What is the difference between FDD and TDD for satellite communication systems?
Satellite Duplex Scheme Comparison
The choice between FDD and TDD has fundamental implications for satellite system design, spectrum allocation, and ground terminal architecture. While FDD dominates traditional satellite communications, TDD is gaining relevance for LEO constellations and advanced antenna systems.
| Parameter | GEO | MEO | LEO |
|---|---|---|---|
| Altitude | 35,786 km | 2,000-35,786 km | 200-2,000 km |
| Latency (one-way) | ~270 ms | 50-150 ms | 1-20 ms |
| Coverage per Sat | Full hemisphere | Regional | Local footprint |
| Handover | None | Periodic | Frequent |
| Path Loss (Ku-band) | ~206 dB | 190-206 dB | 170-190 dB |
Link Budget Allocation
Standard FDD satellite frequency bands: C-band: uplink 5.925-6.425 GHz, downlink 3.7-4.2 GHz (2.225 GHz guard band). Ku-band: uplink 14.0-14.5 GHz, downlink 10.95-11.7 GHz and 11.7-12.2 GHz (1.8-3.05 GHz guard band). Ka-band: uplink 27.5-30.0 GHz, downlink 17.7-20.2 GHz (7.3 GHz guard band). The large frequency separation enables simple diplexing (waveguide filters with >80 dB isolation between bands) and allows the receive and transmit chains to operate independently without timing coordination. Satellite transponder: simultaneous receive (at uplink frequency) and transmit (at downlink frequency) through separate antenna feeds or shared reflector with orthogonal polarization/frequencies. Ground terminal: simultaneous transmit (BUC at uplink frequency, 1-10 W) and receive (LNB at downlink frequency) through a feedhorn diplexer. No timing synchronization required between uplink and downlink.
Propagation Effects
TDD becomes practical for LEO constellations where the propagation delay is much shorter: one-way delay for a 550 km LEO (Starlink altitude): 1.8 ms at zenith, 5 ms at 25° elevation. Guard time: 2× one-way = 3.6-10 ms. For a 5 ms TDD frame: guard time fraction = 3.6/5 = 72% overhead at zenith, unacceptable. For a 100 ms frame: guard time fraction = 3.6% at zenith, acceptable. Solution: use long TDD frames (50-100 ms) with packet-based scheduling. Alternatively, use FDD with paired frequencies as Starlink and OneWeb currently do. TDD has been proposed for some non-geostationary satellite systems where: the satellite uses a phased array with digital beamforming (channel reciprocity enables uplink beam steering based on downlink channel estimation), the traffic is highly asymmetric (e.g., IoT uplink-heavy traffic), or spectrum is limited to unpaired bands.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Terminal Requirements
Some advanced satellite systems combine FDD and TDD: (1) FDD between ground and satellite (traditional paired bands), TDD within the satellite for inter-satellite links (ISL). SpaceX Starlink uses laser ISLs (no RF duplex needed), but RF ISLs at V-band could use TDD in a single allocated band. (2) TDD for uplink access control: multiple ground terminals share the uplink using TDMA (time division multiple access), which is a form of TDD but with FDD between the aggregate uplink and downlink bands. This is standard in VSAT systems (DVB-RCS2 return channel uses MF-TDMA). (3) Flexible duplex: dynamic allocation of spectrum between uplink and downlink based on instantaneous traffic demand, using the same frequency band. This is TDD at the system level but may operate in FDD mode for individual user terminals (each terminal uses a fixed time slot for uplink and a fixed time slot for downlink within the TDD frame).
Frequently Asked Questions
Why do most satellites use FDD?
Three dominant reasons: (1) Long propagation delay makes TDD extremely inefficient for GEO (480 ms guard time) and moderately inefficient for MEO/LEO (3-30 ms guard time). FDD has zero overhead from propagation delay. (2) Continuous operation: FDD allows the satellite HPA to transmit continuously at maximum power, maximizing spectral efficiency and power utilization. TDD wastes HPA capacity during receive periods. (3) ITU band allocation: satellite frequency allocations are historically defined as paired uplink/downlink bands (C, Ku, Ka), making FDD the natural choice. TDD would require new unpaired allocations or repurposing existing paired bands.
Does Starlink use TDD or FDD?
Starlink uses FDD with Ku-band downlink (10.7-12.7 GHz) and Ka-band uplink (14.0-14.5 GHz). The user terminals use electronically steered phased arrays that simultaneously transmit at Ka-band and receive at Ku-band. The frequency separation (2.5-3.3 GHz) enables effective diplexing without the timing overhead of TDD. For the inter-satellite links: Starlink V2 uses laser ISLs at optical frequencies, avoiding the FDD/TDD question entirely for satellite-to-satellite communication.
Could 5G NR TDD be adapted for satellite?
5G NR defines both FDD and TDD modes. The TDD mode uses short frame structures (0.5-10 ms) designed for terrestrial cells with <1 km range. For satellite NR (defined in 3GPP Release 17 as NTN, Non-Terrestrial Networks): FDD is the primary mode for GEO and MEO. TDD is considered only for LEO with frame extensions to accommodate propagation delay. The timing advance mechanism in 5G NR must be extended from the terrestrial maximum of 2 ms to 20+ ms for LEO and 480 ms for GEO, requiring significant protocol modifications. The 3GPP NTN specifications address these modifications, enabling standard 5G NR modems to connect to satellite base stations with software-only changes.