If we figure out inverse of [c] and apply it to the transmission (below figure), the equation will look like,
[r] = [c] . [c]-1 . [t] + [n]
[r] = [t] + [n]
Fig "Precoding"
After removing the noise, we get back transmitted signal.
The inverse of [c] is called Precoding matrix.
Channel sounding
Catch is, how to find out [c] and its inverse.
One way to determine Precoding matrix is to get a feedback from receiver about [c].
For receiver to figure out [c], it will require a (reference) signal without precoding applied.
That will be "s" in figure below.
Fig "Reference signals"
"d" is a reference signal, needed for RX processing.
As the name suggests, it is reference for transmitted signal.
Due to its purpose, it is precoded along with the signal to be transmitted.
If you notice, the number of s signals or antenna ports is equal to the number of transmission antenna ports.
Also, the number of input antenna ports (to precoding) need not be same as the number of output antenna ports; it can be same or lesser (L <= T).
