MU-MIMO - do all antennas stream in different frequencies?

Question

I have few questions on how MU-MIMO in WiFi works.

1. Do all antennas stream in different frequencies or stream in same channel but different sub carriers(OFDMA?)
2. Do MIMO antennas always work in OFDM technology?

Answer 1

There is no such thing as a MIMO antenna. The antenna itself isn't "mimo". The system is. All antennas transmit a different signal for all the frequencies in the system!

If different antennas were assigned disjunct frequencies, you would not have MIMO! A subcarrier is a different frequency, so that distinction you make makes little sense, either.

So neither your two statements in (1) nor your statement in (2) are true.

Ignore OFDM/multiple frequencies for a moment. MIMO works on the idea that if you transmit N signals over the medium (air), each from a different antenna, then they will combine. If the channel is flat, that just means that each transmit signal is multiplied with a different complex number, and then they are all summed up and the sum is observed at the receive antenna.

Now you have also multiple receive antennas. Because the channel between these and you transmit antennas are all different, each of these antennas observes a different sum, I.e. the same N transmit signals, but weighted with different complex factors, summed.

When you write down that table of N×M coefficients, you get a matrix with which you can multiply a vector of transmit signals, and get the vector of receive signals. Simple!

Through a bit of linear algebra (singular value decomposition), you can build a "virtual" set of channels from this, which look like they are parallel and do not add and influence each other.

So that's what you do: you take your one medium air, you transmit multiple signals at once, but you use math to be able to have multiple independent transmission that the receivers cab calculate from the multiple "mixed up" signals they receive. How you do that division is up to you - it can be done to have multiple channels to one receiver with many antennas, just to maximize data rate, or you can have independent channels to different receivers.

That's it, really. The fact that OFDM is done only helps because it allows us to consider one wideband channel that can't be represented by a complex number as many narrowband channels that can. So, you get many of these MIMO systems in parallel, in essence one for every subcarrier.