What is Frequency Division Duplexing (FDD)?

Frequency Division Duplexing (FDD) is a method that is used for establishing a full-duplex communications link by using two different radio frequencies for transmit and receive operations. The transmit and receive frequencies are separated by a defined frequency offset.

In frequency-division duplexing (FDD), two disjoint frequency bands are provided to users for Tx and Rx. In FDD, two different carrier frequencies, one from each band, are assigned to a user; one carrier frequency for transmission from the user (also known as upstream, return link, or uplink) and one carrier frequency for reception by the user (also known as downstream, forward link, or downlink).

The difference between the two assigned frequencies, also known as the frequency split, remains constant, and must be large enough to allow the use of low-cost techniques to separate the two signals; otherwise, the transmit and receive signals may be adversely affected by one another.

FDD is used in a wide range of applications like Microwave and mm-Wave Links and 4G/LTE networks. Click here to see FDD Bands assigned to LTE.

Advantages of FDD

• The full data capacity is always available in each direction because the send and receive functions are separated.
• It offers very low latency since transmit and receive functions operate simultaneously and continuously.
• It can be used efficiently in licensed and license-exempt bands.
• Most licensed bands worldwide are based on FDD.
• Due to regulatory restrictions, FDD radios used in licensed bands are coordinated and protected from interference, though not immune to it.

Disadvantages of FDD

• FDD systems are complex to install. Any given path requires the availability of a pair of frequencies; if either frequency in the pair is unavailable, then it may not be possible to deploy the system in that band.
• Frequencies are allocated separately for uplink and downlink. This leads to wastage of spectrum when it is not used.
• Radios require pre-configured channel pairs, making sparing complex.
• Any traffic allocation other than a 50:50 split between transmit and receive yields. inefficient use of one of the two paired frequencies, lowering spectral efficiency.
• FDD can not be deployed where the spectrum is un-paired.