Frequency division duplex (FDD) is a communication technique where the connected parties can communicate with each other in both directions through use of separate frequency bands for transmitting and receiving.
Since FDD uses different frequency bands for upstream data and downstream data, the sending and the receiving signals do not interfere with each other.
Cellular networks use FDD to separate the channels. One block of the electromagnetic spectrum is allocated for uplink, which carries data from mobile phones to a base station. A different block of the spectrum is allocated to downlink, carrying data from a base station to mobile phones. Each of the blocks are divided into a number of channels.
In advanced mobile phone systems (AMPS), 832 full-duplex channels are used, each comprising of a pair of simplex channels, one for uplink and the other for downlink. The uplink channels are separated from the downlink channels through guard bands.
Asymmetric digital subscriber line (ADSL)
Very-high-bitrate digital subscriber line (VDSL)
IEEE 802.16 WiMax(Worldwide Interoperability for Microwave Access) Frequency Division Duplexing mode