How Stuff Works – 802.11n and Spatial Multiplexing

This is the third post in my “How Stuff Works” series.  The first two posting discussed MIMO and channel bonding.  This post looks at another technical improvement that leads to greater speed in 802.11n networks – spatial multiplexing.

It is helpful to take a quick look at a classic 802.11 transmitter.

802.11 Classic Transmitter

802.11 Classic Transmitter

In this scenario, only one data stream is sent from the transmitter to the receiver (represented by the orange line).

With spatial multiplexing, multiple data streams are transmitted at the same time.  They are transmitted on the same channel, but by different antenna.  They are recombined at the receiver using MIMO signal processing.  This is represented in the diagram above with two spatial streams – an orange colored one and a navy blue colored one.

Spatial Multiplexing - Two Streams

Spatial Multiplexing - Two Streams

Spatial multiplexing doubles, triples, or quadruples the data rate depending on the number of transmit antennas.   Remember, you may hear three numbers when referring to 802.11n or MIMO networks – the first is the number of transmit antenna, the second is the number of receive antenna, and the third is the number of spatial streams.   For example, a 3×3x2 system has two spatial streams.

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15 comments so far

  1. farhan on

    great article sir

  2. deena on

    When I was looking for information on how does MIMO improves the capacity. this one gives me clear picture. Simple and Good.


  3. Anonymous on

    i am doing a presentation on spacial multiplexing in my class this article help me understand it better thank you

  4. minhaz on

    Really great.

  5. Anonymous on

    very successful

  6. DAX Associates on

    A crystal clear explaination of spatial multiplexing!

  7. Andrea on

    Hi Douglas,
    I have just a doubt. In the “spatial multiplexing” configuration, I didn’t figure if every single rx antenna can receive both streams simultaneously, or only one at time.

    Greeting from Italy

  8. hemanshu on

    what is recursive spatial multiplexing

  9. Sarfaraz Hussain Smee on

    it gives me clear knowledge about spetial multiplexing…

  10. thisha on

    it was a nice articles, thanks for sharing,

    is MRC and spatial multiplexing same ?

  11. Giovanni on

    I’m just curious: why would such thing as a 3x3x2 exist? I mean: if both the transmitter and the receiver have 3 antennas, why would they only use 2 spatial streams, and not the maximum of 3 spatial streams allowed by such configuration? Thanks to anyone willing to answer!

  12. Greg on

    Supporting more spatial streams involves more than just the number of Tx and Rx antenna. It increases, significantly, the complexity of the baseband processing. Having the extra TX and RX antenna still provide for antenna diversity, which is important to improving the 2 streams SNR and thus data rates at range.

  13. Anonymous on

    Pity that nobody explains (in a simple way) how a spatial stream work, i.e. how is collision and interference avoided with spatial streams.

  14. Anonymous on

    Why is there no interference when multiple signals are transmitting on multiple antennae on the same channel?

  15. Alexander Cerna on

    I too am perplexed by the idea of, say, 2 spatial streams both essentially combined as they’re flying across on the same frequency channel–how do the receivers pick out the 2 spatial streams when they’re all on the same frequency channel? It’s as if 2 flashlights of exactly the same light color are pointed at your 2 eyes, and how would your brain figure out which flashlight did photons come from? Can someone please explain?

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