Duplexing is the process of achieving 2-way communication in a channel. Half-duplex is one-way communication at a given time. Full-duplex, on the other hand, is simultaneous communication between the two parties. 5G NR supports both FDD and TDD duplexing schemes.

Frequency Division Duplexing (FDD) is a technique to achieve full-duplex communication by using two different radio frequencies for transmitter and receiver operations. Both the uplink and downlink can transmit at the same time at different spectrum frequencies.

Time Division Duplexing (TDD) is a technique to emulate full-duplex communication over a half-duplex communication link. Both uplink and downlink use the same frequency but the transmission changes by switching in time.

Differences between them and their area of application :

1. FDD is implemented on a paired spectrum where downlink and uplink transmissions are sent on separate frequencies. TDD is implemented on an unpaired spectrum, implying the usage of only one frequency for both downlink and uplink transmissions.
2. In FDD, Diplexer is needed and hence cost is higher. In TDD, Lower cost as no diplexer is needed to isolate the transmitter and receiver.
3. In FDD, lower latency because Tx/Rx transmission at the same time. In TDD, switching between Tx/Rx transmission leads to higher latency.
4. FDD leads to lower spectral efficiency. TDD has single frequency which causes higher spectral frequency.
5. FDD is not suited for very dense environments. TDD can be used in very dense environments.
6. FDD is preferable for lower frequency bands. TDD is preferable for higher frequency bands usually above 10 GHz.
7. In FDD, system transmits on a continuous basis. In TDD, system transmits periodically(discontinuously).
8. In FDD, Downlink and uplink channel responses would not match perfectly due to different frequency bands used in both these directions. In TDD, it matches and hence TDD delivers better performance in MIMO/Beamforming algorithms compare to FDD.