UE does not see gNB

[luyuanfan]

Problem Description

I've been trying to set up a gNB with srsRAN, Open5GS, and a X410 USRP device. However, the network does not show up in my UE's list of networks. I'm think that the problem could be in the configuration files. I'm also not sure if the way I connect the antenna could cause the problem.

Setup Details

Core Platform: Open5GS
RF device: USRP X410 (connected via ethernet cable with ip address set to 192.168.20.2)
UHD version: 4.8.0
OS: Ubuntu 24.04.1 LTS
UE: Nothing phone 2

Attached Files

• open5gs-config.txt
• gnb-config.txt
• open5gs-log.txt
• gnb-log.txt
• 

[luyuanfan]

I omitted some of the gNB log file. It says Real-time failure in RF: underflow

[andrepuschmann]

Hey - if you see constant lates or underflows it's an indication for network config issues, perhaps rx/tx buffering on the NIC, kernel?
Please run ./benchmark_rate --rx_rate 30.72e6 --tx_rate 30.72e6 and see if that works without issues on your machine. If it does, try ./benchmark_rate --rx_rate 122.88e6 --tx_rate 122.88e6 as well.

[luyuanfan]

Hey - if you see constant lates or underflows it's an indication for network config issues, perhaps rx/tx buffering on the NIC, kernel? Please run ./benchmark_rate --rx_rate 30.72e6 --tx_rate 30.72e6 and see if that works without issues on your machine. If it does, try ./benchmark_rate --rx_rate 122.88e6 --tx_rate 122.88e6 as well.

Hi! Thank you for the response. I did forget to run the srsran performance tuning script (for network buffer tuning). After running that script and your suggestion, it seems to have fixed the underflow problem. However, the UE still does not see the network.

I don't see much warning when I set the gnb log: all_level: to warning, so I'm taking that as there's no network config issues. So I'm guessing that there might still be problems with the configurations. Is there anything that might be problematic in what I have?

[robertfalkenberg]

I've seen your PLMN is 999/70. Unless you are 100% confident the UE supports that network (e.g. you successfully tested the same UE + PLMN with different equipment), I suggest you configure the network with Test-PLMN 001/01.

Further details in #100 (reply in thread)

[luyuanfan]

I've seen your PLMN is 999/70. Unless you are 100% confident the UE supports that network (e.g. you successfully tested the same UE + PLMN with different equipment), I suggest you configure the network with Test-PLMN 001/01.

Further details in #100 (reply in thread)

Hi - Yes I believe I have tried this combination. My UE would not allow me to add an APN 00101. However, even if this is true, the UE should still be able to see the network. I've tried many combinations including this but it doesn't work.

[robertfalkenberg]

However, even if this is true, the UE should still be able to see the network

Unfortunately no; Some UE already filter blacklisted networks from that list - they are just omitted (unlike blocked networks via USIM, which are usually listed but tagged as "(not allowed)").

[robertfalkenberg]

Another example is found here:

#1348

[robertfalkenberg]

Hi @luyuanfan,

I've seen you closed the issue. Do you have any hints for the community how you resolved it?

[luyuanfan]

Hi @luyuanfan,

I've seen you closed the issue. Do you have any hints for the community how you resolved it?

I, in fact, have not solved the problem yet! I ran a spectrum analyzer (uhd_fft) which could be a pointer to where the problem lies.

According to this tutorial, if network is not visible to the UE, it could be because the gNB's signal is unhealthy. A healthy (whatever that takes) spectrum should have a plateau shape around the center frequency. It attached this example for reference:

Here's the spectrum that I got (I'm not using the waterfall mode in this image, but this is clearly not plateau-like):

This is the gNB config for this run:
gnb_rf_b200_tdd_n78_20mhz.yml

So my currently direction is to find out:

1. If the spectrum should have a plateau shape even before the UE connects (I assume the answer to this is yes)
2. What could be the cause of this unhealthy shape.

If you have any idea, please let me know!

[luyuanfan]

Hi @luyuanfan,

I've seen you closed the issue. Do you have any hints for the community how you resolved it?

The only other thing I found out to be important is that I should have programmed the SIM card so that its IMSI starts with 00101 (referencing this). I chose 00101 because I'm using a OnePlus 13R, and it seems to work with other OnePlus models.

[luyuanfan]

There's one line in the gNB output that might be useful. So it says:

Cell pci=1, bw=20 MHz, 1T1R, dl_arfcn=368500 (n3), dl_freq=1842.5 MHz, dl_ssb_arfcn=366970, ul_freq=1747.5 MHz

I believe dl stands for downlink. The spectrum analysis I have above uses dl_freq=1842.5 MHz.

I tried to run the analysis around ul_freq=1747.5 MHz as well. Attached here is the result:

It seems like the signal is just very weak but TBH I don't know what the expected behavior should be.

[luyuanfan]

I also do not know how to make sense of the periodic little spikes when I run the analysis around downlink frequency. It this expected behavior?