How Stuff Works – 802.11n MIMO
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:
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:
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.