What is Channel Reciprocity?

Hi All,
What is Channel Reciprocity?
And how it use for TDD to be more beneficial for mMIMO?
Why we do not use FDD with mMIMO?

Channel Reciprocity is a physical phenomena, because DL/UL bands coincide.

Both Uplink and Downlink complement each other.
As such, easy to apply link adaptation on DL once get the channel estimate from UL.
Reciprocity only possible in TDD, some of the problems to achieve reciprocity in FDD is the channel aging effect as the no of MIMO order increases.