Technical Reference
Non-Terrestrial Networks
Deep Technical Reference
Architecture, RF link budgets, 3GPP Rel-17/18 protocol adaptations, waveform design, and mobility management — for researchers and RF engineers who need the details, not the pitch deck.
3GPP Rel-17 / Rel-18
LEO · MEO · GEO
NR-NTN · IoT-NTN
S-band · Ka-band · Ku-band
Regenerative & Bent-pipe
Section 01
NTN Topology & Reference Architecture
An NTN system consists of two fundamental RF links: the Service Link (UE ↔ satellite) and the Feeder Link (satellite ↔ ground gateway). Click any card on the right to build the architecture layer by layer and read the detailed explanation.
Transparent (Bent-pipe) Payload
- Satellite is a pure RF repeater — amplify & frequency-shift only
- Protocol stack (RLC/MAC/PHY) fully at ground gNB
- Inter-satellite latency determined by feeder round-trip
- Simpler design, lower cost; Rel-17 baseline assumption
Regenerative Payload
- On-board demodulation & re-modulation (full baseband)
- gNB or split gNB function can reside on satellite
- Eliminates feeder link latency from UE's perspective
- Enables ISL (Inter-Satellite Link) routing
Service Link (UE ↔ Satellite)
- S-band (2 GHz) for NR-NTN; L-band for IoT-NTN
- Wide-beam → narrow-spot (steerable phased array)
- Direct-to-device (D2D) target: handheld UE class
- EIRP-limited by UE class (Class 3: 23 dBm)
Feeder Link (Satellite ↔ GW)
- Ka-band (20/30 GHz) typical; Ku-band also used
- High G/T gateway, large aperture antenna
- Subject to rain fade — fade margin & ACM essential
- Multiple gateways for geographic redundancy
Section 02
Link Budget & Propagation
NTN link closure demands careful budget accounting across five impairment categories. Unlike TN, elevation angle, satellite altitude, and atmospheric path length dominate the margin.
Free-Space Path Loss
FSPL = 20·log₁₀(d) + 20·log₁₀(f) + 92.45 [dB]
// d in km, f in GHz
d = √( h² + 2·h·R_E·cos²(ε) ) – R_E·sin(ε) + R_E²·sin²(ε)
// ε = elevation angle, R_E = 6371 km
// d in km, f in GHz
d = √( h² + 2·h·R_E·cos²(ε) ) – R_E·sin(ε) + R_E²·sin²(ε)
// ε = elevation angle, R_E = 6371 km
Doppler Shift (worst-case LEO)
Δf = f_c · v_sat / c · cos(ψ) [Hz]
// v_sat ≈ 7.56 km/s @ 600 km
// Max Δf ≈ ±47 kHz @ 2 GHz (S-band)
// Max Δf ≈ ±480 kHz @ 20 GHz (Ka-band)
// v_sat ≈ 7.56 km/s @ 600 km
// Max Δf ≈ ±47 kHz @ 2 GHz (S-band)
// Max Δf ≈ ±480 kHz @ 20 GHz (Ka-band)
| Impairment | Typical value | Orbit |
|---|---|---|
| FSPL | ~163 dB | LEO 600km / 2GHz |
| FSPL | ~209 dB | GEO / 20GHz |
| Atmospheric (O₂, H₂O) | 0.5–3 dB (El>20°) | all |
| Rain fade (Ka-band) | 3–30 dB (climate-dep.) | GEO/Ka |
| Scintillation | 0.2–2 dB (S-band) | LEO |
| Ionospheric delay | 0.5–50 m (frequency-dep.) | all |
| Doppler rate (LEO) | ±350–480 Hz/s @ S-band | LEO |
Rain fade margin: Ka-band feeder links require adaptive coding & modulation (ACM)
with ~10–20 dB fade margin. On-board processing (OBP) isolates the feeder from the service link budget.
