Instability in staying camped only on the home network

Knowledge Base Power
1

Hi.
I’m starting a project with 5G and I still have little experience in this telecommunications area. In my lab I have a Quectel RM520N-GL board and a USRP B210 board. Everything is connected and working relatively well. That is, my 5G modem is camped on my 5G test network (MCC = 001 and MNC = 01) and is registered. See:

+QENG: “servingcell”,“NOCONN”,“NR5G-SA”,“TDD”,001,01,000000E00,0,1,641280,78,6,-84,-11,22,1,-

However, if I move the gNB a few centimeters away from the modem, the RM520N will camp on a carrier operator’s network. So the modem’s connection to the 5G network is very sensitive. In a stable situation, the gNB and 5G modem are only 1 meter apart.

What can cause this instability? What should I check on the modem or on the gNB?

Notes: my 5G modem has 3 antennas, but there is room to connect one more. With 3 antennas, could I have problems registering on the lab network? Does that make sense? Also, my modem has a USB cable with 2 plugs to connect to my computer. I connected only one of the plugs to the machine where I run AT commands. The other plug (not connected) is for extra power to the 5G modem? Should I connect it as well? Could not connecting it cause instability of the link with the gNB?

On my USRP B210 I connected only one antenna to TX/RX. I did not connect RX2. Should I connect another antenna to RX2?

Thank you very much for any tips.
Best regards.

2

Deviating from the questions you’ve asked, I’ll just mention that you can lock the modem to your network with the AT command: AT+COPS=1,2,"00101"

3

Hi.

Thank you for your reply. I had tried that last Friday. But this command returns the response “+CME ERROR: 30” 99% of the time. So I also tried the other two formats of this command:

0 → Long format alphanumeric up to 16 characters.
1 → Short format alphanumeric .

That is:

AT+COPS=1,0,“Long format alphanumeric ”
AT+COPS=1,1,“Short format alphanumeric ”

These last two commands return OK as a response, but they have no effect on the RM520N-GL module.

For these reasons, I suspected a problem with signal strength or antenna positioning.

4

The “soft” lock option AT+COPS=4,2,"00101" is also worth a try.

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