TDD vs FDD
FDD (Frequency Division Duplex) uses paired spectrum: uplink and downlink each get their own band, so a device can transmit and receive at the same time. TDD (Time Division Duplex) uses a single band that uplink and downlink take turns on, switching between directions slot by slot.
That one difference drives almost everything else. FDD needs two matched blocks of spectrum, which is easy to find at low frequencies but scarce higher up. TDD only needs one block, so it fits the wide contiguous channels available in mid-band — and it lets you change the downlink-to-uplink split to match traffic, which is heavily downlink-skewed on most networks. The trade-offs land on latency (TDD adds a switching gap), coverage (FDD gives the uplink a continuous channel), and antenna design (TDD's channel reciprocity makes Massive MIMO beamforming far easier).
| Aspect | TDD | FDD |
|---|---|---|
| Spectrum | Unpaired — one band shared by UL and DL. | Paired — separate UL and DL bands with a fixed duplex gap. |
| UL/DL separation | In time: slots alternate between uplink and downlink. | In frequency: uplink and downlink run on different carriers, simultaneously. |
| DL/UL ratio | Configurable — e.g. 4:1 or 7:3 DL-heavy patterns, set per cell. | Effectively fixed near 50/50 by the paired allocation. |
| Spectral efficiency | High in DL-heavy traffic; some loss to guard periods. | No switching overhead, but symmetric split wastes UL when DL dominates. |
| Latency | A switching/guard period sits between DL and UL; the pattern adds scheduling delay. | No turnaround gap — UL and DL are always available. |
| Coverage / UL link budget | Weaker — the UE only transmits part of the time, limiting average UL power. | Stronger — the UE can transmit continuously, helping cell-edge uplink. |
| Channel reciprocity | Yes — UL and DL share the band, so UL soundings estimate the DL channel. | No — UL and DL are on different frequencies, so the channel differs. |
| Massive MIMO fit | Excellent — reciprocity gives DL beamforming weights without heavy CSI feedback. | Harder — relies on codebook-based CSI feedback from the UE. |
| Interference | Needs network sync — unsynced neighbours cause DL-to-UL cross-link interference. | Lower risk — fixed UL/DL bands keep the two directions apart. |
| Typical bands | NR n78 (3.5 GHz); LTE B40/B41. Most 5G mid-band. | NR n1 (2.1 GHz), n3 (1.8 GHz); LTE B1/B3. Most low/mid FDD. |
| Use cases | 5G mid-band capacity, Massive MIMO, DL-heavy mobile broadband. | Low-band wide-area coverage, voice, latency-sensitive symmetric links. |
Why most 5G mid-band is TDD
The capacity story for 5G lives in mid-band — roughly 2.5 to 4.2 GHz, with n78 around 3.5 GHz being the workhorse globally. Two things push that spectrum towards TDD.
First, you can't easily find paired blocks up there. FDD needs two matched bands with a duplex gap between them; mid-band allocations are large single chunks (often 80–100 MHz per operator), which suit one shared TDD carrier.
Second, mobile traffic is lopsided — far more downlink than uplink. TDD lets the operator pick a DL-heavy pattern (a 4:1 split is common) and even retune it as demand shifts, so the spectrum tracks real usage instead of sitting half-idle on the uplink the way a symmetric FDD pair would.
TDD's reciprocity advantage for Massive MIMO
Massive MIMO needs the gNB to know the channel to each user before it can steer narrow beams. In FDD the uplink and downlink sit on different frequencies, so the channel a base station measures on the uplink doesn't match the downlink — the UE has to report channel state (CSI) from a codebook, which is coarse and costs airtime, and gets expensive as antenna counts climb.
TDD shares one band, so the channel is the same in both directions within the coherence time. The gNB estimates the downlink channel from uplink sounding (SRS) and computes beamforming weights directly. That channel reciprocity is the main reason large antenna arrays — 32T32R, 64T64R — are deployed almost entirely on TDD mid-band.FDD's coverage and latency strengths
FDD keeps two advantages that matter at the edge of the network. Coverage is the big one: a UE on FDD can transmit continuously, so it puts more average power into the uplink — usually the limiting link for cell range. A TDD UE only transmits in its uplink slots, which caps average uplink power and shrinks the cell. That's a large part of why low-band coverage layers stay FDD.
Latency is the other. FDD has no DL-to-UL turnaround, so there's no guard period and no waiting for an uplink slot to come round. TDD adds a switching gap and some scheduling delay tied to its slot pattern, though short NR slots and DL-heavy configurations keep it modest in practice.
The bottom line
There's no single winner — they sit in different parts of the band plan. TDD owns 5G mid-band: it fits the wide unpaired channels, lets operators bias capacity toward the downlink, and its channel reciprocity is what makes Massive MIMO practical. FDD still owns low-band, where paired spectrum is plentiful and the continuous uplink gives the coverage and latency that wide-area and voice layers need. A real 5G network runs both — TDD mid-band for capacity, FDD low-band for reach — often tied together with carrier aggregation.
Frequently asked questions
- Is 5G TDD or FDD?
- 5G NR supports both. Mid-band (around 3.5 GHz, like n78) and mmWave are TDD, while most low-band 5G reuses paired FDD spectrum such as n1 (2.1 GHz) and n3 (1.8 GHz). Networks typically combine TDD for capacity and FDD for coverage.
- Why is TDD better for Massive MIMO?
- TDD uses one band for uplink and downlink, so the channel is reciprocal: the gNB estimates the downlink channel from uplink sounding (SRS) and steers beams directly, without heavy CSI feedback. FDD has to rely on codebook-based feedback that scales poorly with large arrays.
- Does TDD need network synchronisation?
- Yes. Neighbouring TDD cells should switch between uplink and downlink at the same time. If they are out of sync, one cell's downlink can interfere with another's uplink (cross-link interference), so operators align the slot pattern and use phase/time sync such as GPS or PTP.
- Which has better coverage, TDD or FDD?
- FDD generally has the better uplink link budget because the device can transmit continuously, giving more average uplink power — and the uplink usually limits cell range. A TDD device only transmits in its uplink slots, so its coverage tends to be tighter at the same frequency.
Related terms
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