Complete NTN Downlink Budget (example: LEO 600 km, NR-NTN, S-band)
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Carrier frequency | f_c | 2.0 | GHz |
| Satellite Tx EIRP | EIRP_sat | 54 | dBW |
| Free-space path loss | FSPL | −163.4 | dB (el=30°) |
| Atmospheric loss | L_atm | −0.7 | dB |
| Polarization loss | L_pol | −0.5 | dB |
| UE receive gain | G_UE | −3 | dBi (omni) |
| UE noise figure | NF | 7 | dB |
| System noise temp (290K) | kTB | −174 | dBm/Hz |
| Channel BW | BW | 10 | MHz → +70 dB·Hz |
| Received C/N₀ | C/N₀ | ≈ 58.4 | dB·Hz |
| Required C/N₀ (QPSK r=1/3) | C/N₀_req | ~52 | dB·Hz |
| Link margin | M | ≈ 6.4 | dB |
Section 03
3GPP Protocol Adaptations TS 38.300 / 38.211 / 38.321
Rel-17 introduced NTN-specific modifications to NR's timing, HARQ, scheduling, and RRC procedures to handle propagation delays up to 600 ms (GEO). Rel-18 extends these for IoT-NTN and mobility enhancements.
Click Next to walk through HARQ timing
| Feature | TN (NR baseline) | NTN Adaptation | 3GPP Spec |
|---|---|---|---|
| Timing Advance (TA) | N_TA ≤ 3846 × 64·Ts (~0.67 ms max) | Extended ≤ 3846 × 64·Ts + N_TA_offset (up to ~680 ms GEO) | TS 38.211 §7.4 |
| HARQ process count | 8 processes (UL/DL) | Up to 16 processes; HARQ may be disabled for NTN | TS 38.321 |
| HARQ RTT | 8 slots (μ=0), ~8 ms | 10–600 ms one-way prop; RTT can exceed 1.2 s GEO | TS 38.300 |
| K1 / K2 offset | 0–15 slots | Extended to 0–1706 slots to cover propagation delay | TS 38.213 |
| UE pre-compensation | Not required | UE pre-compensates TA and Doppler using GNSS + ephemeris | TS 38.331 |
| Random Access (RACH) | Preamble → RAR window: 10–40 ms | RAR window extended; msg3 delayed by propagation | TS 38.321 §5.1 |
| PDCP SN size | 12 or 18 bit | 18-bit preferred to avoid SN wrap-around with large RTT | TS 38.323 |
| RLC AM window | Default (4096 SN) | Larger window needed; configured per link delay | TS 38.322 |
HARQ disabling (Rel-17): With GEO RTTs > 600 ms, running full HARQ chase-combining
is impractical. 3GPP optionally disables HARQ feedback — the MAC layer falls back on
RLC ARQ retransmissions for reliability. gNB configures this per bearer via
pdsch-HARQ-ACK-Codebook IE.
Section 04
Waveform & Signal Design for NTN
NR's CP-OFDM waveform requires modifications to handle the extreme Doppler and delay spread of NTN channels. Subcarrier spacing, CP length, and pre-compensation are the primary design levers.
AUTO
ICI criterion (Doppler vs SCS)
f_D_max << Δf (SCS)
// f_D_max = f_c · v_sat/c [worst case]
// S-band 2GHz, LEO: f_D ≈ 50 kHz
// NR SCS options: 15/30/60/120 kHz
ICI ≈ (f_D / Δf)² × SNR → irreducible floor
// f_D_max = f_c · v_sat/c [worst case]
// S-band 2GHz, LEO: f_D ≈ 50 kHz
// NR SCS options: 15/30/60/120 kHz
ICI ≈ (f_D / Δf)² × SNR → irreducible floor
CP length vs. delay spread
T_CP > τ_max + ΔT_prop
// Normal CP: 4.7 μs (15 kHz SCS)
// NTN: prop. delay variation matters
// within-cell delay var. ~ altitude / c
// Earth-fixed beam: ~1 ms variation
// Satellite-fixed beam: < 0.1 ms
// Normal CP: 4.7 μs (15 kHz SCS)
// NTN: prop. delay variation matters
// within-cell delay var. ~ altitude / c
// Earth-fixed beam: ~1 ms variation
// Satellite-fixed beam: < 0.1 ms
| SCS | Symbol dur. | Max Doppler (ICI<1dB) | Suitable orbit |
|---|---|---|---|
| 15 kHz | 66.7 μs | ~750 Hz | GEO only |
| 30 kHz | 33.3 μs | ~1.5 kHz | MEO/GEO |
| 60 kHz | 16.7 μs | ~3 kHz | LEO (after pre-comp.) |
| 120 kHz | 8.3 μs | ~6 kHz | LEO preferred |
Pre-compensation (UE-side): The UE uses GNSS position and satellite
ephemeris (provided via SIB or broadcast) to estimate and pre-compensate both
Doppler and TA before transmission. Residual error after pre-comp is typically
< 1 ppm, within NR receiver tolerance.
