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 there is a safety frequency band, the guard zone. During transmission, data can be received and transmitted 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.WiMAX. Since the transmission channels have equal transmission widths, this technique 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 timedivision multiplexing for the two links and uses the same frequencies. A third technique, Hybrid Frequency Division Dup lex (HFDD), is a combination of both techniques.