FDD (Frequency Division Duplex) is a technique in mobile networks that uses separate frequency bands for uplink and downlink communication; TDD (Time Division Duplex) is a technique that uses the same frequency band for uplink and downlink but only communicates in one direction at a time.

• Both FDD and TDD are two spectrum usage techniques used in mobile communication networks such as Mobile WiMax, LTE, 5G etc. • In FDD mode, both uplink and downlink can transmit at the same time at different spectrum frequencies. • In TDD mode, both uplink and downlink use the same spectrum frequencies but at different times.

Both FDD and TDD are used in 5G and LTE. Each has its advantages and disadvantages. FDD is older than TDD, less flexible, and more expensive. It's faster though, and can transmit both uplink and downlink at the same time, and needs fewer base stations. TDD is mostly used for WiFi and Bluetooth but is being used more and more in general cellular.

Both FDD and TDD are two spectrum usage techniques, both forms of duplex, used in mobile or fixed wireless broadband links. It is essential to these links that transmission can occur in both directions simultaneously so that data can flow downlink (DL) and uplink (UL) at the same time without any noticeable interruptions.