TDD
Time Division Duplex: a duplex mode where uplink and downlink share the same frequency band but are separated in time, with flexible slot configurations in NR.
TDD shares one frequency band between uplink and downlink by splitting them in time, and it's the dominant duplex mode for 5G mid-band — the 3.5 GHz spectrum that carries most sub-6 capacity is TDD. The reason it fits 5G so well is twofold: unpaired spectrum is more available at those frequencies, and TDD's channel reciprocity (uplink and downlink on the same frequency) lets massive MIMO derive downlink beamforming from uplink SRS sounding.
The lever an operator tunes is the slot pattern — the ratio of downlink to uplink slots. Most consumer traffic is download-heavy, so patterns are typically skewed toward downlink (a 4:1 DL:UL ratio is common). That asymmetry is a feature, but it has costs: uplink coverage and latency suffer when uplink opportunities are sparse, and there's a guard period at the DL-to-UL switch to let signals clear. A real gotcha is interference between operators or cells using different TDD patterns on the same band — DL from one cell can hit UL reception of another — which is why TDD networks in a region usually need synchronised, aligned configurations.
Frequently asked questions
- What's the difference between TDD and FDD?
- FDD uses two separate frequency bands — one for uplink, one for downlink — so the device transmits and receives at the same time on different frequencies. TDD uses a single band and alternates between uplink and downlink in time. FDD suits paired spectrum and gives symmetric, low-latency links; TDD suits the unpaired mid-band spectrum where most 5G capacity lives and allows a flexible, usually downlink-heavy, split.
- Why does most 5G use TDD instead of FDD?
- The mid-band spectrum that carries the bulk of 5G capacity (around 3.5 GHz) is largely unpaired, which suits TDD. TDD also lets operators skew the uplink/downlink ratio toward downlink to match real traffic, and its channel reciprocity makes massive MIMO beamforming far more practical by letting the base station learn the downlink channel from uplink sounding. FDD is still used, mainly on the lower paired bands inherited from LTE.
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