What is FDD?
FDD is a duplexing technique that uses two separate paired frequency bands simultaneously — one exclusively for uplink (UE to gNB) and one for downlink (gNB to UE) — enabling full-duplex operation. FDD is widely used for 5G NR in sub-6 GHz paired spectrum bands and is the traditional duplex mode of LTE.
How Does FDD Work?
The UL and DL bands are separated by a duplex gap large enough to prevent self-interference. Because they operate at different frequencies simultaneously, there is no need for guard time between DL and UL as in TDD. FDD is particularly suited to symmetric traffic patterns and historically allocated paired spectrum (e.g. 700 MHz, 1800 MHz, 2.1 GHz).
Use Cases
5G NR sub-6 GHz deployments on paired bands (n1, n3, n7, n28, n66, n71), 4G/5G coexistence bands, voice-centric deployments requiring symmetric UL/DL.
3GPP / Standards Reference
3GPP TS 38.101-1 (FR1 FDD NR Band Definitions)
Related Terms
TDD | UL | DL | NR | FR1 | Carrier aggregation
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