---
title: Non-Terrestrial Networks Overview
url: https://www.emergentmind.com/topics/non-terrestrial-networks-ntns
type: topic
---

# Non-Terrestrial Networks Overview

Non-Terrestrial Networks (NTNs) refer to integrated space- and airborne communication infrastructures—primarily satellite constellations (LEO, MEO, GEO), high-altitude platforms (HAPs), and unmanned aerial vehicles (UAVs)—offering radio access and backhaul beyond the traditional terrestrial network, extending wireless coverage, capacity, and resilience worldwide. NTNs are a cornerstone of 5G/6G architecture, providing direct-to-device, relay, and multi-layer connectivity for broadband, IoT/mMTC, mission-critical, and disaster scenarios, especially in rural, remote, and underserved regions [2305.04590][2412.00820][2310.10213][2601.14883][2109.14581].

## 1. Taxonomy and System Architectures

### Platform Types and Orbits

Non-terrestrial platforms span:

- **Geostationary Orbit (GEO):** ≈35,786 km altitude, fixed with respect to Earth, round-trip delay ≈500 ms, very large footprint (radius ≈2,500–3,500 km), negligible Doppler [2305.04590][2109.14581].
- **Medium Earth Orbit (MEO):** 2,000–35,000 km, intermediate delay (RTT ≈ 70–150 ms), moderate Doppler, regional coverage [2305.04590][2109.14581].
- **Low Earth Orbit (LEO):** 300–2,000 km, RTT ≈20–50 ms, high mobility (orbital speed ≈7.5 km/s), significant Doppler (tens of kHz at GHz bands), small and rapidly moving beams [2305.04590][2412.00820].
- **High-Altitude Platforms (HAPs):** ≈20 km altitude, RTT ≈10 ms, stationary/controlled mobility, footprint ≈100–200 km [2305.04590].
- **UAVs:** 0.1–5 km, low latency, limited coverage, extreme flexibility [2305.04590].

Architecture integrates these platforms with terrestrial base stations (TNs) via unified core networks (3GPP NG-Core/5GC/6GC), supporting hybrid access modes (direct, relay, multi-connectivity) and backhaul via feeder/gateway stations [2601.14883][2305.04590].

### Payload Models and Integration

- **Transparent ("bent-pipe") payload:** Simple frequency translation/amplification; all routing at ground stations. Standard in earlier GEO/MEO/LEO deployments [2305.04590][2109.14581].
- **Regenerative payload:** On-board demodulation, routing/switching, ISL support; allows flexible routing, traffic isolation, and lower end-to-end latency [2305.04590][2601.14883].
- **3GPP NR NTN-compliant:** Both direct access and relay are implemented; hybrid functional splits (e.g., DU on satellite, CU on ground) extend terrestrial RAN with satellite/air segment [2601.14883][2109.14581].

NTNs form the upper layers of the "Space–Air–Ground Integrated Network" (SAGIN), where all segments share resources and control [2403.07763].

## 2. Physical Layer and Propagation: Key Characteristics

### Channel Impairments

- **Free-space path loss (FSPL):** For distance \(d\) (m) and carrier frequency \(f\) (Hz),
  \[
  L_{\mathrm{fs}} = 20\log_{10}(4\pi d f/c)
  \]
  Large-scale, elevation- and frequency-dependent [2305.04590][2601.14883].
- **Doppler shift:** LEO satellites at 2 GHz can induce \(f_d ≈ 50\) kHz, requiring Doppler estimation and compensation, particularly for OFDM [2310.10213][2601.14883].
- **Propagation delay:** \(τ = d/c\). GEO: \(τ ≈ 120\) ms one-way; LEO: \(τ ≈ 2–4\) ms [2310.10213].
- **Beam dynamics:** Fast-moving spot beams in LEO require frequent handovers (every few minutes) and fine-grained timing/synchronization.

