than one spatial stream. The primary spatial stream is generally the line-of-sight path from transmitter to receiver. There’s no guarantee that more than one spatial stream can be acquired. Additional spatial streams may be acquired if the signal reflections in the environment are suitable but there’s not guarantee that multiple stream paths will be available. In the real-world, it’s generally easy to acquire a second spatial stream (total of two counting the line-of-sight path). As the number of spatial streams goes up it becomes less and less likely that an additional stream can be acquired. Whether or not a spatial stream is available depends on the instantaneous reflective nature of the environment. Moving a receiving antenna even a few inches (at 5 GHz) can completely change the spatial relationships in the environment. While it’s possible to acquire more than two spatial streams, in the end it’s a statistical probability, not a certainty.
802.11ac specifies up to 8 MIMO spatial streams (compare to 802.11n which offers up to 4) but 802.11ad uses only 1 stream. Recall that 802.11n specifies up to 4 spatial streams but only 3X3 (3-stream) MIMO has made it to the market. Even with 802.11n 3-stream MIMO you typically only find it implemented in access points; clients typically max out at two streams. 8-stream 802.11ac is will not be seen in the initial 11ac offerings (if it makes it at all). Without 8-stream MIMO it’s not possible to realize the maximum throughput rates specified for 802.11ac. 802.11ac includes antenna beamsteering in the standard but does not require its use, and beamsteering is not expected in early 11ac equipment. 802.11ad requires standardized beamsteering to provide required directional antenna gain.