What is Frequency Division Duplexing (FDD)?

Frequency Division Duplexing (FDD)

Frequency Division Duplexing (FDD) is a method used in communication systems where the transmit and receive functions operate on different carrier frequencies. The assigned frequencies have a constant difference, known as the frequency split, and this split must be large enough to prevent adverse effects between the transmit and receive signals. FDD is utilized in various applications, including Microwave and mm-Wave Links and 4G/LTE networks. Here are some advantages and disadvantages of FDD:

Advantages of FDD

1. Full Data Capacity: The full data capacity is always available in each direction because the send and receive functions are separated.
2. Low Latency: FDD offers very low latency since transmit and receive functions operate simultaneously and continuously.
3. Efficiency in Licensed Bands: It can be used efficiently in licensed and license-exempt bands. Most licensed bands worldwide are based on FDD.
4. Regulatory Protection: Due to regulatory restrictions, FDD radios used in licensed bands are coordinated and protected from interference, although not completely immune.

Disadvantages of FDD

1. Complex Installation: FDD systems are complex to install. Each path requires a pair of frequencies, and if either frequency is unavailable, deploying the system in that band may not be possible.
2. Separate Frequency Allocation: Frequencies are allocated separately for uplink and downlink, leading to spectrum wastage when not in use.
3. Radio Configuration: Radios require pre-configured channel pairs, making sparing complex.
4. Inefficient Spectrum Use: Any traffic allocation other than a 50:50 split between transmit and receive results in inefficient use of one of the two paired frequencies, lowering spectral efficiency.
5. Unavailability in Unpaired Spectrum: FDD cannot be deployed where the spectrum is unpaired.

In summary, while FDD offers advantages such as constant data capacity and low latency, it comes with complexities in installation, spectrum wastage, and limitations in unpaired spectrum deployment.