### Channel Modeling

- **Large-scale fading:** Dominant LoS in HAPs/LEO/MEO, Ricean or Shadowed-Rician fading [2302.03376].
- **Coherence time:** Rapid satellite movement (LEO) yields short coherence time (\(T_c \sim 0.1–10\) μs at mmWave); LOS fraction crucial [2405.07005].
- **Atmospheric impairments:** Rain fade, gaseous absorption, and scintillation are significant at mmWave/FSO; ITU models standard [2305.04590][2601.14883].
- **Beamforming:** Active phased arrays on satellites/HAPs enable flexible spot-beam formation and tracking [2412.00820].

## 3. Radio Resource, Access, and Network Slicing

### Multi-Connectivity and Scheduling

- **Multi-connectivity (MC):** PDCP-layer MC (MR-DC) allows UE to connect to multiple nodes (satellite & terrestrial BSs, or multiple satellites). MC in NTN faces new challenges: delay asymmetry, Doppler, out-of-order delivery, frequent beam/sat handovers, and buffer sizing [2310.10213].
- **Resource allocation:** Critical parameters include dynamic bandwidth fission (\(\varepsilon\)), power adaptation, BS activation, and association, with optimization frameworks (BLASTER, log-sum utility, proportional-fair) balancing TN/NTN capacity, energy, and fairness [2405.14053][2310.02002][2601.19078].
- **Network slicing:** SDN/NFV-enabled NTN virtualizes resources into eMBB, URLLC, mMTC slices. Slices are isolated at radio, transport, and core. End-to-end constraints include latency, reliability, rate, and power [2403.07763][2412.00820].

### MAC and HARQ Adaptations

- **Random Access (RA):** Window and timer extensions (add 2× propagation delay). Pre-compensation needed due to timing uncertainty [2601.14883].
- **HARQ:** Large RTT requires increasing number of HARQ processes (e.g., 32+ for LEO), or switching to RLC-ARQ as fallback, with throughput/latency trade-offs [2601.14883][2412.01570].

### Duplexing and TDD

- **TDD in NTN:** Guard period set by maximum cell one-way delay (e.g., LEO at 800 km: >5 ms GP). Enhanced slot allocation (ESSA) methods fill idle guard periods for higher channel utilization [2412.01570].

## 4. Mobility Management, Handover, and Orchestration

- **Mobility:** LEO/MEO satellites produce frequent (minutes-scale) handovers. Optimized algorithms use predictive handover via GNSS/ephemeris, RA-less SN addition, elevation/location-based candidate selection, connected-mode pre-establishment [2310.10213][2412.00820].
- **Handover decision metrics:** Composite utility functions (\(U = α \log(1+\mathrm{SINR}) − β \mathrm{latency} + γ T_\mathrm{stay}\)) account for channel, latency, and dwell time in satellite cell [2412.00820].
- **Inter-satellite links (ISLs):** Regenerative payloads and ISLs enable mesh networking, optimized routing (e.g., time-expanded graphs) for latency reduction [2412.00820].

### Control and Artificial Intelligence

- **AI/ML for NTN:** RL/DNN-based traffic steering, beam scheduling, dynamic slicing, and federated learning for control plane optimization: essential for complex, dynamic topologies and shadowing [2403.07763][2207.11028][2305.10273].

## 5. System-Level Performance and Optimization

### Analytical and Simulation Results

- **Throughput–coverage–fairness trade-offs:** Integration of NTN boosts mean data rates by >200% in rural scenarios, reduces coverage holes by >90%, and enhances energy efficiency by up to 45% [2310.02002][2405.14053][2601.19078].
- **Stochastic geometry models:** System-level performance (coverage, capacity, association probabilities) is rigorously analyzed using spherical PPP, BPP, Cox process models, with closed-form metrics for coverage probability, SINR, and beam clustering [2503.13455][2302.03376].
- **Slot usage:** ESSA in TDD can increase channel utilization by 3–10× over naïve schemes, especially with delay-based user grouping to minimize differential delays [2412.01570].
- **IoT over NTN:** Hybrid offloading (probabilistic association to gateway vs. satellite) significantly improves success probability in mMTC [2212.01614].

