How Stuff Works – 802.11n MIMO

If you have been following my blog, you know that I have a poll regarding the greatest challenges to 802.11n deployment in the enterprise.  (Here’s your chance to rock the vote!)

One challenge for replacing desktop Ethernet with wireless is speed.  Perhaps the widest publicized enhancement to 802.11n is that of MIMO (“my-moe”) antennas, which stands for “Multiple Input, Multiple Output”.  How does MIMO work?

To answer that question, let’s look at how a classic 802.11 wireless transmitter operates:

802.11 Classic Transmitter

802.11 Classic Transmitter

In this case, the signal is sent out of one antenna.  The signal is received by both antennas at the other end, but only one signal is processed and sent up to the MAC layer.   Antenna diversity helps in the fact that the best signal is the one that is processed, but remember that it is still a single antenna that processes the receives and processes the RF energy.

Let’s compare that to a MIMO antenna structure:

MIMO Signal Processing

MIMO Signal Processing

In this case, we have three transmit antennas and three receive antennas (often referred to as 3×3 MIMO).  The black, green, and red lines above each represent their own signal.   With MIMO all three signals are received and processed up the stack.   This significantly improves the receiver’s “ability to hear” and it represented in the graph above by the orange line.

You may hear different implementations of MIMO such as 2×3 and 3×3.  The first number is the number of transmit antennas and the second number is the number of receive antennas.   If you hear 3x3x2, the last number refers to the number of spatial streams, which I will discuss in another post.


7 comments so far

  1. Kara on

    When is the next Haider Hundred?

    Feel free to delete this comment, but shoot me an email; I can’t find an email address for you on your blog. 🙂

  2. Anonymous on

    So what did you say? i’m not sure this reaches the level of “how it works”

  3. Anonymous on

    what is recursive spatial multiplexing

  4. fetflex on

    So there is essentially no Mimo without Spatial Multiplexing. MIMO is the antenna’s and Spatial Multiplexing is the way they communicate?

    • wifijedi on

      You can have MIMO antenna with a single spatial stream.

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