Mobile Technology 

Except some special type of communication like GPS, most of the communication is operating in bi-directional way. That is, one device transmit as well as receive signal. This kind of bidirectional communication is called 'duplex communication'. There are two ways of implementing the duplex communication. One is FDD (Frequency Division Duplex) and the other one is TDD(Time Division Duplex) as illustrated above.

3G, 4G, 5G all support both FDD and TDD. In 3G, FDD based implementation is called WCDMA and TDD based implementation is called TDSCDMA. In 4G, FDD based implementation is called FDD LTE(or simply LTE) and TDD based implementation is called TDD LTE. In 5G, at least for now we don't have any separate name for FDD and TDD version even though 5G also support both FDD and TDD. In 5G, FDD and TDD is associated with a specific band.