FDD vs TDD Demystified – Exploring 5G Use Cases
Contents
What is FDD and TDD
Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) are methods to separate uplink (UL) and downlink (DL) transmissions so a base station (gNB/eNB) can send and receive simultaneously:
- FDD: UL and DL occupy two fixed, paired frequency bands.
- TDD: UL and DL share one unpaired band but take turns in time‑slots.
How FDD works, Pros/Cons and Use Cases
- Paired Spectrum: A UL band and a DL band are licensed in pairs (for example: 703–748 MHz UL / 758–803 MHz DL).
- Simultaneous UL/DL: FDD ensure real Simultaneous Traffic. Because UL and DL are on different frequencies, both directions can transmit at once.
Pros
- Continuous traffic flow, Low‑Latency UL/DL: No need to wait for a switch‑point, which benefits real‑time services.
- Mature Ecosystem: Decades of deployment; highly optimized RF front‑ends and filters.
- Simpler Interference Control: UL and DL are frequency‑isolated, minimizing self‑interference.
Cons
- Paired Spectrum Requirement: Operators must have two contiguous bands.
- Fixed UL/DL Capacity: UL/DL ratio is locked by spectrum allocation, even if traffic skews heavily DL centric.
- Guard Bands: FDD Requires guard bands to isolate UL/DL, wasting a bit of spectrum.
FDD Use Cases
- Coverage‑Centric Low‑Bands (e.g 700 MHz) for wide area and deep indoor penetration.
- Voice‑Heavy or UL‑Intensive Applications (example: video streaming) where symmetric link performance is key.
How TDD works, Pros/Cons and Use Cases
- Frequency Spectrum: TDD leverage a single, unpaired band is shared.
- Time‑Slots: Frames are divided into slots; some are scheduled for DL, others for UL, with flexible ratios.
Pros:
- Flexible DL/UL Ratio: Dynamically adapt (e.g. 80% DL / 20% UL) to traffic demands.
- Efficient Use of Spectrum: No need for paired bands or guard frequencies.
Cons:
- Switching Overhead between UP/Downlink Timeslot & Latency: Need guard periods between UL and DL slots, this adds tiny dead times and can increase overall latency.
- Inter‑Cell Interference: Adjacent cells must align UL/DL slot patterns or implement advanced interference coordination.
TDD Use case:
- Hotspots & Dense Urban Areas needing flexible, high‑capacity downlink channels.
FDD vs TDD Comparaison
| Feature | FDD | TDD |
|---|---|---|
| Frequency Allocation | Separate frequency bands for uplink and downlink | Same frequency band, different time slots |
| Spectrum Requirement | Paired spectrum | Unpaired spectrum |
| Latency | Generally lower | Potentially higher |
| Asymmetric Traffic Handling | Less flexible | More flexible |
| Simultaneous TX/RX | Yes | No (time‑multiplexed) |
| UL/DL Flexibility | Fixed | Dynamic |
| Latency | Lower (no switching) | Slightly higher (guard periods) |
| Interference control | Easier (frequency separation) | More complex (time synchronization) |
| Ideal Bands | Low‑band coverage (e.g. 600–900 MHz) | Mid‑band capacity (e.g. 2.5–4 GHz) |
| Typical Use Cases | Voice, IoT, rural coverage | eMBB, hotspots, urban high‑speed |
5G Duplexing
5G NR supports both FDD and TDD duplexing.
- FDD uses paired spectrum blocks for simultaneous UL/DL.
- TDD uses a single unpaired block and divides it in time into DL/UL intervals.
Both modes coexist in commercial 5G: low bands (< 1 GHz) often FDD, mid‑bands (sub‑6 GHz) and mmWave predominantly TDD. Each offers distinct benefits, and operators frequently aggregate FDD and TDD carriers to blend coverage and capacity.
Fundamentals: Numerology, Slots & Subframes
- Numerology (µ): Defines 5G subcarrier spacing = 15 kHz × 2^µ.
- Symbol duration (useful): 1/SCS (example: µ=0 → 66.7 µs; µ=1 → 33.3 µs; µ=2 → 16.7 µs).
- Slot: 14 symbols ⇒ slot duration = 14 × (1/SCS) (example: µ=1 → 0.5 ms; µ=2 → 0.25 ms).
- Subframe: Always 1 ms = grouping of slots (µ=1 → 2 slots; µ=2 → 4 slots).
- Frame: 10 ms (10 subframes).
5G Frequency‑Division Duplex (FDD)
Spectrum & Bandwidth
- Bands: Paired (e.g. n71: 617–652 MHz DL + 663–698 MHz UL).
- Carrier size: Up to 100 MHz per CC in FR1 (sub‑7 GHz).
- Aggregate: Up to 16 DL + 16 UL CCs (Rel‑15/16).
Frame Structure & Timing
- UL and DL run simultaneously every subframe/slot.
- No guard periods needed.
- PCell/SCell: PCell carries control channels (PDCCH, PUCCH); SCells are additional CCs for data.
Carrier Aggregation in 5G FDD
- Additive UL/DL: Sum each CC’s paired bandwidth.
- Example:
- CC1: 20 MHz DL + 20 MHz UL
- CC2: 100 MHz DL + 100 MHz UL
→ Aggregate = 120 MHz DL + 120 MHz UL.
5G Time‑Division Duplex (TDD)
Spectrum & Bandwidth
- Bands: Unpaired (e.g. n77: 3300–4200 MHz; n257: 26.5–29.5 GHz).
- Carrier size: Up to 100 MHz in FR1; up to 400 MHz in FR2 per CC.
- Aggregate: Up to 16 CCs.
Frame Structure, Slot Alignment & Guard
- TDD pattern specified per subframe (1 ms): e.g. 3 DL → 1 UL → 1 Guard → repeat.
- Slot mapping:
- All CCs follow the same DL/UL decision at subframe boundaries.
- Within a DL subframe, each CC uses all its slots for DL; in an UL subframe, all slots for UL.
- Guard: Silent subframe or reserved symbols between DL→UL (and UL→DL) to absorb propagation delays & RF switch time.
Carrier Aggregation in 5G TDD
- Common DL/UL timing: The gNB marks subframes DL or UL once; applies to all CCs.
- Numerology diversity: CCs can differ in SCS (e.g. one at µ=1, another at µ=3), yet slot boundaries align at 1 ms.
- Example:
- CC1: 100 MHz @30 kHz (µ=1 → 2 slots/subframe)
- CC2: 200 MHz @60 kHz (µ=2 → 4 slots/subframe)
→ Both switch DL/UL together each 1 ms, using full 300 MHz for the chosen direction.
Read more about 5G Carrier aggregation:
