nRF9160 - Improve slow cellular registration

[Vincent44]

Hello,

I was wondering if there if there is way to improve the the network registration process on a custom board with nRF9160.

I currently have a typical "engineer setup" to work on the firmware (custom board equipped with an nRF9160 and a 1NCE SIM card, sitting on my desk).
It works great. However, when uploading a new firmware, the duration to re-connect to the network is very inconsistent : sometimes it connects within less than 30 sec, and sometimes it is still not connected after 3 minutes. In both cases, the device it at the same location, on my desk.

Because my device might be placed in a location where network is not reachable, I have implemented a timer that will switch off the device in case it takes too long to connect. This will prevent draining too much battery. The question is : how long is too long ? Is there a way to improve that ? For example, being able to detect that there are no cells around, so there is not point waiting for too long ?

Thanks.

Vincent

[Achim Kraus]

My experience:

If the modem searches for a network, that may take a while.

The modem keeps the last network successfully registered.

If the modem is restarted (cfun=0/cfun=1), the modem persist that last successful network and uses that for the first try. If the network is still there, the registration is quite fast. If not, it again takes a while to search a network.

If the modem isn't switched off (cfun=0) before the SoC restarts/reboots, then it depends on how long the modem was on. According the doc:

"The command configuration is stored to NVM approximately every 48 hours and when
the modem is set to minimum functionality mode with the +CFUN=0 command."

So, if you reboot the SoC, then you may either use a CFUN=0 before, or you may depend on how long the modem is running after it successfully registers to a network. If none is true, then the found network may be not persisted and that may end in longer search for a network after the modem restarts with the SoC.

My app measure the time to register and if that's more than 60s, then I do a CFUN=0/CFUN=1 in order to ensure, the found network is persisted.