Satellite & HAPS MAC: TA pre-compensation, HARQ adaptations, extended RACH & scheduling offsets
NTN platforms span from HAPS (20 km) to GEO (35,786 km), creating RTTs from <1 ms to 600+ ms — compared to 1-8 ms for terrestrial. This fundamentally impacts HARQ, TA, scheduling, and RACH.
Select a platform and watch packets travel. See how RTT varies from sub-ms (HAPS) to 600ms (GEO).
| Platform | Altitude | One-Way Delay | RTT | Cell Radius |
|---|---|---|---|---|
| Terrestrial | Ground | 0.5-4 ms | 1-8 ms | 0.1-10 km |
| HAPS | 20-50 km | 0.07-0.17 ms | 0.3-1 ms | 50-200 km |
| LEO | 500-2000 km | 1.7-6.7 ms | 20-40 ms | 50-300 km |
| MEO | 8,000-20,000 km | 27-67 ms | 80-150 ms | 100-500 km |
| GEO | 35,786 km | 119 ms | 500-600 ms | 200-1000 km |
NTN TA = Common TA (K_offset) + UE-Specific TA. UE pre-computes TA using GNSS position and satellite ephemeris (from SIB19) before first UL transmission.
See how TA is computed from satellite altitude and UE position. Adjust altitude and UE offset.
NTN HARQ RTT ranges from 30 ms (LEO) to 600+ ms (GEO) — far exceeding terrestrial 8 ms. Options: extend processes (up to 32), disable HARQ (use RLC ARQ), or use blind retransmissions.
Compare HARQ round-trip times. See how GEO needs HARQ disabled or blind retransmissions.
Up to 32 processes. Keeps pipeline full despite 30-50 ms RTT (LEO). Still limited for GEO.
For GEO: RTT too long for meaningful retx. RLC ARQ handles reliability (slower, but works).
Send N copies without waiting for ACK. Reduces latency vs stop-and-wait. Wastes resources on success.
Scheduling offsets (K_offset) are added to K0/K1/K2 to compensate propagation delay. RACH RAR window extended up to 480 slots. Total RACH time: 50-100 ms (LEO) to 1-3 seconds (GEO).
| Parameter | Terrestrial | LEO | GEO |
|---|---|---|---|
| K_offset (slots) | 0 | 20-40 | 280-560 |
| RAR window (slots) | 10 | 40-80 | 240-480 |
| RACH total time | 10-20 ms | 50-100 ms | 1-3 sec |
| HARQ processes | 16 | 32 | Disabled |
| TA pre-comp | No | Yes (GNSS) | Yes (GNSS) |
| Scenario | Platform | Key MAC Config |
|---|---|---|
| Rural broadband (fixed CPE) | GEO | HARQ disabled, large K_offset, eDRX |
| Maritime / Aviation | LEO constellation | Extended HARQ (32), frequent TA update |
| IoT tracking (battery) | LEO (IoT-NTN) | eDRX (minutes), blind retx |
| Emergency / disaster | HAPS | Near-terrestrial params, standard HARQ |
| Hybrid NTN+TN | LEO + terrestrial | NTN as SCG, TN as MCG anchor |
| Aspect | LTE | 5G NR NTN |
|---|---|---|
| Satellite support | None (proprietary) | Standardized (Rel-17+) |
| TA pre-comp | No | Yes (GNSS + ephemeris) |
| HARQ adaptation | Fixed 8 proc | Up to 32, disable option |
| Regenerative payload | N/A | Rel-18 (gNB on sat) |
10,000+ LEO satellites with THz ISL. Multi-hop space routing. MAC coordinates satellite mesh.
AI predicts satellite positions → pre-schedules resources → predictive HO with no measurement gaps.
Single MAC entity: terrestrial + HAPS + LEO + GEO. Dynamic path selection per packet.
Compare NTN capabilities across generations.
| Dimension | 4G (LTE) | 5G NR NTN | 6G (Target) |
|---|---|---|---|
| Satellite | None | LEO + GEO | Mega-constellation |
| Payload | N/A | Transparent + Regen | Full gNB + ISL |
| HARQ | None | Extended / Disabled | AI-predictive |
| Mobility | N/A | Ephemeris-based | AI-predicted HO |
| Coverage | Ground only | 2D + space | 3D ubiquitous |