What is TDD?
TDD is a duplexing method where uplink and downlink transmissions share the same frequency band but are separated in time — alternating between DL and UL in a defined pattern. TDD is the primary duplex mode for 5G NR in the C-band (3.5 GHz — the world’s most deployed 5G band) and all mmWave (FR2) spectrum.
How Does TDD Work?
In NR TDD, each slot is configured as DL (Downlink), UL (Uplink), or Flexible (configurable) via the TDD UL-DL configuration signalled by the gNB in system information (SIB1) and via RRC. A typical 5G TDD pattern is DDDSU (3 DL slots, 1 Special slot containing DL + guard + UL symbols, 1 UL slot) on a 5 ms periodicity. The DL/UL ratio can be adapted dynamically to match traffic asymmetry.
Use Cases
All 5G NR C-band deployments (n77, n78, n79), all FR2 mmWave deployments, dynamic capacity adjustment for download-heavy (streaming) vs. upload-heavy (video conferencing) traffic patterns.
3GPP / Standards Reference
3GPP TS 38.213 (TDD UL-DL Configurations for NR), TS 38.101-1
Related Terms
FDD | UL | DL | NR | FR1 | FR2 | Numerology
Learn More
This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