NTN Channel Model Parameters TR 38.811
S-band (2 GHz) — Service Link
- Urban/suburban: K-factor 7–13 dB (LOS dominant)
- Shadowing σ: 1.8–3.4 dB (elevation-dependent)
- Delay spread: ~100 ns (negligible vs CP)
- Building entry loss: 20–25 dB (indoor UE challenge)
Ka-band (20/30 GHz) — Feeder Link
- Molecular absorption: O₂ at 60 GHz, H₂O at 22 GHz
- Rain attenuation: ITU-R P.618 model, up to 30 dB/km
- Scintillation: amplitude/phase variance, Tatarski model
- Tropospheric delay: ~2.5 m zenith, elevation-scaled
Reference Signals & Estimation
- CSI-RS / DMRS density: may need enhancement for fast LEO
- Phase tracking RS (PTRS) critical at Ka-band
- Doppler estimation via pilot interpolation across time
- Channel coherence time: ~1 ms (LEO, 2 GHz)
IoT-NTN (NB-IoT / eMTC)
- Single-tone NPUSCH: carrier spacing 3.75/15 kHz
- Doppler pre-comp mandatory; lower UE complexity
- L-band (~1.6 GHz) primary allocation
- Coverage target: MCL 164 dB (NB-IoT class)
Section 05
Handover & Mobility Management
Satellite motion creates a fundamentally different mobility model from TN. Cells move across the Earth (satellite-fixed beams) or the Earth moves under static coverage areas (Earth-fixed beams). Both require predictive, ephemeris-driven handover.
Click Next to walk through satellite handover
| HO type | Trigger | Latency |
|---|---|---|
| Intra-beam | RSRP drop within same satellite | < 100 ms |
| Inter-beam (same sat.) | Beam boundary crossing | ~50–200 ms |
| Inter-satellite | Satellite elevation < threshold (5–10°) | Predictive, scheduled |
| Satellite–HAPS | Coverage overlap zone | Application dependent |
| NTN → TN | Terrestrial coverage regained | NAS-triggered or A3/A5 event |
Visible pass duration (LEO)
T_pass ≈ (2/ω_E) · arccos(R_E·cos(ε_min) / (R_E+h))
// ε_min = min elevation (e.g. 10°)
// h = 600 km → T_pass ≈ 6–10 min
// Satellite angular velocity ≈ 1°/s
v_beam ≈ v_sat × (R_E / (R_E+h))≈ 6.8 km/s
// Ground beam sweep speed (Earth-fixed)
// ε_min = min elevation (e.g. 10°)
// h = 600 km → T_pass ≈ 6–10 min
// Satellite angular velocity ≈ 1°/s
v_beam ≈ v_sat × (R_E / (R_E+h))≈ 6.8 km/s
// Ground beam sweep speed (Earth-fixed)
Ephemeris-Assisted Handover (Rel-17/18)
Ephemeris Broadcast
- SIB19 (Rel-17): orbital parameters or state vectors
- Update rate: typically every 1–10 min via RRC
- UE uses GNSS + ephemeris to compute TA, Doppler, beam-ID
- Reduces measurement-based HO overhead significantly
Conditional Handover (CHO)
- HO prepared ahead of time using predicted trajectory
- Execution triggered when satellite reaches threshold elevation
- Eliminates HO failure during satellite switching
- Key for LEO constellations (6–10 min pass durations)
Beam Management (BM)
- P1/P2/P3 procedures adapted for fast beam sweeping
- CSI-RS beam-sweep at satellite scan rate (<1 ms dwell)
- Beam-failure detection (BFD): threshold on L1-RSRP
- BFR (Beam Failure Recovery): expedited RA procedure
Timing & Context Preservation
- UE AS context retained across inter-satellite HO (Rel-18)
- RRC_INACTIVE state preserved: avoids full reconnection
- X2/Xn interface: used for context transfer between gNBs
- Key challenge: ISL latency < UE HO preparation time