Frequency Division Duplexing (FDD) is a technique where the transmitter and receiver operate at different carrier frequencies. For instance, in mobile wireless networks, one block of spectrum is allocated for uplink, which carries data from mobile phones to a base station while another is allocated for downlink.
The FDD technique is used in 2G/UMTS/WCDMA/LTE FDD mode within wireless networks where it powers voice and data services, and in fixed networks, it is used in asymmetric digital subscriber line (ADSL) for data services.
Within the 4G LTE wireless network, FDD provides a migration path for the 3G service, and the technology works alongside Time Division Duplex (TDD) which caters for the evolution or upgrade path for TD-SCDMA.