Long Term Evolution Time Division Duplex (LTE TDD) is the version of LTE that uses the TDD modulation scheme in unpaired spectrum, whereby the same frequency channel is used for both downlink and uplink communication and signals are timed to enable smooth delivery of data. In contrast, LTE uses frequency division duplex (FDD) in paired spectrum, with a separate channel for uplinks and downloads.
FDD spectrum has traditionally been used for cellular voice and data services. TDD-based services are typically considered as more spectrum-efficient and better suited for "bursty" data services.
The most popular frequencies for which LTE TDD is being considered are 2.3GHz and 2.5GHz. The two most significant markets for LTE TDD developments are China and India. China Mobile is the chief proponent of LTE TDD.