### Table: Sample LEO/GEO Performance Metrics (S- and Ka-band, single-sat, 3GPP TR 38.821 calibration) [2601.04882]

| Orbit       | Band  | Max Throughput | One-way Delay | Peak SNR  | Area Capacity Density |
|-------------|-------|---------------|---------------|-----------|----------------------|
| LEO 600 km  | Ka    | 302.7 Mbps    | 4–8 ms        | 8.5 dB    | 170 kbps/km²         |
| GEO 35,786  | Ka    | 469.9 Mbps    | 120 ms        | 11.6 dB   | 1.4 kbps/km²         |
| LEO 600 km  | S     | 76.4 Mbps     | 4–8 ms        | 6.6 dB    | —                    |
| GEO 35,786  | S     | 21.7 Mbps     | 120 ms        | 0 dB      | —                    |

Key observations: LEO offers order-of-magnitude latency advantage and higher area capacity density; GEO saturates at higher absolute throughput due to larger BW/EIRP but performance is challenged by path loss and delay [2601.04882][2511.01484].

## 6. Open Challenges and Future Research Trajectories

- **HARQ/ARQ protocol design:** Large RTT/propagation delays stretch conventional mechanisms. New schemes and extended timers (buffering, window, redundancy).
- **Reliance on GNSS:** Satellite/UE synchronization and Doppler pre-compensation depend on GNSS, which is vulnerable to spoofing/jamming. GNSS-resilient operations and alternative timing protocols are required [2601.04882].
- **Interference and coexistence:** Multi-layer (GEO/LEO/HAP/UAV/TN) spectrum sharing, rate-splitting, and cognitive access, dynamic interference management across systems [2412.00820].
- **Security and privacy in network slicing:** Lightweight isolation and authentication mechanisms per slice topologies, especially for dynamic SD-NTN [2412.00820].
- **Cross-layer and distributed AI:** Federated/distributed ML for resource control, dynamic switching among centralized, decentralized, and federated learning according to topology, latency, privacy, and compute constraints [2207.11028][2305.10273][2403.07763].
- **E2E orchestration:** Digital-twin-based network control, on-board edge processing, O-RAN adoption for flexible NTN orchestration [2305.10273][2403.07763].
- **Next-gen physical-layer:** Design of THz/optical ISLs, RIS/IRS for reconfigurable coverage, adaptive waveforms robust against ultra-high Doppler and delay [2109.14581][2511.01484].

## 7. Standardization and Practical Deployment

- **3GPP NR NTN (Rel-17–Rel-20):** Ongoing inclusion of satellite direct access, regenerative payloads, enhanced frequency bands (L/S/Ka), GNSS-resiliency, advanced mobility & handover, RedCap/IoT extensions [2601.14883][2601.04882][2310.10213].
- **Interoperability and scaling:** Focus on practical issues (gateway placement, beam/user density, feeder/gateway link variability, dual connectivity form-factors, uplink power constraints) and the massive scaling expected in NTNs (e.g., >10,000 LEO satellites) [2305.04590][2601.14883].
- **Industry/field validation:** LEO and GEO constellations (Starlink, OneWeb, HughesNet) have demonstrated >100 Mbps/user, LEO round-trip delays <40 ms, with full 3GPP NR integration and open-source evaluation suites (e.g., ns3-NTN) available for E2E validation [2601.04882][2107.06881].

NTNs, as a fundamental 6G pillar, are rapidly advancing toward seamless, software-defined, AI-orchestrated, energy-efficient, and globally available space–air–ground communication fabrics [2412.00820][2403.07763][2310.10213][2601.14883][2503.13455].

Source: https://www.emergentmind.com/topics/non-terrestrial-networks-ntns