What's up with MIMO?

So what's up with 3 transmitters and 2 receivers??? Given how much I enjoy music, let me use home audio as an analogy. Remember the days of mono speaker systems (yeah, I know... I'm dating myself somewhat here). Well, in a mono system, the audio system sends all of the audio signals through a single speaker. The sound takes the fastest path to the listener, which I would claim in most home environments would be direct line of sight. While this does provide a full audio experience, there is no directionality in the sound which lessens the entertainment experience. And here is where 5 channel digital audio comes into the picture. In such setups, you will have three front speakers; center, left and right. First off, you will notice a similarity between MIMO and home audio technologies, since there are three front speakers, but most listeners that I know are restricted to two ears. Of course, the question that remains is whether lack of a third ear on our forehead decreases the total experience. These three speakers send independent audio signals, each with directional sensitivity. If a plane is moving from the left to right of the screen, the sound will match.

Unfortunately, that's where the comparison of MIMO and audio systems will end. As with the audio example, an 802.11n access point has multiple antennas, and they are typically referred to in terms of transmitters and receivers. So when you hear about a 3x2 802.11n device, this would mean three transmitters and two receivers. However, unlike the audio system which sends a different audio stream on each front antenna, each transmitter in the 802.11n access point transmits the same signal. So why bother sending the same signal on multiple antennas? Well... here's where WLAN also differs from a typical audio system. Most audio systems are used in a single, typically square or rectangular room, which provides for an optimal experience. However, an 802.11 home network is expected to be available throughout the home, and this requires the signals to traverse through various building materials, including walls and doors. Interestingly enough, much to my mother's dismay, when I was young I also seemed to think audio should be enjoyed in a similar manner.... but that's another story.

Going back to WLANs, office environment have many metal obstructions, including filing cabinets and but also building materials found in walls, ceilings and even door frames. These metalic materials do not absorb the signal and therefore cause it to be reflected, and unlike the human ear, electronics are much more sensitive to reception delays. These reflections are called multipath, which historically would significantly hamper network performance because a single antenna would receive multiple reflected signals at different times - causing receiver interference. However, since an 802.11n device has multiple antennas, the separation between these antennas causes the multiple signals to follow a different path to the receiver, basically using the properties of multipath to its advantage. These signals are then received on multiple antennas, which is where the magic of MIMO comes in. These multiple received signals are recombined into a single stream to increase the strength of the signal.

Well... I gotta run so next time maybe we can talk about what happens when that third ear grows on your forehead.

PatC