Time division duplex (TDD) refers to duplex communication where uplink is separated from downlink by the allocation of different time slots in the same frequency band. 

While Frequency Division Duplex (FDD) has been the primary choice for most 2G and 3G networks including GSM and UMTS, 4G LTE and 5G NR networks support both FDD and TDD modes. The Time division duplex (TDD) technique is used in IEEE 802.16 WiMAX, 3G TD-SCDMA and 4G TDD LTE, among others. 

The 4G LTE network is defined to accommodate both paired spectrum for FDD, providing a migration path for the 3G services being used around the globe and unpaired spectrum for Time Division Duplex, TDD LTE, providing the evolution or upgrade path for TD-SCDMA. TDD is typically considered as more spectrum-efficient and better suited for bursty data services.

Time division duplex (TDD) is also a fundamental technology for 5G deployments as its spectral efficiency is bolstered by uplink and downlink signals operating over the same spectrum. The 5G network operating in the mid-range bands around 3.5GHz is based on Time Division Duplex (TDD) mode, while the lower-bands of 5G new radio (including 700MHz, 1.8GHz and 2.1GHz) are based on Frequency Division Duplex (FDD) mode.