Does anybody have a good explanation why we use FDD instead of FDM?
I have found a very nice (and easy) explanation of your question.
As explained there:
Multiplexing - Isolate signals to different users in the downlink from base station to user.
Duplexing - Isolate up and down links.
So, we can use FDM and FDD together (in fact, GSM does this when it has more than one carrier)
And we can also use FDM in a TDD system (in TD-LTE we call the FDM aspect “PRBs”)
I believe this image help to understand (anyone agree/disagree)?