How does Doppler shift affect a LEO satellite communication link and how do I compensate for it?
LEO Doppler
Doppler rate of change is also important: at 550 km LEO, the Doppler rate can reach 500 Hz/s at high elevation angles. This means the PLL or AFC loop must have sufficient bandwidth to track the rapidly changing frequency. For OFDM systems (5G NR, DVB-S2X): the subcarrier spacing must be much larger than the residual Doppler to avoid inter-carrier interference. 5G NR in satellite: uses wider subcarrier spacing (30-120 kHz) to accommodate the Doppler.
| 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 |
- 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
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Does Doppler affect data rate?
Doppler shift does not directly affect data rate, but uncompensated Doppler causes frequency offset that degrades demodulation. For coherent modulation (QPSK, QAM): the carrier recovery loop must acquire and track the Doppler. For OFDM: residual Doppler causes inter-carrier interference, proportional to Doppler/subcarrier_spacing.
What about differential Doppler?
For large ground-based phased arrays: different elements see slightly different Doppler shifts because the satellite is at a finite distance. This differential Doppler must be compensated per element or per sub-array. For a 10 m array at 28 GHz with a satellite at 550 km at 30° elevation: the differential Doppler across the array is approximately 100 Hz, negligible for most systems.