Questions Regarding NTN Integration, EPC Connectivity, and Channel Modeling in 5G-LENA

[Nikhil Gowda]

Dear 5g-Lena community developers and users,

I hope you are doing well.

I am currently working on an NTN-based simulation scenario using 5G-LENA(I am completely new to this ) where satellite nodes are propagating around Earth using an SGP4 mobility model. While implementing the setup, I encountered a few architectural and modeling questions for which I would greatly appreciate your guidance.

1. Connectivity of Non-gNB Nodes to EPC (SGW/PGW/MME) and IP Traffic Generation

From the documentation and examples available in 5G-LENA (NR module), it appears that all gNB nodes are directly connected to the EPC entities (SGW/PGW and MME).

My question is:

• Is it possible to connect a non-gNB node directly to the EPC (SGW/PGW/MME) and transmit IP traffic (e.g., UDP) through it?

• If yes, what would be the correct architectural approach to achieve this?

• Would this require creating a custom NetDevice or modifying the EPC helper configuration?

• Are there any example scripts or references that demonstrate such a configuration?

2. Dynamic UE Association in NTN Scenario (ConnectToGnb / ConnectToClosestGnb)

In my NTN scenario:

• Satellites (with and without gNB functionality) are moving dynamically using SGP4.

• Visibility and Line-of-Sight (LoS) between UE and gNB change during simulation time.

From what I observe, the ConnectToGnb() and ConnectToClosestGnb() methods seem to perform association at setup time rather than dynamically during simulation.

My concern is:

• In an NTN scenario, the UE should connect to a gNB only when it becomes visible (within LoS) during runtime, not statically before simulation starts.

• Is there built-in support for dynamic cell selection / re-association based on mobility and visibility?

• If not, what would be the recommended approach?

  • Trigger manual re-association events?

  • Extend the RRC layer?

  • Implement a custom handover/selection mechanism?

3. Channel Modeling Between gNB and UE (ThreeGppNtnChannel)

Regarding the PHY/channel layer:

• I would like to model the channel between gNB and UE using the ThreeGppNtnChannel model.

• Is it possible to directly use this model in place of the default terrestrial 3GPP channel model?

• If yes:

  • What modifications are required?

  • Does it require changes in the spectrum/channel helper configuration?

  • Are there specific attributes that must be enabled for NTN operation?

4. Can we override other antenna elements and does it support beam steering if we use other antenna patterns apart from UPA(isotropic antenna) ?

Additionally, if there are example configurations, papers, or implementation notes related to NTN channel usage in 5G-LENA, I would be grateful if you could share them.

To summarize, I am primarily looking for guidance on:

1. Direct EPC connectivity for non-gNB nodes and IP traffic routing.

2. Dynamic UE-gNB association in an NTN mobility scenario.

3. Proper integration and configuration of the ThreeGppNtnChannel model.

Any pointers, references, or architectural suggestions would be extremely helpful for progressing with this work.

Thank you very much for your time and support.

Kind regards,

Nikhil GS

[Kuan-Po Chiu (Jesse)]

2) Dynamic UE–gNB association
There is an example in
/examples/three-gpp-leo-constellation.cc
that performs runtime association based on LoS. In this setup, the UE dynamically connects to satellites as visibility changes, so it is not purely static like ConnectToGnb().

That said, the current mechanism is relatively simple (LoS-based). If you need more advanced selection (e.g., SINR-based, hysteresis), you would likely need to extend it via custom handover logic or RRC modifications.

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3) NTN channel model (ThreeGppNtnChannel)
Yes, you can use the built-in ThreeGppNtnChannel instead of the terrestrial model.

The main point to watch is geometry consistency. There is currently a known issue in the ns-3 LEO mobility model affecting small-scale fading due to coordinate inconsistencies—we are working on fixing this. So the model works, but validation of fast fading should be done carefully.

---

4) Antenna / beamforming
I have tested both isotropic antennas and the 3GPP TR 38.901 antenna model with orientation enabled, and both work within the current framework. Not sure if it's you're looking for or you want a custom antenna pattern.

---

I have an integrated ns-3 LEO + 5G-LENA setup in a local repository (mobility + channel + 5G link), though it still needs some cleanup. Happy to discuss or share details—feel free to reach out.

Best regards,
Jesse

[Nikhil Gowda]

Dear Jesse,

Thank you very much for your detailed response and for taking the time to clarify these points, I truly appreciate your support.

Regarding the channel and antenna aspects, I can confirm that the ThreeGppNtnChannel and antenna configurations are working well in my current setup.

For context, I am currently using an ns-3 + 5G-LENA setup (without the ns-3 LEO integrated module), where I am modeling satellite mobility using SGP4.

On the dynamic UE–gNB association, I attempted to locate the example you mentioned (/examples/three-gpp-leo-constellation.cc), but I was unable to find it in my current repository. It would be very helpful if you could provide more details or point me to the correct source or branch where this example is available.

I am particularly interested in understanding how runtime UE association based on LoS is implemented and I could be adapted to my current setup

I also found your mention of the integrated ns-3 LEO + 5G-LENA setup very interesting. I’m particularly curious about how you handled EPC integration in that scenario—specifically:

• How did you connect the EPC core (SGW/PGW) to the satellite mesh?

• Was the EPC involved in routing, or did you rely on a separate routing mechanism within the satellite network?

Thanks again for your guidance, and I would be glad to learn more about it.

Best regards,

Nikhil GS

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