Open-Source 5G NR Non-Terrestrial Network (NTN) Testbed

A reproducible, fully open-source testbed for 5G NR Non-Terrestrial Networks, demonstrating a complete end-to-end GEO satellite attach (from cell sync through to an established data session), using only open-source components and no SDR hardware.

Built on OpenAirInterface (OAI) and Open5GS, with a GEO satellite link emulated in OAI's RFsimulator. Every dependency is pinned by commit; every configuration change and design decision is recorded.

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What works today

A full 3GPP Release-17 GEO NTN attach, verified end to end against a live 5G core over an emulated 238.74 ms one-way (~477 ms round-trip) satellite link:

cell sync -> SIB19 acquisition -> 4-step RACH -> RRC Setup -> NAS registration -> PDU session establishment -> IP connectivity

Component	Implementation
UE	OAI nr-uesoftmodem (GNSS-aware, computes NTN timing advance)
gNB	OAI nr-softmodem (CU+DU; broadcasts SIB19, K-offset 478)
RF	OAI RFsimulator emulating the GEO propagation channel (no SDR)
Core	Open5GS 5G SA, containerised (Docker Compose)
NTN features	SIB19 ephemeris, K-offset scheduling, ephemeris-based timing advance, extended RRC timers, HARQ feedback disabled (Rel-17)

Verified results

Random access -> registration (UE side):

PDU session + data radio bearer (gNB <-> Open5GS):

The complete 4-step contention-based RACH succeeds over the GEO round trip, the UE registers with the core, and a PDU session with a dedicated DRB is established.

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Reproduce it

Tested on Ubuntu 22.04. Requires the pinned OAI and Open5GS versions. See external/VERSIONS.md.

1. Bring up the 5G core (Open5GS via Docker Compose). See lab-notes.md Phase 1.
2. Build OAI at the pinned commit (tag 2026.w16) and apply the three patches in patches/oai-2026.w16/.
3. Run gNB and UE under real-time scheduling (propagation delay is set in the conf files):

# gNB
sudo chrt -f 80 env RFSIMULATOR=server ./ran_build/build/nr-softmodem \
  -O <repo>/configs/oai-gnb/gnb.sa.band254.u0.25prb.rfsim.ntn.conf \
  --rfsim --gNBs.[0].min_rxtxtime 6

# UE (after gNB shows "Running as server ...")
sudo chrt -f 80 env RFSIMULATOR=127.0.0.1 ./ran_build/build/nr-uesoftmodem \
  -O <repo>/configs/oai-ue/ue.conf \
  --band 254 -C 2488400000 --CO -873500000 \
  -r 25 --numerology 0 --ssb 60 --rfsim

Configurations are tracked in configs/, kept separate from the upstream OAI clone so every change is visible.

📄 A complete step-by-step deployment guide, covering core, gNB and UE with every command and a validation checklist, is available in docs/.

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Known limitations

• RFsimulator only. No real radio / SDR.
• Wall-clock latency does not reflect GEO delay. The GEO propagation delay is modelled correctly in the sample/timing domain, which is precisely why it drives the NTN protocol behaviour (SIB19 timing, K-offset, the RAR-window fix above). However, OAI's RFsimulator is not real-time: it processes samples as fast as the CPU allows, so a ping over the link returns in tens of milliseconds rather than the ~477 ms a physical GEO round trip would take. The delay is real where it matters for protocol correctness; it is not a wall-clock latency emulator. This is a known property of the RFsimulator, not a configuration issue.
• Single host, no scale/performance testing, no certification or regulatory work.

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Roadmap

Phases 1, 2, and 3 are complete. Later phases are planned and will be added incrementally; see PLAN.md for full detail.

Phase	Description	Status
1	5G Core in containers (Open5GS)	Complete
2	gNB + UE, GEO NTN end-to-end attach	Complete
3	Reference architecture document	Complete
4	SMS over NAS (SMSoNAS) over NTN	Planned
5	D2D transport comparison (IP / Non-IP / SMSoNAS)	Planned
6	Voice-over-NTN delay-budget analysis	Planned
7	NTN-terrestrial roaming architecture	Planned
8	Cloud-native deployment (K3s)	Planned
9-10	LEO primer, AI-RAN sketch (optional)	Planned

A LEO testbed is planned as a separate repository.

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Author

Sabbir Ahmed, Doctoral Researcher, 6G Flagship, University of Oulu, Finland.

Previously at the 5G Test Network (5GTN) Oulu, where I led 6G-XR deliverable D1.1 ("Requirements and Use Case Specifications", Horizon Europe SNS-JU, 2023) and worked on 5G/B5G testbed deployment with open-source stacks (OpenAirInterface, srsRAN, Open5GS, free5GC, UERANSIM) and commercial Nokia RAN.

• Email: sabbiraw.ahmed@gmail.com
• GitHub: https://github.com/sabbir-uoulu

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Licensing

• Documentation, diagrams, configurations: CC-BY-4.0. See LICENSE
• Code, scripts, automation: MIT. See LICENSE-CODE

Attribution

This work integrates and depends upon:

• OpenAirInterface 5G (OAI) (https://gitlab.eurecom.fr/oai/openairinterface5g), GNU OAI Public License (EURECOM
• Open5GS), https://open5gs.org/ (AGPL-3.0), Sukchan Lee and contributors
• docker_open5gs (https://github.com/herlesupreeth/docker_open5gs), BSD (Supreeth Herle
• OAI NTN GEO config), https://github.com/ngkore/OAI-5G-NR-NTN, community contributions, ngkore

Pinned versions and commits in external/VERSIONS.md.

Citation

If you use this work, please cite it via its DOI:

Ahmed, S. (2026). Open-Source 5G NR Non-Terrestrial Network (NTN) Testbed (v1.0.0). Zenodo. https://doi.org/10.5281/zenodo.20588732

A CITATION.cff is included in the repository; GitHub shows a "Cite this repository" button generated from it.