MIMO, antennas and streams - oh, my!

* How do 802.11n specs translate into the real world?

802.11n's intimidating nomenclature contains such mysterious terms as MIMO, spatial streams and spatial multiplexing, which can quickly freeze a network manager's brain. What do these specs really mean in the context of a real-world 802.11n wireless networking environment?

802.11n's intimidating nomenclature contains such mysterious terms as multiple-input, multiple-output (MIMO), spatial streams and spatial multiplexing, which can quickly freeze a network manager's brain. What do these specs really mean in the context of a real-world 802.11n wireless networking environment?

Generally (and in an admittedly oversimplified sense), MIMO and spatial stream numbers work together to indicate throughput potential at given ranges. Generally, the greater the number of receive antennas, the greater the distance that a particular data rate can be sustained.As in most things wireless, of course, the actual throughput and range you will experience with 802.11n depends on your environment – whether the floor plan is open or chopped up; what the building and window construction materials are; and the configuration of the client devices involved in the transmission. So consider the MIMO and spatial stream numbers that appear on a vendor’s data sheet in that context.MIMO is just one part of the equation. It refers to the number of transmit and receive antennas involved in exchanging wireless signals across a propagation channel. (“Propagation channel” is a fancy word for the wireless path that signals take through the air.)

2x2 MIMO, for example, indicates two antennas at the transmit end and 2 antennas at the receive end, the minimum required by the draft 802.11n standard. 2x3 MIMO indicates two transmitting antennas and 3 receiving antennas. And so forth.

Spatial multiplexing is a mandatory component of the 802.11n standard, and MIMO is required in order for spatial multiplexing to take place. So the two work hand in hand.

But what’s spatial multiplexing? It’s a technique whereby multiple antennas separately send different flows of individually encoded signals (called spatial streams) over the air in parallel; in essence, reusing the wireless medium or “multiplexing” the signals to shove more data through a given channel. At the receiving end, each antenna sees a different mix of the signal streams. In order to decode them accurately, the receiving device needs to separate the signals back out (or “demultiplex” them).

Note that the number of spatial streams that can be multiplexed over the air is dependent on the number of transmitting antennas. So while 2x3 MIMO has an additional receive antenna compared to 2x2 MIMO, only two spatial streams can be supported in both configurations. So why have three antennas at the receiving end?

We’ll take a look at that next time.

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