Electronic – MIMO: How Can Two Antennas So Close Together Receive Different Signals

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I'm trying to understand how MIMO (as used by 802.11n wi-fi and some upcoming cellular data standards) works in practice. I understand at least the gist of the theory of spatial multiplexing. What I don't get is how it works in practice. Let's say you have two TX antennas ~30 cm apart, two RX antennas ~30 cm apart and the distance between the transmitter and receiver is about 30 meters. The angular separation of the transmit antennas as seen from the receiver is tiny. From the perspective of each RX antenna the two TX antennas are about the same distance away. How is the difference between the signals at the two RX antennas measurable and not buried in noise? I would think that in such a configuration they would be nearly identical.

EDIT: Both answers I've received have mentioned phase. This doesn't make sense to me. Assume a coordinate system where the two TX antennas are separated only along the X-axis. The Y component of their separation is zero. As the Y-axis distance between the transmitter and receiver goes to infinity, the difference in distance between the two TX antennas as seen from the receiver goes to zero. If the distance between the TX antennas is 30 cm and most of the distance between the transmitter and receiver is along the Y-axis, then 30 m is for all practical purposes infinite separation between the transmitter and receiver.

Best Answer

There are different types of MIMO. Those are Precoding, Spatial multiplexing, and Diversity Coding.

Precoding

The idea behind MIMO is that at the frequencies being used, the wavelength is small enough that even 30 cm apart is enough to receive the signal at different phases. As Brain said, the wavelength is about 12.5cm for 2.4 GHz. This means that regardless of how far you are from the two antennas, the delay (or phase delay) between the two antennas will always be fixed for any given angle.

You are able to take advantage of this phase difference to create beam steering. The math and actual implementation of this is complex, but the general idea is actually relatively simple. If the two signals are in phase, then you know that the source of that signal is the same distance from each antenna which means that your source has to be somewhere along the line of symmetry.

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As the source begins to move around, the signal will get to one or the other antennas first and the angle from the receiver can be determined based off of the amount of delay between the two. This then allows you to setup "sectors" or beams based of off how much delay is applied to the incoming signal.

Now technically the drawing I showed is only MISO (Multi in single out), but the logic holds true when you add another antenna to create a full MIMO. Also, on the transmitting side, you can do the same thing I talked about with receiving, but instead a delay is applied to one or the other antenna to create a beam in specific direction out of the transmitter.

The accuracy of the angle in and out of each pair of antennas is determined by both the spacing of the antenna and the accuracy of electronics to produce and detect a specific phase shift.

Also things get more complex as you start to account for the fact that at some locations the signal might appear to get to the antennas at the same time but are actually 1 full cycle apart. Also there has to be a control system setup to know what direction you should be directing you beam at, especially when you have a moving device.

But to get to your question directly, it doesn't matter if your source has 2 antennas or not, it is treated the same on the receiving end. What maters is the angle that the source is from the destination. You essentially end up with a source directing its beam in the general direction of the receiver and then the receiver is steering its beam in the general direction of the transmitter.

The big advantage of using MIMO is that you are not creating a lot of extra noise for neighboring devices and so you are able to get more devices in to a small area. Also, since the signal is more directional there is less to bounce off of which results in less issues with multipath.