FDD

FDD gives uplink and downlink their own separate frequency bands, so a device transmits and receives at the same time without any switching. That simultaneity is its strength: symmetric capacity in both directions, consistently low latency (no waiting for an uplink slot to come around), and simpler interference behaviour than TDD's time-shared scheme. It needs paired spectrum, with a duplex gap between the two bands so a device's own transmitter doesn't swamp its receiver.

In 5G, FDD lives mostly on the lower bands — the sub-1 GHz coverage spectrum often refarmed from LTE, where paired allocations are the norm and the symmetric, reliable uplink helps coverage. The trade-off versus TDD is rigidity: the uplink/downlink split is fixed by the band plan, so you can't reallocate capacity toward whichever direction is busy, and FDD can't exploit channel reciprocity for massive MIMO the way TDD does. Plenty of networks run both — FDD low bands for reach, TDD mid-band for capacity — and aggregate them.