MIMO relationship with OFDM and why are the two that much linked to each other?
All your MIMO textbooks describe the MIMO channel as a matrix of complex values, right?
And you realize that these matrix entries in (row, column) represent the channel coefficient between then row.th antenna and the column.th antenna on the other side.
Now, you know that wireless channels usually cannot just be represented as a single complex coefficient. Usually you need to have the full channel impulse response, which is way more than just one value!
Yet, all the usual MIMO techniques require that each channel is just a complex number!
Therefore, only channels that are a single complex number are &quot;useful&quot; for these methods.
Such channels are flat channels (in contrast to frequency-selective channels).
To use MIMO, you need to take the frequency-selective channels you find in reality and convert them to flat channels.
That is exactly what OFDM is designed to do.
Therefore, you first apply OFDM, then you get as many flat channels as you have OFDM subcarriers, and then to each of these, you apply MIMO techniques.
That's all there is to that.