Frequency Division Duplex(FDD) is a frequency division multiplex( FDM) for implementing duplex connections. With this radio technology, the radio channels of the uplink and downlink are separated between the receiving station and the base station. In the FDD method, the uplink( UL) and the downlink( DL) are implemented on two different carrier frequencies. Between the two transmission bands is a security frequency band, the guard zone. During transmission, data can be received and sent simultaneously.
FDD technology is relatively complex, since appropriate filters are required to separate the frequency bands in the upstream and downstream directions, and it is difficult to cover both frequency ranges with some antennas. In addition, the guard band between the two transmission bands must be at least as wide as the transmission bands. As a result, more than 50% of the available frequency spectrum is lost.
FDD technology is used in applications that require symmetrical bandwidth in the uplink and downlink, for example in GSM, UMTS and 802.16, but also in Long Reach Ethernet( LRE) and WiMAX. Since the transmission channels have equal transmission widths, this technology is well suited for voice applications. The situation is different for asymmetric traffic such as Internet traffic, where transmission capacities in the uplink remain unused.
Another technique, time division duplex( TDD), uses time division multiplexing for the two links and uses the same frequencies. A third technique, Hybrid Frequency Division Duplex( HFDD), is a combination of both techniques.