TDD and FDD — Sharing a frequency in time, or using two
RF and system
Where it sits
What it is
What they are. FDD gives each direction its own frequency, permanently. TDD gives both directions the same frequency and alternates in time. NR supports both; the slot configuration for TDD is TS 38.213 clause 11.1.
TDD is the interesting case, and most new NR spectrum is TDD — because paired spectrum is scarce at high frequencies and because the traffic is asymmetric, so a fixed 50/50 split would waste half of it. TDD lets the downlink/uplink ratio be configured, and reconfigured.
It costs a guard period. Every switch from downlink to uplink needs silence long enough for the last downlink symbol to arrive from the cell edge before the first uplink symbol departs. That silence is spectrum nobody uses.
And it buys reciprocity. Same frequency, both directions, moments apart — so an uplink measurement is a downlink measurement. This is the single fact the entire sounding note rests on, and it is why SRS antenna switching is a TDD story and has no FDD equivalent.
So the duplexing choice decides how CSI is obtained. In TDD the network can measure the uplink and infer the downlink (SRS). In FDD the fine structure differs between the two bands, so the device must measure and report (CSI-RS). Two completely different machineries, chosen by one property of the spectrum licence.
Read on
This concept was first written up in ref-system, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: