Getting Started with NTN (Non-Terrestrial Networks) — Beginner's Guide

What is NTN?

Non-Terrestrial Networks (NTN) is 3GPP's framework for integrating satellites, HAPS (High-Altitude Platform Stations), and UAV platforms into the 5G architecture. The goal: extend 5G coverage to places terrestrial towers can't reach — oceans, deserts, polar regions, disaster zones, aircraft, and rural areas.

Before NTN, satellite communication and cellular were completely separate worlds with incompatible standards. NTN bridges them by making satellites act as 5G base stations (or relays), so a standard 5G UE can connect to a satellite using the same NR air interface it uses on the ground.

Why NTN Matters

Coverage: ~90% of Earth's surface has no terrestrial cellular coverage
Resilience: Satellite backhaul survives natural disasters that destroy ground infrastructure
IoT at scale: Sensors in remote agriculture, shipping containers, wildlife tracking — all need connectivity without nearby towers
Direct-to-device: Modern smartphones (starting with Rel-17) can connect directly to satellites without special hardware

NTN Evolution at a Glance

Release	Phase	What Happened
Rel-15	Study	TR 38.811 (channel models), TR 38.821 (protocol solutions) — pure research
Rel-16	IoT Study	TR 36.763 — extending NB-IoT/eMTC over satellite
Rel-17	First Deployable Spec	Normative NR NTN in TS 38.300/331/321 — transparent payload, LEO/GEO, bands n255/n256
Rel-18	Enhancements	FR2-NTN (Ka-band), RACH-less handover, discontinuous coverage, 30 MHz BW
Rel-19	Advanced	Regenerative payload, inter-satellite links, store-and-forward, NTN-TN mobility
Rel-20	6G NTN Vision	Harmonized TN/NTN design, VLEO, up to 800 MHz BW

The Big Picture — Key NTN Concepts You Must Understand

1. Satellite Types and Orbits

Everything in NTN design flows from how far away the satellite is. Orbit altitude determines latency, Doppler shift, visibility window, and cell size.

Orbit	Altitude	One-Way Delay	Doppler (S-band)	Visibility
VLEO	300-600 km	1-5 ms	~24 kHz	5-12 min
LEO	600-2000 km	2-9 ms	~15 kHz	10-20 min
MEO	2000-35000 km	33-54 ms	~1 kHz	2-6 hr
GEO	~35786 km	119-139 ms	~0 Hz	Continuous

Start here: Understand that LEO = low latency but fast-moving satellites (frequent handovers), GEO = high latency but stationary (simple mobility).

2. Transparent vs Regenerative Payload

This is the most fundamental architecture decision in NTN:

Transparent Payload (Rel-17 baseline):

Satellite is a "bent pipe" — just amplifies and forwards the radio signal
The gNB sits on the ground behind the NTN Gateway
Total RTT = service link + feeder link (both ways)
Simple satellite hardware, but higher latency

Regenerative Payload (Rel-19):

Satellite hosts the full gNB (or gNB-DU) on board
Terminates the NR air interface in space
RTT = service link only (much lower latency)
Enables inter-satellite links (ISL) for mesh routing
Requires significant on-board processing power

3. The Three Cell Types

Cell Type	Satellite	Behavior	Mobility Impact
Earth-fixed	GEO	Cell footprint is permanent on the ground	Simplest — cells don't move
Quasi-earth-fixed	NGSO (steerable beams)	Beams steered to keep covering same area	Beam handover between consecutive satellites
Earth-moving	NGSO (fixed beams)	Cell footprint slides with the satellite	Frequent handovers, multiple TACs per cell

4. Timing and Frequency — The Hardest Part

This is where NTN gets genuinely complex. In terrestrial NR, propagation delay is <1 ms. In NTN:

LEO RTT: ~25 ms (transparent) — manageable
GEO RTT: ~541 ms (transparent) — HARQ completely breaks at normal operation

The UE must:

Have a GNSS fix before transmitting (mandatory — no GNSS = no transmit)
Know satellite ephemeris (broadcast in SIB19, SIB31, SIB32)
Pre-compensate Timing Advance autonomously using GNSS position + ephemeris
Pre-compensate Doppler on uplink (up to 24 kHz shift at S-band for LEO)

Key timing parameters:

Common TA: Network-broadcast baseline timing advance for all UEs in the cell
K_offset: Scheduling offset to accommodate service link RTT (extends DCI-to-PUSCH, PDSCH-to-HARQ-ACK timing)
K_MAC: Additional offset for feeder link delay

5. HARQ Adaptations

Normal NR HARQ (8 processes, ~8 ms RTT assumption) completely stalls with 25+ ms RTT. NTN solutions:

Up to 32 HARQ processes (vs 16 in terrestrial NR)
HARQ feedback can be disabled entirely — falling back to RLC ARQ for retransmissions
Network chooses per-process: feedback enabled or disabled

6. Mobility in NTN

NTN mobility is more complex than terrestrial because cells move:

Intra-satellite beam handover: UE moves between beams of the same satellite
Inter-satellite handover: Satellite flies away, next satellite takes over
Conditional Handover (CHO): Pre-configured with event A4, time-based, or location/distance-based triggers (CondEventD2)
RACH-less handover (Rel-18): Reduces handover interruption
Satellite switch with re-sync: For quasi-earth-fixed cells, avoids full L3 handover
NTN-to-TN and TN-to-NTN handover: Seamless transition between satellite and ground

7. NTN Frequency Bands

Band	Name	Frequencies	Use Case
n255	L-band	UL 1626.5-1660.5 / DL 1525-1559 MHz	IoT, narrowband, maritime
n256	S-band	UL 1980-2010 / DL 2170-2200 MHz	Direct-to-device, broadband
n254	S-band MSS	UL 1610-1618 / DL 2483-2500 MHz	MSS services
Ka-band	-	UL 27.5-30 / DL 17.7-20.2 GHz	High-throughput, feeder links
Ku-band	-	UL 14-14.5 / DL 10.7-12.75 GHz	VSAT, maritime broadband

Reading Plan — Specs You Need to Master NTN

Phase 1: Foundations (Start Here)

If I were to start fresh, I'd Read these first. They give you the "why" and the big picture before you dive into normative detail.

Priority	Spec	Title	What You Learn	Key Clauses
1	TR 38.821	Solutions for NR to support NTN	The original study — architecture options, protocol adaptations, link budgets, all the design trade-offs	Full document
2	TR 38.811	Study on NR to support NTN	Channel models, propagation characteristics, deployment scenarios	Clauses 4-6
3	TR 21.917	Release 17 Description	High-level summary of what Rel-17 NTN delivers	Clauses 5.1.1, 5.1.2, 5.2
4	TR 38.822	NR NTN UE Feature List	What UE capabilities are defined for NTN	Full document

Phase 2: Core Normative Specs (The Meat)

These are the specs that define how NTN actually works. Read them in this order.

Priority	Spec	Title	What You Learn	Key Clauses
5	TS 38.300	NR and NG-RAN Overall Description	The NTN bible — architecture, timing, mobility, HARQ, all NTN procedures	Clause 16.14 (entire section)
6	TS 38.331	NR RRC Protocol	RRC signalling for NTN: SIB19 (ephemeris), SIB31/32, handover commands, measurement config	Clause 5.2.2 (SIBs), clause 5.3.5 (mobility), clause 5.5 (measurements)
7	TS 38.321	NR MAC Protocol	HARQ adaptations, K_offset MAC CE, scheduling offsets, RACH for NTN	Clause 5.2 (TA maintenance), clause 5.18.24 (Differential K_offset)
8	TS 38.213	Physical Layer Procedures for Control	Timing advance adjustments, HARQ-ACK timing, DCI-to-PUSCH timing with K_offset	Clause 4.2 (timing adjustments)
9	TS 38.304	UE Procedures in Idle/Inactive	Cell selection, reselection, PLMN selection for NTN cells	Clause 5 (cell selection/reselection)

Phase 3: RF and Performance

Priority	Spec	Title	What You Learn	Key Clauses
10	TS 38.101-5	UE Radio Tx and Rx — Satellite Access	NTN UE RF requirements, bands n255/n256/n254	Clauses 5-7
11	TS 38.108	Satellite Access Node Radio Tx and Rx	SAN (satellite access node) RF requirements	Full document
12	TS 38.133	RRM Requirements	Measurement timing for NTN, cell identification with SMTC, relaxed requirements	Clause 9 (measurements)

Phase 4: Enhancements (Rel-18 / Rel-19)

Once you've mastered the baseline, read the enhancement specs.

Priority	Spec	Title	What You Learn	Key Clauses
13	TR 21.918	Release 18 Description	Rel-18 NTN enhancements summary	Clauses 5.2, 5.3
14	TS 38.300 (Rel-18/19 updates)	NR Overall Description	FR2-NTN, regenerative payload, ISL, discontinuous coverage	Clause 16.14 (updated sections)
15	TR 38.863	Study on NTN Enhancements	Rel-18 study items: coverage, mobility, capacity enhancements	Full document

Phase 5: 6G NTN Vision (Forward-Looking)

Priority	Spec	Title	What You Learn	Key Clauses
16	TR 38.914	6G Study on Requirements	6G NTN deployment scenarios, VLEO-GSO, 800 MHz BW, harmonized TN/NTN	Clause 4.10, clause 1
17	TR 22.870	6G Use Cases and Service Requirements	NTN role in ubiquitous connectivity, maritime, aviation, disaster	Clauses 5, 6

Topic Checklist — if you would like to track your NTN learning progress

Use this as a self-assessment.

Architecture and Deployment

Transparent vs regenerative payload architecture
Service link vs feeder link
NTN Gateway role and placement
Orbit types: VLEO, LEO, MEO, GEO — propagation characteristics
Earth-fixed / quasi-earth-fixed / earth-moving cells
NTN frequency bands (n255, n256, Ka-band, Ku-band)
5GC integration (AMF placement, TAC management, Mapped Cell ID)

Timing and Synchronization

GNSS requirement for NTN UEs
Satellite ephemeris formats (state vector, orbital elements)
SIB19, SIB31, SIB32 content and purpose
Common TA, ta-CommonDrift, ta-CommonDriftVariation
UE-specific TA pre-compensation (T_TA)
K_offset — scheduling offset for DL/UL alignment
K_MAC — MAC-level offset for feeder link delay
Doppler pre-compensation (open-loop UE-side)
Timing drift management and continuous TA update

HARQ and MAC

Extended HARQ processes (up to 32)
HARQ feedback disable mode
RLC ARQ fallback when HARQ feedback is disabled
DRX adaptations for NTN (extended timers)
BSR/SR behaviour with long RTT
MAC CE for Differential K_offset

Idle Mode and Camping

PLMN selection with satellite RAT types
Cell selection criteria for NTN
Cell reselection in NTN (NTN-to-NTN, NTN-to-TN)
SIB19 NTN assistance info for idle mode
Extended DRX for NTN idle mode
Paging adaptations

Mobility (Connected Mode)

Intra-satellite beam handover
Inter-satellite handover
Conditional Handover (CHO) — event A4, time-based, CondEventD2
RACH-less handover (Rel-18)
Satellite switch with re-synchronization
NTN-to-TN and TN-to-NTN handover
Feeder link switchover (soft/hard)
SMTC configuration for NTN (multiple SMTCs per carrier)

Measurements

SSB-based measurements with SMTC alignment
Multiple SMTCs per carrier for multi-satellite
Measurement events for NTN (A1-A5, D1, D2)
Measurement gaps and NTN considerations
UE location verification (multi-RTT single satellite)

Rel-18/19 Enhancements

FR2-NTN (Ka-band direct access)
PUCCH repetition and DMRS bundling for coverage
30 MHz bandwidth support
Network-verified UE location
Discontinuous coverage handling
Regenerative payload normative support
Inter-satellite links (ISL)
Store-and-forward for discontinuous coverage

Suggested Study Order for Beginners

Big Picture
  Read TR 38.821 (study item) — understand WHY each NTN adaptation exists
  Read TR 21.917 clause 5 — Rel-17 summary

Architecture & Timing
  Read TS 38.300 clause 16.14.1 (architecture)
  Read TS 38.300 clause 16.14.2 (timing and synchronization)
  Read TS 38.300 clause 16.14.7 (O&M parameters, ephemeris)

Procedures
  Read TS 38.300 clause 16.14.3 (mobility)
  Read TS 38.331 SIB19, handover IEs
  Read TS 38.321 NTN-related MAC procedures

Idle Mode + Measurements
  Read TS 38.304 (cell selection/reselection for NTN)
  Read TS 38.133 clause 9 (RRM measurement requirements)

Enhancements
  Read TR 21.918 clause 5 (Rel-18 NTN)
  Read updated TS 38.300 clause 16.14 sections for Rel-18/19

RF + Advanced
  Read TS 38.101-5 (UE RF for satellite access)
  Read TR 38.914 clause 4.10 (6G NTN vision)